Neutronics Assessment of the Spatial Distributions of the Nuclear Loads on the DEMO Divertor ITER-like Targets: Comparison between the WCLL and HCPB Blanket

Abstract

The Plasma Facing Components (PFCs) of the divertor target contribute to the fundamental functions of heat removal and particle exhaust during fusion operation, being subjected to a very hostile and complex loading environment characterized by intense particles bombardment, high heat fluxes (HHF), varying stresses loads and a significant neutron irradiation. The development of a well-designed divertor target, which represents a crucial step in the realization of DEMO, needs the assessment of all these loads as accurately as possible, to provide pivotal data and indications for the design and structural performance prediction of the PFCs. In a particular way, this study is fully devoted to the comprehension of the distributions on the divertor target of the main nuclear loads due to neutron irradiation, performed for the first time using an extremely detailed approach. This work has been carried-out considering the latest configuration of the DEMO reactor, including the updated design of the divertor and ITER-Like PFCs geometry, varying the blanket layout (Water Cooled Lithium Lead—WCLL and Helium Cooled Pebble Bed—HCPB), thus evaluating the impact of the different blanket concept on the above-mentioned distributions. Neutronics analyses have been performed with MCNP5 Monte Carlo code and JEFF3.3 nuclear data libraries. 3D DEMO MCNP models have been created, focusing in particular on a thorough representation of the divertor and PFCs, allowing for the assessment of the distributions of the main nuclear loads: radiation damage (dpa/FPY), He-production rate (appm/FPY) and nuclear heating density (W/cm³) and total nuclear power deposition (MW). These results are presented by means of 2D maps and plots for each PFCs sub-component both for WCLL and HCPB blanket case: W-monoblocks, Cu-interlayers\CuCrZr-pipe and PFC-CB (Cassette Body) supports made of Eurofer steel.

1. Introduction

In the recent European roadmap [1,2] drafted for realizing commercially viable fusion power generation, reliable power handling was defined as one of the most critical missions. In this regard, the divertor is the key in-vessel component, as it is responsible for power exhaust and impurity removal via guided plasma exhaust. Due to the intense bombardment of energetic plasma particles, the Plasma Facing Components (PFCs) of the divertor are exposed to extreme heat flux loads, which being localized in a narrow layer, they can achieve values up to 20 MW/m² as peak condition [3,4,5,6,7,8,9,10,11]. The thermal stresses generated from such power densities can lead to plastic deformation, which causes the cracking of the surface of the wall materials, due to low cycle fatigue failure. Furthermore, the pulsed nature of a fusion reactor results in cyclic loads, leading to the creep-fatigue phenomenon, which leads to the deformation of the plasma facing materials under repeated stresses at high temperature, thus influencing the component lifetime [4,7,8,9]. In addition, charged particles collide with atoms of the target material, and exchange momentum. This could cause the ejection of atoms from the solid surface, through the erosion process of sputtering [3,4,6,7]. If neutron irradiation is added, the operational conditions of DEMO divertor targets become even more critical, underlining the necessity to deal with these challenging issues [9,12].

Neutrons penetrate deeply in the machine damaging the whole volume of the reactor. The main primary effects of neutron irradiation on plasma facing materials, can be expressed in terms of nuclear loads: radiation damage, which leads to atomic displacements; transmutation due to neutron interaction with matter, which leads to the formation of He and H gasses and other chemical elements, some of which can induce a high induced radioactivity; nuclear heating, caused by the above-mentioned phenomena and all the possible nuclear reactions that can occur in the interaction between radiation and matter, with a consequent release of energy.

They can lead to consequent secondary microscopic and macroscopic material’s responses, which generally include: material overheating, swelling, change in electrical resistivity, dimensional instabilities, change in yield strength and the loss of ductility with the increase of embrittlement behavior, change in creep rate, change in fatigue life, the loss of fracture toughness, induced radioactivity and decay heat, change in thermal conductivity, chemical composition change and others [7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24], as shown in Figure 1 [13].

This entails that due to their location, the PFCs receive the highest damage during operation and thus these are the parts with the shortest lifetime in the reactor. In this regard, it appears evident how performing a detailed and accurate neutronics assessment of the distribution on the targets of the nuclear loads and quantities of interest, represents a crucial aspect for the design of PFCs. This kind of analysis can provide input data and pivotal indications essential to evaluate the PFCs performance during the operation and maintenance phases, to estimate its feasible structural lifetime, as well as to estimate the type of treatment to which this component will be undergone in the decommissioning phase. The PFCs of a divertor cassette consists of three parts, namely, the straight leg of the vertical target, liner and reflector plates (replaced by the dome in some previous configurations) and baffle region [24,25,26]. The 2019 DEMO divertor cassette concept has been considered for this study and its layout is shown in Figure 2 [25,27]. In addition, the latest design of the ITER-Like PFCs has been taken into account [25,26,27,28,29,30,31] (Figure 3). This work is focused on an extensive neutronics comparative analysis mainly concerning the assessment of the spatial distribution of the principal nuclear loads on the DEMO divertor PFCs targets, considering both the WCLL and HCPB blanket configurations, during operations. The different type of blanket has an impact on the neutron spectrum (water vs. He as coolant) impinging on the targets [32]. Therefore, it is reasonable to expect an effect on nuclear loads levels and intensity distribution, if these two blanket options are compared. In particular, this study consists of the following parts:

• The calculation and assessment of the nuclear damage (dpa/FPY), nuclear heating (W/cm³) and He-production rate (appm/FPY) distributions on W-monblocks, Cu-interlayers\CuCrZr-pipe and PFC-CB supports made of Eurofer steel, of inner and outer target, considering separately, both the WCLL and HCPB blanket configurations.
• Direct comparison between the above-mentioned nuclear loads distributions among the two blanket concepts, in order to evaluate the blanket impact.
• Assessment, both for WCLL and HCPB cases, of the total nuclear power (MW) on PFCs and breakdown in each component, comparing the results between the two DEMO configurations under analysis.

Two MCNP models have been reproduced: WCLL [33,34,35,36,37,38] and HCPB [39] Single Module Segment layered blankets, both with the new MCNP model of the divertor 2019 configuration. Detailed MCNP representations of the whole ITER-Like (IL) PFCs inner and outer targets have been recreated and integrated in both the above-mentioned models [25]. Neutron and gamma transport simulations have been performed using MCNP5v1.6 Monte Carlo code [40] and the Joint Evaluated Fusion File JEFF 3.3 nuclear data libraries [41].

2. WCLL and HCPB DEMO MCNP Models with Divertor IL PFCs: Description and Integration

The analyses have been performed using two models: starting from the EU DEMO1 2017 reference configuration MCNP model (representing a 11.25° toroidal sector of the tokamak, with plasma parameters shown in

Table 1. Main parameters of the DEMO baseline configuration [42].

No. of Toroidal Field Coils	16
Major radius (m)	8.938
Minor radius (m)	2.883
Aspect ratio	3.1
Plasma elongation	1.65
Plasma triangularity	0.33
Fusion power (MW)	1998
Average neutron wall loading (MW/m2)	1.04
Net electric power (MW)	500

) [42] two MCNP geometrical models consisting of semi-heterogeneous blanket representation of WCLL and HCPB blanket, both with the MCNP model of the divertor 2019 configuration [43], have been generated (Figure 4). Moreover, the detailed MCNP model of the entire ITER-Like PFCs targets have been reproduced and integrated in the above-mentioned divertor model (Figure 5) both with WCLL and HCPB blanket representation (Figure 6). The detailed CAD model [27] of ITER-Like PFCs targets is shown in Figure 7.

More details about the ITER-Like PFCs design and MCNP model description, the layout and modeling approach chosen for the WCLL and HCPB blankets herein discussed, are better explained in the reference paper [25].

Note that the colors in Figure 4, Figure 5 and Figure 6 are representative of the material mixtures assigned to every geometrical cell of the MCNP models when these are observed through the MCNP geometry viewer. Therefore, these colors are shown only for a symbolic scope, not having any specific meaning. The pre-processing and preparation of the IL CAD model has been performed by means of the 3D modeling software ANSYS SpaceClaim 2019 [44] in order to generate a CAD model suitable for neutronic analyses. The simplified CAD model has been converted into the equivalent MCNP geometrical representation using the CAD-to-MCNP interface of SuperMC [45]. To create a correct coupling between the neutronics model of the PFC and that of the cassette body and to further simplify the model integration phase, it has been necessary to integrate 31 (inboard) and 43 (outboard) CuCrZr pipes in the DEMO divertor 2019 MCNP model, which is shown in Figure 5 and Figure 6. An example of a full outer target array of 43 PFCs is shown in Figure 8 [46].

An individual pipe can be arranged 78 and 70 W-monoblocks and the related Cu-interlayers, on inboard and outboard, respectively. Regarding the PFC-CB support design, in these models has been integrated 16 and 14 PFCs-CB supports on inboard and outboard, respectively (as shown in Figure 9), for each CuCrZr pipe. It should be noted that the arrangement pitch of the supports in the baffle regions has not been specified in the reference divertor CAD model [27], therefore it has been assumed to be the same as that on the straight leg. A summary of the numbers of PFCs elements towards the vertical and horizontal directions (poloidal and toroidal abscissa, respectively) of both the targets, in accordance with the representation shown in Figure 7, is highlighted in

Table 2. Number of PFCs elements towards the horizontal and vertical direction of the inner target (see Figure 7).

Inner Target	Number of Components towards the Horizontal Direction (Toroidal Abscissa)	Number of Components towards the Vertical Direction (Poloidal Abscissa)	Total (Horizontal × Vertical)
CuCrZr-pipe	31	1	31
Cu-interlayer	31	78	2418
W-monoblock	31	78	2418
PFC-CB support	31	16	496

and

Table 3. Number of PFCs elements towards the horizontal and vertical direction of the outer target (see Figure 7).

Outer Target	Number of Components towards the Horizontal Direction (Toroidal Abscissa)	Number of Components towards the Vertical Direction (Poloidal Abscissa)	Total (Horizontal × Vertical)
CuCrZr-pipe	43	1	43
Cu-interlayer	43	70	3010
W-monoblock	43	70	3010
PFC-CB support	43	14	602

.

3. Nuclear Analyses

The developed MCNP DEMO models, with a fully heterogeneous representation of the 2019 divertor and IL PFCs, has been used to assess the nuclear damage (dpa/FPY), nuclear heating (W/cm³) and He-production rate (appm/FPY), spatial distributions on W-monblocks, Cu-interlayers/CuCrZr-pipe and PFC-CB supports made of Eurofer steel, on inner and outer target. These results are presented through 2D-meshes with fine resolutions:

• 31 × 78 for Cu/CuCrZr and W of the inner target.
• 31 × 16 for Eurofer of PFC-CB supports of the inner target.
• 43 × 70 for Cu/CuCrZr and W of the outer target.
• 43 × 14 for Eurofer of PFC-CB supports of the outer target.

In addition, a direct comparison between the above-mentioned nuclear loads distributions between the two blanket concepts, in order to quantify the blanket impact has been performed. The results are provided using 2D plots which show the nuclear quantity variation along the poloidal profiles as function of a curvilinear abscissa which starts from the upper part of the targets (baffle region) and directed towards the lower part (Figure 10). Every graph shows the poloidal profiles comparing both the external (highlighted in purple in Figure 7) and internal (black in Figure 7) sides of the target loads, both for WCLL and HCPB blanket. Each point is representative of the nuclear load value on each component (W, Cu/CuCrZr and Eurofer) along the profile. The number of components is 78 (IB) and 70 (OB) W-Monoblocks/Cu-Interlayers/CuCrZr pipe, 16 (IB) and 14 (OB) Eurofer supports as shown in Table 2 and Table 3.

Furthermore, the total nuclear power (MW) on PFCs and breakdown in each component, both for WCLL and HCPB blanket case, has been assessed, comparing the values among the two configurations. The materials properties and composition have been selected according to the specifications for neutronic analyses in the EUROfusion framework [47]. In particular, the reference composition of tungsten can be found in the article [25]. The reference compositions of Eurofer steel and CuCrZr alloy are shown in

Table 4. Reference Eurofer steel composition used for neutronics analyses [47].

EUROFER-97 Steel	Reference for Neutronics Analysis
Density (g/cm3)	7.87
Composition	wt%
Fe	88.698
C	0.11
Mn	0.4
Cr	9
V	0.2
Ta	0.12
W	1.1
N	0.03
Ti	0.02
P	0.005
Si	0.05
S	0.005
Ni	0.01
Mo	0.005
Cu	0.01
Nb	0.005
Al	0.01
B	0.002
Co	0.01
As	0.05
Sn	0.05
Sb	0.05
Zr	0.05
O	0.01

and

Table 5. Reference CuCrZr alloy composition used for neutronics analyses [47].

CuCrZr	Reference for Neutronics Analysis
Density (g/cm3)	8.90
Composition	wt%
Cu	98.71
Cr	0.75
Zr	0.11
Co	0.05
Ta	0.01
Nb	0.1
B	0.001
O	0.032
Mg	0.04
Al	0.003
Si	0.04
P	0.014
S	0.004
Mn	0.002
Fe	0.02
Ni	0.06
Zn	0.01
As	0.01
Sn	0.01
Sb	0.011
Pb	0.01
Bi	0.003

, according to [47]. The results have been normalized to 1998 MW fusion power (neutron yield: 7.094 × 10²⁰ n/s), according to the plasma parameters specified in Table 1. The simulations have been performed using standard MCNP cell-based tallies (F4) with proper multiplier to calculate the nuclear quantities of interest for the PFCs design development, performance assessment and verification of design requirements [48]. The results have statistical uncertainties in the range ±0.5–5% (depending on the position and dimensions of the cell under analysis) with a maximum of ±7% in a few limited zones. The calculations have been performed on the ENEA CRESCO cluster [49].

3.1. Spatial Distributions of the Nuclear Loads on the Divertor Targets, Both for WCLL and HCPB Blanket

Figure 11 and Figure 12 show the nuclear heating (W/cm³) distribution on W-monoblocks of the inner and outer target, both for WCLL and HCPB, respectively. From the maps shown in Figure 11 the following considerations can be pointed out: the absolute peak value amounts to around 21.40 W/cm³ with the WCLL blanket and it is achieved in correspondence of the 47th monoblock along the vertical direction of the 1st and 43rd columns (external sides) on the outer target. The maximum value on the inner target amounts to around 21.20 W/cm³, achieving it in correspondence of the 58th monoblock along the vertical direction of the external edges of the target (1st, 2nd, 30th and 31st columns). More in general, for both the targets, the highest values of nuclear heating are reached on the baffle regions than the straight leg of the vertical target, this latter area being less exposed to neutron irradiation (smaller plasma view factor). In this regard, the minimum values amount to about 12.80 W/cm³ for the inner target, achieved in correspondence of the 4th monoblock on the innermost arrays (columns: 14th, 15th and 16th) and 8.31 W/cm³ on the 4th monoblock arranged on the innermost arrays (columns: 21st, 22nd, 23rd arrays) of the outer target. In addition, a symmetrical trend of the distribution with respect to the target central axis is highlighted for both the targets. It can also be noted that the intensity of the nuclear heating tends to be greater on the external sides of the target than on the internal ones. This effect on the toroidal distribution of the loads is much more accentuated on the inner target than the outer one and it is probably since the external sides (especially on the baffle region) are subjected to higher irradiation levels due to the radiation streaming in the gaps between the cassettes. From the maps shown in Figure 12 the following highlights can be underlined: the absolute peak value amounts to around 19.00 W/cm³ with the HCPB blanket both for the outer and inner target. They are achieved in correspondence of the 47th monoblock along the vertical direction of the 1st and 43rd columns (external sides) for the outer target and of 58th monoblock towards the vertical direction of the external edges of the inner target (columns: 1st, 2nd, 30th and 31st arrays). Once again, for both the targets, highest values of nuclear heating are reached on the baffle regions than the straight leg of the vertical target, being the minimum values about 10.44 W/cm³ for the inner target, reached in correspondence of the 4th monoblock on the innermost arrays (columns: 14th, 15th and 16th) and 7.84 W/cm³ on the 4th monoblock arranged on the innermost arrays (21st, 22nd, 23rd columns). The symmetrical trend of the distribution with respect to the target central axis for both the targets is confirmed also with the HCPB blanket. The intensity of the nuclear heating tends to be greater on the external sides of the target than on the internal ones (especially for the inner target), also considering the HCPB case.

The nuclear heating density distribution (W/cm³) on Cu\CuCrZr components of the inner and outer target, are shown in Figure 13 and Figure 14, for the WCLL and HCPB case, respectively. Looking at the maps of Figure 13, the following highlights can be underlined: the absolute maximum value amounts to 8.77 W/cm³, considering the WCLL blanket and it is achieved in correspondence of the 60th copper element along the vertical direction of the external sides of the inner target (1st, 2nd, 30th and 31st arrays). The peak value on the outer target is reached in correspondence of the 49th copper element placed along the vertical direction, again on the external edges (1st, 2nd, 42nd and 43rd arrays), amounting to around 8.28 W/cm³. The minimum values on the inner and outer target are 3.88 W/cm³ and 2.94 W/cm³, respectively. In the same order, they are achieved both on the 1st row of Cu/CuCrZr elements and in the innermost columns (inner: 14th, 15th and 16th, outer: 21st, 22nd and 23rd). Considering the maps shown in Figure 14, related to the HCPB case, the following evaluations can be done: the absolute peak value amounts to 8.14 W/cm³ and it is observed on the 60th Cu\CuCrZr element along the vertical direction of the inner target external sides, as before seen. Regarding the outer target, the maximum nuclear heating value is reached in correspondence of the 52nd Cu\CuCrZr segment on the vertical abscissa, again on the same external sides already stated, amounting to 7.66 W/cm³. The minimum values on the inner and outer target are 3.22 W/cm³ and 2.48 W/cm³, respectively. In the same order, they are achieved both on the 1st row of Cu/CuCrZr elements and in the innermost columns (inner: 14th, 15th and 16th, outer: 21st, 22nd and 23rd). Once again, the symmetrical trend of the heat load distribution with respect to the target central axis is evident, as well as the more intense values on the baffle regions of both the targets than the vertical straight leg and strike zones, as can be expected. This is confirmed for both the blankets.

Figure 15 and Figure 16 show the nuclear heating (W/cm³) distribution on the Eurofer PFC-CB supports of the inner and outer target, both for WCLL and HCPB, respectively. As far as the maps shown in Figure 15 are concerned, the following main evaluations can be stated: the absolute peak value amounts to 7.29 W/cm³, which is observed in correspondence of the 13th row of the PFC-CB supports, arranged on the more external sides of the inner target (columns: 1st, 2nd and 30th and 31st). The maximum value on the outer target PFC-CB supports is 6.69 W/cm³ and it is reached in correspondence of the 10th row and external columns (1st and 43rd arrays). The minimum values amount to 2.63 W/cm³ and 1.96 W/cm³, respectively, reached on the 2nd row of the inner target (innermost columns: 14th, 15th and 16th) and 1st row of the outer target (innermost columns: 21st, 22nd and 23rd). Looking at maps shown in Figure 16, which are related to the HCPB blanket case, the following assessments can be made: the absolute maximum value is 6.45 W/cm³, which is reached on the 13th row of the PFC-CB supports, placed on the more external edges (columns 1st and 31st) of the inner target. The peak value on the outer target is 6.12 W/cm³, which is in correspondence with the 11th row of the PFC-CB supports (external columns of the array: 1st and 43rd). The minimum values amount to 1.62 W/cm³ and 2.09 W/cm³, respectively, reached on the 1st row and 2nd row and innermost columns: 21st, 22nd and 23rd (outer target), 14th, 15th and 16th (inner target). Additionally in this case, the symmetrical trend of the nuclear heating distribution with respect to the target central axis is highlighted, as well as the more intense values on the baffle regions of both the targets than the vertical straight leg and strike zones, as reasonable. This is verified for both blankets.

Figure 17 and Figure 18 show the nuclear damage (dpa/FPY) distribution on W-monoblocks of the inner and outer target, both for WCLL and HCPB, respectively. Considering the maps shown in Figure 17, related to the WCLL case, the following observations can be noted: the absolute peak value is 2.10 dpa/FPY, achieved in correspondence of the 67th row of the W-monoblock arrangement, and in particular non the external columns (1st, 2nd and 30th, 31st) of the inner target. The maximum value on the outer target amounts to 2.07 dpa/FPY, which appears to be quite similar to the inner target case, and it can be observed on the 53rd row and external columns (1st, 2nd and 42nd, 43rd) of the W-monoblock array. The absolute minimum value of nuclear damage on W is 0.43 dpa/FPY which is reached on the 1st row and internal columns (21st, 22nd, 23rd) of the outer target. The minimum value on the inner target is 0.52 dpa/FPY, reached on the innermost columns and lowest row of the array (1st row; 15th, 16th and 17th columns). Observing Figure 18, which show the maps of the nuclear damage (dpa/FPY) distribution on W-monoblocks for both the target with the HCPB blanket, the following assertion can be made: the absolute peak value amounts to 1.84 dpa/FPY, and it can be observed on the 53rd row and external columns (1st, 2nd and 42nd, 43rd) of the outer target. The maximum value on the inner target is 1.83 dpa/FPY dpa/FPY, reached on the 66th row and external columns (1st, 2nd and 30th, 31st) of the inner target. The absolute minimum value is 0.39 dpa/FPY, which is reached on the 1st row and innermost columns (21st, 22nd and 23rd) of the outer target. The lowest value on the inner target is 0.44 dpa/FPY, once again on the 1st row and innermost columns (15th, 16th and 17th). Additionally in this case, the highest values of the nuclear damage are achieved on the external edges of both the targets and in particular on the baffle region, showing overall a symmetrical distribution with respect to the central column of both the targets.

Figure 19 and Figure 20 show the maps of the nuclear damage (dpa/FPY) distribution of the Cu/CuCrZr components on the inner and outer target, both for WCLL and HCPB, respectively. In particular, looking at the maps of Figure 19, which are referred to the DEMO configuration with WCLL blanket the following highlights can be noted: the absolute peak value amounts to 8.75 dpa/FPY and it is achieved in correspondence of the 60th row and external columns (1st, 2nd, 30th and 31st) of the inner target Cu/CuCrZr components. The maximum value on the outer target Cu/CuCrZr components, is 8.47 dpa/FPY, which is reached on the 54th row and external columns (1st, 2nd and 42nd, 43rd) of the array.

The absolute minimum value is 1.42 dpa/FPY, observed on the 1st row and innermost columns (21st, 22nd and 23rd) of the Cu/CuCrZr components of the outer target. The lowest value of the inner target is 1.78 dpa/FPY, once again reached on the bottom and central part of the target (1st row and 15th, 16th, 17th columns). Regarding the HCPB blanket, the same maps of the nuclear damage distribution on Cu/CuCrZr components, can be seen in Figure 20, pointing out the following highlights: the absolute peak value is reached on the 60th row and external columns (1st, 2nd and 30th, 31st) of the inner target, and it amounts to 7.31 dpa/FPY.

The peak value on the outer target is achieved on the 53rd row and external columns (1st, 2nd and 42nd, 43rd) and it is 7.22 dpa/FPY. The absolute minimum value is 1.28 dpa/FPY, on the 1st row and innermost columns (21st, 22nd and 23rd) of the Cu/CuCrZr components of the outer target. The lowest value of the inner target amounts to 1.47 dpa/FPY, once again reached on the bottom and central part of the target (1st row and 15th, 16th, 17th columns). The nuclear damage distributions, as in the previous cases, show a greater intensity, both with WCLL and HCPB, on the baffle region and external sides of the targets, as it is reasonable to expect.

Figure 21 and Figure 22 show the maps of the nuclear damage (dpa/FPY) distribution of the Eurofer PFC-CB supports on the inner and outer target, both for WCLL and HCPB, respectively. If the maps of Figure 21 are observed, which are related to the configuration with WCLL, the following considerations can be stated: the absolute peak value amounts to 5.07 dpa/FPY and it is achieved in correspondence of the 14th row and external columns (1st, 2nd, 30th and 31st) of the inner target array. The maximum value of damage on the outer target is 4.99 dpa/FPY and it is reached in correspondence of the 11th row and external columns (1st and 43rd) of the array. The absolute minimum value amounts to 0.77 dpa/FPY and it can be observed on the 1st row and innermost columns (21st, 22nd and 23rd) of the outer target arrays. The minimum value on the inner target is 0.86 dpa/FPY, reached on the 1st row and innermost columns (15th, 16th and 17th) of the array.

Regarding the HCPB blanket, the same maps of the nuclear damage distribution on Eurofer PFC-CB supports, can be seen in Figure 22, highlighting the following main considerations: the absolute peak value is reached on the 11th row and external columns (1st and 43rd) of the outer target array, and it amounts to 4.41 dpa/FPY. The peak value on the inner target is achieved on the 13th row and external columns (1st and 31st) and it is 4.40 dpa/FPY, showing an identical peak value of the outer target. The absolute minimum value is 0.68 dpa/FPY, on the 1st row and innermost columns (21st, 22nd and 23rd) of the outer target array. The lowest value of the inner target amounts to 0.72 dpa/FPY, once again reached on the bottom and central part of the array (1st row and 15th, 16th, 17th columns). These nuclear damage distributions, as for the other components already analyzed, show higher levels of magnitude, both with WCLL and HCPB, on the baffle region and external sides of both the targets, as already underlined. Figure 23 and Figure 24 show the maps of the He-production rate (appm/FPY) distribution of W-monoblocks arranged on the inner and outer target, both for WCLL and HCPB, respectively. The maps of Figure 23 (WCLL) highlight the following main considerations: the absolute peak value amounts to 1.88 appm/FPY, reached in correspondence of the 53rd row and external columns (1st and 43rd) of the outer target array. The maximum value on the inner target is 1.83 appm/FPY and it is achieved in correspondence of the 67th row and external columns (1st and and 31st) of the array. The minimum values amount to 0.32 appm/FPY (inner target) and 0.37 appm/FPY (outer target), both reached on the 1st row and innermost edges (15th, 16th and 17th; 21st, 22nd and 23rd; inner and outer, respectively).

In the maps of Figure 24, which are referred to the HCPB case, the following main considerations can be stated: the absolute peak value is again reached in correspondence of the 53rd row and external columns (1st and 43rd) of the outer target, amounting to 1.90 appm/FPY. The maximum value on the inner target is 1.84 appm/FPY, achieved on the 67th row and external sides (columns 1st and 31st of the array). The minimum values are substantially the same already observed for the WCLL case. The He-production distributions, as for the other components already analyzed, highlight higher intensities, both with WCLL and HCPB, on the baffle region and external sides of both the targets and a symmetrical distribution with respect to the target central axis. In addition, comparing the maximum and minimum values, as well as the overall distributions, no significant differences between the WCLL and HCPB can be noted, outlining a very similar response in terms of He-production. Figure 25 and Figure 26 highlight the maps of the He-production rate (appm/FPY) distribution of Cu/CuCrZr components arranged on the inner and outer target, both for WCLL and HCPB, respectively. The maps of Figure 25 (WCLL) show the following main conclusions: the absolute peak value amounts to 55.56 appm/FPY, reached in correspondence of the 52nd row and external columns (1st and 43rd) of the outer target array. The maximum value on the inner target is 55.20 appm/FPY and it is achieved in correspondence of the 60th row and external columns (1st and and 31st) of the array. The minimum values amount to 6.76 appm/FPY (inner target) and 8.04 appm/FPY (outer target), both reached on the 1st row and innermost sides (15th, 16th and 17th; 21st, 22nd and 23rd; inner and outer, respectively).

If the maps of Figure 26, which are referred to the HCPB case, are seen, the following main considerations can be stated: the absolute peak value is again reached in correspondence of the 54th row and external columns (1st and 43rd) of the outer target, amounting to 56.28 appm/FPY. The maximum value on the inner target is 54.65 appm/FPY, achieved on the 60th row and external sides (columns 1st and 31st of the array). The lowest values amount to 6.91 appm/FPY (inner target) and 8.14 appm/FPY (outer target), both reached on the 1st row and innermost sides (15th, 16th and 17th; 21st, 22nd and 23rd; inner and outer, respectively). The He-production distributions, as for the other components already assessed, highlight higher intensities, both with WCLL and HCPB, on the baffle region and external sides of both the targets and a symmetrical distribution with respect to the target central axis. In addition, comparing the maximum and minimum values, as well as the overall distributions, a very similar response in terms of He-production, can be outlined both with WCLL and HCPB.

Figure 27 and Figure 28 highlight the maps of the He-production rate (appm/FPY) distribution of Eurofer PFC-CB supports arranged on the inner and outer target, both for WCLL and HCPB, respectively. The maps of Figure 27 (WCLL) lead to the following main conclusions: the absolute peak value amounts to 58.03 appm/FPY, reached in correspondence of the 13th row and external columns (1st and 31st) of the inner target array. The maximum value on the outer target is 57.15 appm/FPY and it is achieved in correspondence of the 11th row and external columns (1st and and 43rd) of the array. The minimum values amount to 10.24 appm/FPY (inner target) and 9.25 appm/FPY (outer target), reached respectively on the 2nd and 1st row and innermost edges (15th, 16th and 17th; 21st, 22nd and 23rd; inner and outer, respectively). The maps of Figure 28 are referred to the HCPB case, and the following main considerations can be stated: the absolute peak value is achieved in correspondence of the 11th row and external columns (1st and 43rd) of the outer target, amounting to 55.81 appm/FPY. The maximum value on the inner target is 54.64 appm/FPY, achieved on the 13th row and external sides (columns 1st and 31st of the array). The lowest values amount to 8.83 appm/FPY (inner target) and 8.50 appm/FPY (outer target), reached respectively on the 2nd and 1st row, on the innermost columns (15th, 16th and 17th; 21st, 22nd and 23rd; inner and outer, respectively). Once again, the He-production distributions highlight higher intensities, both with WCLL and HCPB, on the baffle region and external sides of both the targets and a symmetrical distribution with respect to the target central axis. In addition, comparing the maximum and minimum values, as well as the overall distributions, a quite similar response in terms of He-production in Eurofer, ca be outlined both with WCLL and HCPB, although less evident than W-monoblocks and Cu/CuCrZr components.

3.2. Summary of the Nuclear Load’s Spatial Distributions Analyses

In this section, the main highlights of the analysis related to the assessment of the nuclear load’s spatial distributions on the inner and outer target, both with WCLL and HCPB blanket, are outlined as follows:

• The maximum and minimum values and their positions (in terms of row (R) and column (C) of the array) of nuclear heating (W/cm³), nuclear damage (dpa/FPY) and He-production (appm/FPY) on the main PFC elements (W-monoblocks, Cu/CuCrZr, Eurofer of PFC-CB supports) are well summarized in

  Table 6. Maximum and minimum values and related position (in terms of row (R) and column (C) of the array) of nuclear heating density (W/cm³), nuclear damage (dpa/FPY), He-production (appm/FPY) on the PFCs of the WCLL inner target.

  WCLL—Inner Target
  Nuclear heating (W/cm3)	W monoblocks		Cu/CuCrZr components		Eurofer PFC-CB supports
  	Value	R × C	Value	R × C	Value	R × C
  Max.	21.20	58 × 1	8.77	60 × 1	7.29	13 × 1
  		58 × 31		60 × 31		13 × 31
  Min.	12.80	4 × 16	3.88	1 × 16	2.63	2 × 16
  Nuclear damage (dpa/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	2.10	67 × 1	8.75	60 × 1	5.07	14 × 1
  		67 × 31		60 × 31		14 × 31
  Min.	0.52	1 × 16	1.78	1 × 16	0.86	1 × 16
  He-production (appm/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.83	67 × 1	55.20	60 × 1	58.03	13 × 1
  		67 × 31		60 × 31		13 × 31
  Min.	0.32	1 × 16	6.76	1 × 16	10.24	2 × 16

  (WCLL inner target),

  Table 7. Maximum and minimum values and related position (in terms of row (R) and column (C) of the array) of nuclear heating density (W/cm³), nuclear damage (dpa/FPY), He-production (appm/FPY) on the PFCs of the WCLL outer target.

  WCLL—Outer Target
  Nuclear heating (W/cm3)	W monoblocks		Cu/CuCrZr components		Eurofer PFC-CB supports
  	Value	R × C	Value	R × C	Value	R × C
  Max.	21.40	47 × 1	8.28	49 × 1	6.69	10 × 1
  		47 × 43		49 × 43		10 × 43
  Min.	8.31	4 × 22	2.94	1 × 22	1.96	1 × 22
  Nuclear damage (dpa/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	2.07	53 × 1	8.47	54 × 1	4.99	11 × 1
  		53 × 43		54 × 43		11 × 43
  Min.	0.43	1 × 22	1.42	1 × 22	0.77	1 × 22
  He-production (appm/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.88	53 × 1	55.56	52 × 1	57.15	11 × 1
  		53 × 43		52 × 43		11 × 43
  Min.	0.37	1 × 22	8.04	1 × 22	9.25	1 × 22

  (WCLL outer target),

  Table 8. Maximum and minimum values and related position (in terms of row (R) and column (C) of the array) of nuclear heating density (W/cm³), nuclear damage (dpa/FPY), He-production (appm/FPY) on the PFCs of the HCPB inner target.

  HCPB—Inner Target
  Nuclear heating (W/cm3)	W monoblocks		Cu/CuCrZr components		Eurofer PFC-CB supports
  	Value	R × C	Value	R × C	Value	R × C
  Max.	19.00	58 × 1	8.14	60 × 1	6.45	13 × 1
  		58 × 31		60 × 31		13 × 31
  Min.	10.44	4 × 16	3.22	1 × 16	2.09	1 × 16
  Nuclear damage (dpa/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.83	66 × 1	7.31	60 × 1	4.40	13 × 1
  		66 × 31		60 × 31		13 × 31
  Min.	0.44	1 × 16	1.47	1 × 16	0.72	1 × 16
  He-production (appm/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.84	67 × 1	54.65	60 × 1	54.64	13 × 1
  		67 × 31		60 × 31		13 × 31
  Min.	0.32	1 × 16	6.91	1 × 16	8.83	2 × 16

  (HCPB inner target) and

  Table 9. Maximum and minimum values and related position (in terms of row (R) and column (C) of the array) of nuclear heating density (W/cm³), nuclear damage (dpa/FPY), He-production (appm/FPY) on the PFCs of the HCPB outer target.

  HCPB—Outer Target
  Nuclear heating (W/cm3)	W monoblocks		Cu/CuCrZr components		Eurofer PFC-CB supports
  	Value	R × C	Value	R × C	Value	R × C
  Max.	19.00	47 × 1	7.66	52 × 1	6.12	11 × 1
  		47 × 43		52 × 43		11 × 43
  Min.	7.84	4 × 22	2.48	1 × 22	1.62	2 × 22
  Nuclear damage (dpa/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.84	53 × 1	7.22	53 × 1	4.41	11 × 1
  		53 × 43		53 × 43		11 × 43
  Min.	0.39	1 × 22	1.28	1 × 22	0.68	1 × 22
  He-production (appm/FPY)	W-Monoblocks		Cu/CuCrZr components		Eurofer—PFC-CB supports
  Max.	1.90	53 × 1	56.28	54 × 1	55.81	11 × 1
  		53 × 43		54 × 43		11 × 43
  Min.	0.37	1 × 22	8.14	1 × 22	8.50	1 × 22

  (HCPB outer target).
• All these nuclear quantities show their maximum values generally located on the baffle region and external edges of the inner and outer target (both with WCLL and HCPB). This is since the baffle region has a higher view factor on the plasma than the straight leg of the vertical target: this entails a more intense neutron fluxes, especially on the external edges of the target, where there is an additional effect caused by the neutron and radiation streaming between the divertor cassette bodies.

• The minimum values are achieved in correspondence of the PFCs arranged on the lower part and innermost regions (straight legs) of the inner and outer target (both with WCLL and HCPB), as expected and already observed in past studies [25,26].

• All these nuclear loads show spatial distributions with an overall symmetrical trend along the toroidal abscissa, with respect to the target central axis, as highlighted in Figure 29.
• The thermal load achieves its peak values (10–20 MW/m² as stationary heat load and during slow transient events [2,6,7,10,11,12]) in the lower part of the straight leg of the vertical target (strike zone) as shown in Figure 30; on the contrary, as outlined in this study, the neutron irradiation is maximum on the top part of the target (baffle region). This represents an important aspect for the engineering and technological design of the DEMO divertor targets, being the peak values of the two main and demanding loads (thermal and nuclear) not superimposed. Although, during the divertor project phase, a conservative approach can be chosen (overlapping the two peaks), during the operation phase, it is expected that the most loaded PFCs in terms of thermal power do not correspond to those with the highest irradiation dose.
• As specified in the introduction, nuclear damage represents one of the main and most critical primary irradiation effects, which can seriously affect the secondary material’s responses in terms of physical, chemical and thermo-mechanical properties, thus leading as a last step, to a significant impact on the PFCs structural integrity, if neutron irradiation is considered. In this regard, the limit values for the damage in Eurofer used in the shielding block, as well as in the structural components of the divertor CB and CuCrZr in PFCs, are under evaluation and the following values can be currently considered as reference: 6 dpa for Eurofer and 10–20 dpa for CuCrZr, both at the End-Of-Life (EOL), which is expected to be 1.5 FPY as foreseen design value and at least included in the acceptable range of 0.8–2 FPY [6,7,10,11,12]. Under these conditions, the Eurofer of PFC-CB supports shows about 5 dpa/FPY as peak values with WCLL blanket and 4.4 dpa/FPY with HCPB, implying a potential lifetime of 1.2 and 1.4 FPY of the most loaded fixation systems, respectively, with WCLL and HCPB. These values seem to confirm the fulfillment of the acceptable range of operative life, but not the design lifetime, albeit they are very close to it. If 1.5 FPY is taken as reference lifetime, around 155 PFC-CB supports over 496 (31%) on the WCLL inner target exceed this limit. This percentage becomes about 22% (130 over 602) if the outer target is considered. In the same order, these percentages are around 4% (20/496) for the inner target and 2.7% (16/602) for the outer target, if HCPB blanket is considered. This suggests the following considerations: both with WCLL and HCPB, the portion of target affected by higher neutron irradiation doses, is generally greater on the inner than the outer target; this is probably since the uppermost section of the outer target is well shielded by the lower part of the outboard blanket segment (see Figure 31), which effect is not similarly considerable on the inboard side. In addition, the nuclear loads on inner and outer targets appear to be higher with WCLL blanket than HCPB (except for the He-production rate, which seems to be not substantially affected by the different blanket layout), with a consequent effect on the expected lifetime and operation performance of PFCs. This aspect is better discussed and analyzed in the next sections. Nuclear damage on CuCrZr achieves 8.75 dpa/FPY (WCLL) and 7.31 dpa/FPY (HCPB) as maximum values, which means 13.1 dpa and 11 dpa cumulated over 1.5 FPY, respectively. According to the previous irradiation test data, CuCrZr shows saturation of tensile behavior in the dose range of 0.5–2.5 dpa [50] or 1–10 dpa [51] at 150–300 °C (foreseen operational temperature range of divertor CuCrZr pipes). Thus, the peak dpa values (10–14 dpa) herein calculated, may probably be acceptable [52] and substantially in accordance with the reference limit values of 10–20 dpa.

• Regarding the nuclear heating peak value on inner W monoblock (Table 6 and Table 8) a shift of about 10 positions before is observed, compared with the peak values of nuclear damage and He-production (both for WCLL and HCPB blanket). This effect is less evident considering the outer target W monoblock (row 47 vs. 53) and not highlighted with Cu/CuCrZr and Eurofer components. If we look at Figure 32 showing the poloidal profiles of W-inner target, the nuclear heating density values appear to have a plateau in the baffle region, compared to the nuclear damage and He-production values which peaks region is narrow (Figures 34 and 36). These different poloidal profiles highlight a different behavior for each nuclear quantity, which is due to several factors: the variation of neutron spectra impinging on the target along the poloidal profiles, the different cross-section vs. energy trend of each phenomenon (damage, He-production, reactions which contribute to heating generation), the shape of the targets which is different between inner and outer, the surrounding materials which can affect the radiation transport, etc. The main scope of this work is to quantify as first the nuclear loads distributions on the DEMO DIV PFCs, traying to analyze the main effects which affect these distributions.
• All the analyzed nuclear load’s distributions show similar maximum and minimum values among the PFCs of inner and outer target, both with WCLL and HCPB.

3.3. Nuclear Loads and Thermal Power Deposition on PFCs during Operations: Comparison between WCLL and HCPB

A detailed assessment of the spatial distribution of the nuclear loads on the DEMO divertor targets during operations for both the BB cases under analysis, has been performed, and it is well described in the previous section.

This section is fully devoted to assessing the effects of the different blanket layouts on the nuclear load’s distribution of the DEMO divertor. In particular, the values of the nuclear heating density (W/cm³), radiation damage (dpa/FPY) and He-production (appm/FPY) for W-monoblocks, Cu/CuCrZr elements, PFC-CB supports and CB are shown by means of 2D poloidal profiles as function of a curvilinear abscissa, which starts from the upper part of the targets (baffle region) and is directed towards the lower part (Figure 10). In addition, the total nuclear power (MW) on PFCs and breakdown in each component has been performed, comparing the results between the two blanket concepts. Every 2D plot shows the poloidal profiles comparing both the external (highlighted in purple and red in the Figure 7) and internal (black in Figure 7) sides of the target loads, both for WCLL and HCPB blanket. Each point is representative of the nuclear load value on each component (W, Cu/CuCrZr and Eurofer support) along the profile. The number of components is 78 (IB) and 70 (OB) W-Monoblocks/Cu-Interlayers/CuCrZr pipe, 16 (IB) and 14 (OB) Eurofer supports. The poloidal profiles of nuclear heating density of W-monoblocks are shown in Figure 32 and Figure 33, for the inner and outer target, respectively.

If the comparison between WCLL and HCPB profiles of the inner target (Figure 32), is considered, the following considerations can be stated: the nuclear heating density appears to be higher with the Water Cooled blanket than the Helium Cooled one. The difference between the profiles is around +10/20% for the W-monoblocks arranged on the baffle region (+11.6% comparing the corresponding peak values shown in Table 6 and Table 8), then increasing up to +20/30% on the straight leg of the vertical targets (+22.6% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 32), the following statements can be supported: these plots clearly show a greater level of nuclear loads intensity on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +20/40%), and more attenuated on the baffle region (e.g., around +5/15%). It should be noted that the differences between the profiles highlighted with the two comparisons are due to different reasons: considering the comparison between the two blankets, the variation mainly lies in the impact that the two different layouts (in terms of materials and design) have on the neutron spectra impinging on the target. In the second comparison, the difference is mainly due to the position of the components on the target and therefore to the view factor on the plasma, which is substantially independent of the type of blanket. These aspects are better discussed in the next section.

Nuclear heating density on W-monoblocks appears to be higher with WCLL than HCPB, also observing the outer poloidal profiles (Figure 33). The difference between the profiles is around +10/20% for the W-monoblocks placed on the baffle region (+12.6% comparing the corresponding peak values shown in Table 7 and Table 9), then increasing up to +20/30% on the straight leg of the vertical targets (+6% comparing the corresponding minimum values reported in Table 7 and Table 9). The profiles are almost perfectly overlapped for the first W-monoblocks (1–7), i.e., in correspondence with the extremal part of the baffle region (highlighted in Figure 31). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 33), once again, the plots show a higher level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +10/30%), and more attenuated on the baffle region (e.g., around +5/10%).

Compared to the inner target, the difference between external and internal profiles is generally smaller, especially in the baffle region where the solid and dashed lines are almost overlapped. The poloidal profiles of nuclear damage of W-monoblocks are shown in Figure 34 and Figure 35, for the inner and outer target, respectively. Nuclear damage on W-monoblocks is higher with WCLL than HCPB, as shown by the inner poloidal profiles of Figure 34. The difference between the profiles is around +10/20% for the W-monoblocks placed on the baffle region (+14.8% comparing the corresponding peak values shown in Table 6 and Table 8), then increasing up to +20/30% on the straight leg of the vertical targets (+18.2% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 34), once again, the plots show a higher level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +10/35%), and more attenuated on the baffle region (e.g., around +5/10%). If the poloidal profiles related to the outer target (Figure 35) are considered, the following assessment can be done: as is reasonable to expect nuclear damage is greater with WCLL than HCPB; once again the difference between the profiles is about +10/20% in correspondence of the baffle region (+12.5% comparing the peak values shown in Table 7 and Table 9), remaining substantially unvaried also in the remaining part of the target (+10.3% comparing the minimum values shown in Table 7 and Table 9). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 35), once again, the plots show a higher level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external sides are more pronounced on the straight leg (e.g., about +10/40%), and strongly attenuated on the baffle region (e.g., less than +5%). Indeed, the solid and dashed lines are almost overlapped both in the inner target and especially in the outer target, in correspondence of the baffle region.

The poloidal profiles of He-production of W-monoblocks are shown in Figure 36 and Figure 37, for the inner and outer target, respectively. Compared to what has been noted for nuclear heating density and damage, He-production seems to be not significantly affected by the type of blanket, being the blue and orange lines essentially overlapped both for the inner and outer target (Figure 36 and Figure 37). This effect can be noted also looking at the spatial distribution maps of He-production on W, shown in the previous section, which appear very similar in terms of color scale. The reason for this is better discussed in the next section.

Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner and outer target (Figure 36 and Figure 37), the plots show a higher level of nuclear loads intensity on the components placed more externally on the targets, compared to the central ones. The differences between internal and external sides are more pronounced on the straight leg (e.g., about +10/45% as range), and strongly attenuated on the baffle region (e.g., less than 5%). Indeed, the solid and dashed lines are almost overlapped both in the inner target and especially in the outer target, in correspondence of the baffle region.

The poloidal profiles of nuclear heating density on Cu/CuCrZr components are shown in Figure 38 and Figure 39, for the inner and outer target, respectively. If the comparison between WCLL and HCPB profiles of the inner target (Figure 38) is considered, the following considerations can be done: the nuclear heating density appears to be higher with the Water Cooled blanket than the Helium Cooled one, as already highlighted for W-monoblocks.

The difference between the profiles is around +5/10% for the Cu/CuCrZr components placed on the baffle region (+7.7% comparing the corresponding peak values shown in Table 6 and Table 8), then increasing up to +15/20% on the straight leg of the vertical targets (+20.5% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 38), the following statements can be assessed: these plots clearly show a more intense level of nuclear loads intensity on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +15/20%), and more attenuated on the baffle region (e.g., around +5/15%).

The differences between the profiles highlighted with the two comparisons are due to different reasons: considering the comparison between the two blankets, the variation mainly lies in the impact that the two different layouts (in terms of materials and design) have on the neutron spectra impinging on the target. The second comparison points out a difference which is principally due to the position of the components on the target and therefore to the view factor on the plasma, which is substantially independent of the type of blanket. These aspects are better discussed in the next section.

Nuclear heating density on Cu/CuCrZr components appears to be higher with WCLL than HCPB, also looking at the outer poloidal profiles (Figure 39). The difference between the profiles is around +5/15% for the Cu/CuCrZr components placed on the baffle region (+8.1% comparing the corresponding peak values shown in Table 7 and Table 9), then increasing up to +15/20% on the straight leg of the vertical targets (+19% comparing the corresponding minimum values shown in Table 7 and Table 9). The profiles are almost perfectly overlapped for the first Cu/CuCrZr components (1–7), i.e., in correspondence with the extremal part of the baffle region (highlighted in Figure 31), well shielded by the outboard blanket segment. Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 39), once again, the plots show a greater level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +15/20%), and more attenuated on the baffle region (e.g., around +5/10%). Compared to the inner target, the difference between external and internal profiles is generally smaller, especially in the baffle region where the solid and dashed lines are almost overlapped.

The poloidal profiles of nuclear damage of Cu/CuCrZr components are shown in Figure 40 and Figure 41, for the inner and outer target, respectively. Nuclear damage on Cu/CuCrZr is higher with WCLL than HCPB, as shown by the inner poloidal profiles of Figure 40. The difference between the profiles is around +20/25% for the Cu/CuCrZr located on the baffle region (+19.7% comparing the corresponding peak values shown in Table 6 and Table 8), maintaining the same order of magnitude (+20/30%) on the straight leg of the vertical targets (+21.1% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 40), once again, the plots show a more intense level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +20/30%), and more attenuated on the baffle region (e.g., around +5/15%). If the poloidal profiles related to the outer target (Figure 41) are considered, the following assessment can be made: as is reasonable to expect nuclear damage is greater with WCLL than HCPB; once again the difference between the profiles is about +10/20% in correspondence of the baffle region (+17.3% comparing the peak values shown in Table 7 and Table 9), remaining substantially unvaried also in the remaining part of the target (+10.9% comparing the minimum values shown in Table 7 and Table 9). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 41), once again, the plots show a greater level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The differences between internal and external sides are more pronounced on the straight leg (e.g., about +10/35%), and strongly attenuated on the baffle region (e.g., less than +5%). Indeed, the solid and dashed lines are almost overlapped both in the inner target and especially in the outer target, in correspondence of the baffle region.

The poloidal profiles of He-production on Cu/CuCrZr components are shown in Figure 42 and Figure 43, for the inner and outer target, respectively. Once again, He-production appears to be not significantly affected by the type of blanket, being the blue and orange lines essentially overlapped both for the inner and outer target (Figure 42 and Figure 43). This effect can be noted also looking at the spatial distribution maps of He-production on Cu/CuCrZr, shown in the previous section, which show a very similar behavior in terms of colours scale. The reason why this effect occurs is better discussed in the next section. Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner and outer target (Figure 42 and Figure 43), the plots show a higher level of nuclear loads intensity on the components placed more externally on the targets, compared to the central ones, as already shown in the previous case. The differences between internal and external sides are more pronounced on the straight leg (e.g., about +10/45% as range), and strongly attenuated on the baffle region (e.g., less than +5%). In this regard, the solid and dashed lines are almost overlapped both in the inner target and especially in the outer target, in correspondence of the baffle region.

The poloidal profiles of nuclear heating density on PFC-CB supports made of Eurofer are shown in Figure 44 and Figure 45, for the inner and outer target, respectively. If the comparison between WCLL and HCPB profiles of the inner target (Figure 44) is considered, the following considerations can be assessed: the nuclear heating density appears to be higher with the Water Cooled blanket than the Helium Cooled one, as already highlighted for W-monoblocks and Cu/CuCrZr components. The difference between the profiles is around +5/15% for the supports placed on the baffle region (+13.0% comparing the corresponding peak values shown in Table 6 and Table 8), then increasing up to +15/25% on the straight leg of the vertical targets (+25.8% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 44), the following statements can be assessed: these plots clearly show a higher level of nuclear loads intensity on the components placed more externally on the targets, compared to the central ones. The differences between internal and external edges are more pronounced on the straight leg (e.g., about +20/35%), and more attenuated on the baffle region (e.g., around +5/20%). Once again, considering the comparison between the two blankets, the difference mainly lies in the impact that the two different layouts (in terms of materials and design) have on the neutron spectra impinging on the target. The second comparison highlights a difference which is principally due to the position of the components on the target and therefore to the view factor on the plasma, which is substantially independent of the type of blanket. These aspects are better discussed in the next section.

Nuclear heating density on supports made of Eurofer appears to be higher with WCLL than HCPB, also looking at the outer poloidal profiles (Figure 45). The variance between the profiles is around +0/10% for the supports positioned on the baffle region (+9.3% comparing the corresponding peak values shown in Table 7 and Table 9), then increasing up to +15/20% on the straight leg of the vertical targets (+21.0% comparing the corresponding minimum values shown in Table 7 and Table 9). The profiles are almost perfectly overlapped for the first supports (1–3), i.e., in correspondence with the extremal part of the baffle region (highlighted in Figure 31), well shielded by the outboard blanket segment. Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 45), once again, the plots show a greater level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The variances between internal and external edges are more pronounced on the straight leg (e.g., about +15/20%), and more attenuated on the baffle region (e.g., around +0/10%). Compared to the inner target, the variance between external and internal profiles is generally smaller, especially in the baffle region where the solid and dashed lines are almost overlapped.

The poloidal profiles of nuclear damage on PFC-CB supports made of Eurofer, are shown in Figure 46 and Figure 47, for the inner and outer target, respectively. Nuclear damage on Eurofer supports is higher with WCLL than HCPB, as shown by the inner poloidal profiles of Figure 46. The variance between the profiles is around +15% for the supports positioned on the baffle region (+15.2% comparing the corresponding peak values shown in Table 6 and Table 8), increasing up to 15/30% on the straight leg of the vertical targets (+19.4% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 46), once again, the plots show a more intense level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The variances between internal and external edges are more pronounced on the straight leg (e.g., about +30/60%), and more attenuated on the baffle region (e.g., around +15/30%). If the poloidal profiles related to the outer target (Figure 47) are considered, the following assessment can be made: as is reasonable to expect, nuclear damage is greater with WCLL than HCPB; once again the variance between the profiles is about +5/15% in correspondence of the baffle region (+13.2% comparing the peak values shown in Table 7 and Table 9), remaining substantially unchanged also in the remaining part of the target (+13.2% comparing the minimum values shown in Table 7 and Table 9). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 47), once again, the plots highlight a greater intensity of nuclear loads level on the components placed more externally on the targets, compared to the central ones. The variances between internal and external sides are significantly higher on the straight leg (e.g., about +20/50%), and more attenuated on the baffle region (e.g., +5/15%).

Indeed, it can be noted how the solid and dashed lines are almost overlapped both in the inner target and especially in the outer target, in correspondence of the baffle region.

The poloidal profiles of He-production on PFC-CB supports made of Eurofer, are shown in Figure 48 and Figure 49, for the inner and outer target, respectively. Contrary to the previous cases (W-monoblocks and Cu/CuCrZr components), for the Eurofer supports, the difference between the profiles is not negligible, highlighting higher He-production values with the WCLL blanket compared to the HCPB one, both in the inner and outer target.

Looking at Figure 48, the variance between the profiles is around +5/10% for the supports positioned on the inner baffle region (+6.2% comparing the corresponding peak values shown in Table 6 and Table 8), not varying the order of magnitude (+5/15%) on the straight leg of the inner vertical targets (+16.0% comparing the corresponding minimum values reported in Table 6 and Table 8). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the inner target (Figure 48), once again, the plots show a more intense level of nuclear loads magnitude on the components placed more externally on the targets, compared to the central ones. The deviations between internal and external edges are more pronounced on the straight leg (e.g., about +40/70%), and more attenuated on the baffle region (e.g., around +15/30%). If the poloidal profiles related to the outer target (Figure 49) are considered, the following assessment can be done: the difference between the profiles is about +0/5% in correspondence of the baffle region (+2.4% comparing the peak values shown in Table 7 and Table 9), remaining substantially unchanged (+5/10%) also in the remaining part of the target (+8.8% comparing the peak values shown in Table 7 and Table 9). Regarding the comparison between the poloidal profiles of external and internal arrays (both for WCLL and HCPB) of the outer target (Figure 49), once again, the plots highlight a greater intensity of nuclear loads level on the components placed more externally on the targets, compared to the central ones. The variances between internal and external sides are significantly higher on the straight leg (e.g., about +30/90%), and more attenuated on the baffle region (e.g., +10/30%). The reason why this effect occurs is better discussed in the next section.

The poloidal profiles for the nuclear quantities of interest, on the first layer (Eurofer) of the inboard cassette body (CB) have been assessed, subdividing it into 56 spheres (or segment) both on the external and the innermost side (Figure 50).

The purpose of this additional evaluation is the analysis of the shielding capabilities of the PFCs on the most outward part of the cassette body, as well as the nuclear loads distribution along the poloidal profiles of the CB, considering once again the impact of the different kind of blanket (WCLL and HCPB). Considering the nuclear heating profiles (Figure 51), once again it is greater with the Water Cooled blanket than the Helium cooled one and in particular, the maximum values are in correspondence of the external edge baffle region: 8.47 W/cm³ and 7.41 W/cm³ for WCLL and HCPB, respectively, showing an increase of about 14.3% with WCLL.

The minimum values are 2.37 W/cm³ (WCLL) and 1.90 W/cm³ (HCPB), highlighting an increase of 24.7% with the water coolant, in correspondence with the lower and innermost part of the cassette body. Therefore, the WCLL shows higher nuclear heating loads than HCPB (+15/20% on the baffle region, +20/30% on the straight leg). As already noted, a greater intensity on the external edges than the innermost side (around +15% on the baffle region, +25/30% on the straight leg) is pointed out both for WCLL and HCPB cases. The nuclear damage profiles are shown in Figure 52, highlighting greater values with the WCLL blanket than the HCPB. The peak values are in correspondence of the external edge baffle region: 4.49 dpa/FPY and 3.95 dpa/FPY for WCLL and HCPB, respectively, showing an increase of about 13.4% with WCLL. Considering that the cumulated damage limit in Eurofer (as mentioned in the previous sections) is 6 dpa, this implies that under these conditions the DEMO divertor cassette body could fulfill the expected lifetime of 1.5 FPY only with the HCPB blanket, on the contrary with the WCLL the lifetime expected may be lower (1.34 FPY). The minimum values are 0.61 dpa/FPY (WCLL) and 0.52 dpa/FPY (HCPB), showing an increase of 17.3% with the water coolant, in correspondence with the lower and innermost part of the cassette body. Therefore, the WCLL shows higher nuclear damage loads than HCPB (+15% on the baffle region, +15/20% on the straight leg). A more intense level on the external edges than the innermost side (around +20% on the baffle region, +40/50% on the straight leg) is observed both for WCLL and HCPB cases. The He-production of the first layer of the CB (see Figure 53) appears to be considerably affected by the blanket typology, compared to the PFCs, where the He-production rate appears to be quite similar with both the blanket layouts. In this case, the maximum values are achieved in correspondence of the external edge baffle region: 79.34 appm/FPY (WCLL) and 66.82 appm/FPY (HCPB) pointing out an increase of about 19% with WCLL than HCPB. The lowest values are once again achieved on the lowest and innermost part of the CB layer: 9.91 appm/FPY and 8.83 appm/FPY for WCLL and HCPB respectively (+12.2% with WCLL). WCLL shows higher He-production rates than HCPB (+15% on the baffle region, +25/30% on the straight leg). A more intense level on the external edges than the innermost side (around +30% on the baffle region, +50/60% on the straight leg) is observed both for WCLL and HCPB cases.

The total nuclear power deposited in the 48 DEMO divertor cassettes components (PFCs, CB + liner and reflector plates), including a detailed breakdown for each PFC sub-component (W-monoblocks, Cu-interlayers, CuCrZr-pipe, Eurofer supports and cooling water) is shown in

Table 10. Comparison between the total nuclear power (MW) deposited on the DEMO divertor (showing the breakdown for each single contribution) both for WCLL and HCPB blanket, considering all the 48 divertor cassettes.

WCLL	MW Tot (48 Cassettes)	Distribution (%)	HCPB	MW Tot (48 Cassettes)	Distribution (%)	WCLL/HCPB
PFCs	25.14	14.38	PFCs	21.79	14.65	1.15
Inboard	11.79	6.75	Inboard	10.12	6.80	1.17
W-Monoblocks	9.04	5.17	W-Monoblocks	7.79	5.24	1.16
Cu-Interlayers	0.42	0.24	Cu-Interlayers	0.37	0.25	1.14
CuCrZr-pipes	0.61	0.35	CuCrZr-pipes	0.53	0.36	1.14
Water-coolant	0.79	0.45	Water-coolant	0.63	0.42	1.25
Eurofer-supports	0.94	0.54	Eurofer-supports	0.80	0.54	1.18
Outboard	13.34	7.63	Outboard	11.67	7.85	1.14
W-Monoblocks	10.21	5.84	W-Monoblocks	8.94	6.01	1.14
Cu-Interlayers	0.48	0.28	Cu-Interlayers	0.43	0.29	1.13
CuCrZr-pipes	0.69	0.39	CuCrZr-pipes	0.61	0.41	1.12
Water-coolant	0.84	0.48	Water-coolant	0.71	0.48	1.18
Eurofer-supports	1.11	0.64	Eurofer-supports	0.98	0.66	1.14
CB + Liner and reflector plates	149.67	85.62	CB + Liner and reflector plates	126.89	85.35	1.18
TOTAL	174.80	100.00	TOTAL	148.68	100.00	1.18

, for both the blanket options under analysis. The total amounts to 174.8 MW with WCLL blanket and 148.6 MW with HCPB one (+18% using a water cooled blanket). In both cases, around 85% of the power is distributed in the structure of CB, liner and reflector plates and the remaining 15% inside the PFCs. This 15% is in turn evenly split between inboard and outboard targets (around 7% and 8%, respectively), of which 5% is the contribution of the W-monoblocks, and the rest is attributed to the other components. On average there is +15% with the WCLL compared to the HCPB blanket, for every divertor component, as already observed with the nuclear load’s maps and poloidal profiles (especially nuclear heating density and radiation damage).

3.4. Summary of the WCLL vs. HCPB Comparison

Figure 54 shows the neutron spectra impinging on the W-Monoblock placed in the central zone of the baffle region (position C, see Figure 7 and Figure 9) in correspondence with the external edge of the inboard target, both with WCLL and HCPB. The different layout of the breeding blanket clearly affects the neutron spectrum distribution on the blanket first wall and divertor plasma facing components. In particular, the effect related to neutron moderation is predominant with the water cooled blanket -WCLL, and this is confirmed looking at the neutron spectra of Figure 54, highlighting a higher proportion of low energy neutrons (energy range of 10⁻⁷–1 MeV) in the divertor W-armour, than HCPB. For higher neutron energy values (1–14 MeV), the neutron spectra appear to be similar, showing comparable peak values of neutron fluxes.

This clearly has an impact on the different response and behavior of the nuclear load’s distribution on the DEMO divertor targets, passing from one type of blanket to another, due to the cross-section vs. neutron energy trends of the several phenomena described herein described. Photon production by means of (n,γ) neutron absorption reactions, generally represents the predominant contribution to nuclear heating deposition into materials, under neutron irradiation. The energy-cross sections dependence [32,53,54] of $180W\left(n,\gamma \right)181W$, $182W\left(n,\gamma \right)183W$, $183W\left(n,\gamma \right)184W$, $184W\left(n,\gamma \right)185W$, $186W\left(n,\gamma \right)187W$ for Tungsten, $63Cu\left(n,\gamma \right)64Cu$ and $65Cu\left(n,\gamma \right)66Cu$ for Copper, $54Fe\left(n,\gamma \right)55Fe$, $56Fe\left(n,\gamma \right)57Fe$, $57Fe\left(n,\gamma \right)58Fe$ Iron, which is the principal element of Eurofer, shows increasing cross section with lower neutron energies, entailing a higher probability that these reactions occur in this energy range. The analysis suggests that the greater neutron moderation in the water cooled blanket of the WCLL concept, results in neutron spectra with a more intense level of low energy neutrons in the divertor targets, thereby enhancing the nuclear heating deposition, compared to the HCPB.

The radiation damage values and related cross-section have been assessed according to the NRT method [55,56]. In this regard, the displacement cross-sections [54,55,57,58,59] versus incident neutrons energy for Iron, Copper and Tungsten appear to have two separate regions: lower energies region (10⁻⁹–10⁻³ MeV) where the cross-section generally increases with decreasing energy and higher energies range (10⁻²–100 MeV) where cross-section generally increases (or it shows resonance peaks) with the increasing of incident neutron energy. Although, a more in-depth analysis would be necessary, in the first instance, again in this case the water moderation of the impinging spectrum on the targets, would seem to have an impact on the low energy-displacement cross sections region, resulting in higher fluxes of moderated neutrons, so leading to greater damage levels with WCLL.

The variation in He-production between WCLL and HCPB case appears to be negligible in Copper/CuCrZr and Tungsten, and slightly more noticeable in Eurofer steel used in supports and CB first layers. The main reactions of He production in Copper ($63Cu\left(n,\alpha \right)60Co$, $65Cu\left(n,\alpha \right)62Co$) and Tungsten ($182W\left(n,\alpha \right)179Hf$, $183W\left(n,\alpha \right)180Hf$), which are responsible for most of the He produced in armor, interlayer and heat-sink materials, are threshold reactions [53,54,60]. Only for neutron energies above the threshold does a reaction become possible, and the threshold energies are: 2 and 3.5 MeV for the $\left(n,\alpha \right)$ reactions in $63Cu$ and $65Cu$ respectively; around 1 MeV for $\left(n,\alpha \right)$ reactions in W. From a close examination of the spectra between HCPB and WCLL, it is apparent that there is a trend similarity in the neutron energies above 1 MeV (see Figure 54), which is precisely the range over which the gas production reactions become more likely, thus entailing as much as similar He-production level in Copper as well as in Tungsten components, both with WCLL and HCBP blanket. The main reactions of He production in Iron, as principal element of Eurofer steel, are: first of all, $56Fe\left(n,\alpha \right)53Cr$ and as lower of contribution $57Fe\left(n,\alpha \right)54Cr$. The first reaction has a threshold energy of 3.7 MeV, the second one has a cross-section peak at 10–20 MeV, and a second region where cross-section values increase while neutron energies are reduced. In principle, He-production levels in Eurofer PFC-CB supports, show slight differences among WCLL and HCPB, and a higher variation on the first layers of Eurofer in the CB. Although $56Fe$ shows a cross-section trend with threshold energy, probably the presence of other elements in Eurofer, which could be affected by the different shape of the neutron spectra (Figure 54) regarding the gas transmutation reactions, it may lead to a more substantial dependence of the He-production in steel on the blanket type, compared to Tungsten and Copper, where this effect is not considerable.

4. Conclusions

Starting from the EU DEMO1 2017 reference configuration MCNP model (representing a 11.25° toroidal sector of the tokamak), two MCNP models have been reproduced: with WCLL and HCPB Single Module Segment layered blankets, both with the MCNP model of the divertor 2019 configuration. The detailed MCNP representations of the whole ITER-like PFCs inboard and outboard targets have been developed and integrated in both the above-mentioned models. A detailed comparative neutronics analysis, regarding the nuclear load’s spatial distributions on the Divertor PFCs targets has been performed, leading to the following main remarks:

• The spatial distributions of the main nuclear loads (radiation damage (dpa/FPY), nuclear heating density (W/cm³) and Helium production (appm/FPY)) on the PFC elements (W-monoblock, Cu/CuCrZr and PFC-CB supports made of Eurofer) of the inner and outer targets, have been assessed, for both the blanket configurations under analysis. The maximum and minimum values and the related positions where they are achieved, are summarized in Table 6, Table 7, Table 8 and Table 9.
• All the graphs and maps clearly show a higher level of nuclear load magnitude on the components placed more externally on the targets (inboard and outboard), compared to the central ones. Therefore, a symmetrical behavior with respect to the central part of the target is confirmed. Very similar values are observed on both external target edges (red and purple, in Figure 7). The differences between internal and external edges are more pronounced on the straight leg (e.g., about +50/60% as peak difference), and more attenuated on the baffle region (e.g., around +5/10%). This effect on the toroidal distribution of the loads on the targets is due to the fact that the external sides of the targets (especially on the baffle region) are subjected to higher irradiation levels probably due to the radiation streaming in the gaps between the cassettes.
• The different type of blanket has an impact on the neutron spectrum (water vs. helium as coolant) impinging on the targets (Figure 54). Therefore, it is reasonable to expect an effect on the nuclear load’s distribution during operations.
• Helium production on W, Cu/CuCrZr appears to be not significantly affected by the blanket configuration. He-production in Eurofer seems to be more dependent on the different shape of the neutron spectra, highlighting slightly higher levels with WCLL than HCPB.
• Nuclear damage on W, Cu/CuCrZr and Eurofer show higher levels with the WCLL blanket than HCPB. The average differences amount to about +15%/10% on W, +20% on Cu/CuCrZr, +15% on Eurofer (baffle region) and around +20% on W, +20/30% on Cu/CuCrZr and +20% on Eurofer (straight leg).
• Nuclear heating density shows a trend similar to nuclear damage. This is due to the greater gamma generation, whose production on the divertor targets is favored by the water-cooled blanket (higher neutron fluxes at lower energy). These differences are more reduced in the baffle region (+10% on W, Cu/CuCrZr and Eurofer) compared to the straight leg zone (+20% on W, +15/20% on Cu/CuCrZr, and Eurofer).
• Analyzing cross-section data for radiation damage in Iron, Copper and Tungsten available in literature, it seems that there is a range where cross-sections increase while neutron energies decrease (like (n,γ) reactions), thus contributing to a similar outcome of nuclear heating distribution. He-production cross sections for most of the isotopes of Iron, Tungsten and Copper, are threshold reactions (they occur for neutron energies greater than around 1 MeV). If the neutron spectra comparison (Figure 54) is observed, it is evident that they have a quite similar trend in the energy range above 1 MeV, thus leading to an enough similar response in terms of He-production both for WCLL and HCPB, except for Eurofer, as previously stated, probably due to the presence of other elements.
• Table 10 shows the total nuclear power (MW) deposited on the PFCs and CB and the breakdown in each component during operations. On average there is +15% with the WCLL compared to the HCPB.

In conclusion, the development of a divertor target with a sufficient capability of removal power exhaust and impurity particles coming from the plasma core, is a crucial prerequisite for the realization of the demonstration fusion power plant (DEMO) and for future fusion reactors. In this regard, the main implications of these results are related to the provision of high detailed neutron irradiation load distributions and data on the divertor PFCs, varying the blanket configuration, which can play a key role in the next design and manufacturing stages of the DEMO divertor.