# Analysis of EU-DEMO WCLL Power Conversion System in Two Relevant Balance of Plant Configurations: Direct Coupling with Auxiliary Boiler and Indirect Coupling

^{*}

## Abstract

**:**

^{TM}code, referred to a preliminary design phase, devoted to the sizing of the main components, and to a second phase focused on the cycle optimization. The study demonstrated the feasibility of the two BoP concepts. They are able to produce a satisfactory average electric power (>700 MW) with an acceptable average net electric efficiency (33.6% for both concepts). For each solution, the main strengths and weaknesses are compared and discussed.

## 1. Introduction

_{th}must be removed from the reactor Breeding Blanket (BB) [6], while the power deposited in such a structure during dwell time (i.e., when the plasma is off) approaches the 1% of the nominal value [7]. Eleven pulses per day are foreseen, generating a concern related to the unconventional operation of the DEMO BoP.

^{©}), feeds the steam turbine under the dwell phase with a reduced steam flow rate (around 10%). Such a configuration allows the connection with the electrical grid to be kept, ensuring a minimum energy production for PHTSs and BoP auxiliaries. Furthermore, two possible alternatives for the WCLL BB BoP are considered. The first one is the DCD with AUXiliary Boiler (AUXB), namely, WCLL DCD AUXB BoP, where BZ and FW PHTSs OTSGs directly feed the steam turbine, and a gas-fired boiler provides 250 MW in both the pulse and dwell operation, ensuring the minimum load of 10% to the steam turbine during dwell time. The advantage of such a solution is the use of well-known components already adopted in nuclear and conventional industries, except for the steam turbine whose feasibility should be assessed (this is valid also for the WCLL DCD BoP). The main drawback is the large power required from the auxiliary boiler. On the other hand, an Indirect Coupling Design (ICD) with Intermediate Heat Transfer System (IHTS) and ESS, namely, WCLL ICD BoP, is studied. In this case, only BZ PHTS delivers thermal power directly to PCS, by means of two OTSGs. Instead, the FW PHTS is connected to the IHTS equipped with a large ESS. The thermal coupling is provided by two water/molten salt heat exchangers. The ESS consists of two tanks filled with molten salt (MS) at different temperatures. During the pulse, the ESS accumulates a fraction of the FW thermal power, storing molten salt in the hot tank. Then, during the dwell, this energy is transferred to the PCS through four helicoidally coil steam generators. This configuration guarantees a continuous and near-constant turbine load (100%) in both the pulse and dwell phases. This is the main advantage of such a concept, but the large dimension of the energy storage tanks (around 11,000 m

^{3}) represents a significant concern. It is worth noting that storage systems have already been proved to be effective in improving the operational flexibility of integrated energy systems, for example, in combination with renewable energy sources, as reported in [11].

## 2. DEMO WCLL BB Balance of Plant

#### 2.1. WCLL ICD BoP Power Conversion System

^{©}, respectively), four Helicoidally Coil Steam Generators (HCSG—delivering power to the PCS), the pumps, and their connections [15].

_{th}. During the pulse, the power delivered from the FW PHTS to the ESS amounts to 439.8 MW

_{th}(see Table 1). It is assumed that around 1.25 × 10

^{6}MJ are stored during this phase, which corresponds to a power of 173.9 MW

_{th}. The difference between the two powers is directly driven to the PCS by one of the four HCSGs. Molten salt operates between 280 and 320 °C, while feedwater enters the helical tubes at 238 °C producing steam at 299 °C and 6.41 MPa. The HITEC

^{©}and feedwater mass flow rates are 4261 kg/s and 145 kg/s, respectively.

^{©}mass flow rate is evaluated to 33,436 kg/s (8359 kg/s per component). The whole ESS contains an inventory of 20,062 tons of molten salt; thus, a volume of 11,000 m

^{3}is required per tank.

#### WCLL ICD BoP PCS: GateCycle^{TM} Modeling

^{TM}code. A schematic view of the system modeling is presented in Figure 1. For each component, the first input data have been derived from preliminary energy balance calculations and imposed into the model for the first run of the code. Then, input data have been iteratively refined run by run to chase the finest layout for the system.

^{TM}manual to best fit the several expansion stages and to enhance the convergence of the simulation [13]. HITEC

^{©}is not included in the available working fluids in GateCycle

^{TM}; thus, the MS HCSG is modeled with an equivalent heat exchanger component, where water substitutes the MS. The hot side water flow rate is scaled as if the design power was directly delivered from the FW PHTS to the PCS, assuming a calibrated heat transfer coefficient. The Re-Heater (RH) is also simulated as an equivalent heat exchanger, with calibrated conditions at the hot side, and the moisture separator is reproduced with a sink (Sink1 in Figure 1) and two sources (SRC1 and SRC2 for steam and drain, respectively) boundary conditions. Such boundaries are calculated assuming an efficiency of the component equal to 95%. This modeling approach was adopted since the available version of the code does not provide the moisture separator among the usable components. Concerning the pressure losses, a detailed calculation was not implemented in this study. With the aim of evaluating a plausible pumping power, as a first approximation, input data for pressure drops are derived from [22]. In addition, the pressure losses through the feedwater and steam lines (namely, the feedwater pipeline and steam pipeline in Figure 1) are supposed to be 50 kPa and 100 kPa, respectively. Finally, the pressure losses related to the steam extraction lines are postulated to be 3% of the extraction pressure. In this way, a more realistic pressure value is obtained at the FWHs. Furthermore, heat losses towards the environment are estimated to be 1% of the duty for each component, and the shaft/gearbox losses of the turbine are imposed to be 0.1% of the power. Fouling factors are considered for each HX, assuming 0.085 × 10

^{−3}m

^{2}K/W for FWH and 0.176 × 10

^{−3}m

^{2}K/W for single-phase HX, according to the suggestions of the industry partner (i.e., Ansaldo Nucleare).

^{TM}cases, where PHTS constraints and PCS requirements are kept constant. The formers are summarized in Table 1, whereas the most important input parameters for the PCS operation are collected in Table 2. In addition, some input parameters have been assumed as a starting point for the steam turbine’s resolution. In the present activity, HP and LP turbines are designed with the Spencer Cotton Cannon efficiency method [23] (plus Putman correction for LP turbines [24]) and with the “input extraction pressures” option. Such pressure values were already optimized based on the Ansaldo Energia suggestions and have been assumed as a starting point (see Table 3). In off-design mode (see below), the same efficiency method is selected for the steam turbine, but extraction pressures are calculated with the Stodola ellipse model [25].

#### 2.2. WCLL DCD AUXB BoP Power Conversion System

#### WCLL DCD AUXB BoP PCS: GateCycle^{TM} Modeling

## 3. Results

_{gross}) and the net electric power (W

_{e}), defined as:

_{t}

_{1}and W

_{t}

_{2}represent the shaft power of the HP and LP turbines, respectively, and η

_{gen}is the generator efficiency, assumed equal to 0.98. Equation (2) presents the net electric power, defined as the difference between the gross power and the overall pumping power, calculated including all the pumps (i.e., primary, secondary, and tertiary pumping devices). Per each power, the correspondent efficiency is evaluated as:

_{grass}and η

_{e}are expected to be higher than 100% during dwell time, when ${\dot{Q}}_{reactor}$ is around 1% of the nominal value and the power is delivered to the PCS by the IHTS.

#### 3.1. WCLL ICD BoP PCS: Results

#### 3.2. WCLL DCD AUXB BoP PCS: Results

## 4. Discussion

_{2}or CH

_{4}, instead of natural gas in the AUXB. As a drawback of this solution, the integration of an auxiliary plant able to produce green fuel must be considered.

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

AUXB | AUXiliary Boiler |

BB | Breeding Blanket |

BoP | Balance of Plant |

BZ | Breeding Zone |

CAS | Cassettes |

DA | Deaerator |

DCD | Direct Coupling Design |

DCLL | Dual Coolant Lithium Lead |

DEMO | DEMOnstration Fusion Power Plant |

DIV | Divertor |

DW | Dwell |

ESS | Energy Storage System |

FW | First Wall |

FWH | Feedwater Heater |

HCLL | Helium-Cooled Lithium Lead |

HCPB | Helium-Cooled Pebble Bed |

HCSG | Helical Coil Steam Generator |

HX | Heat Exchanger |

HP | High Pressure |

ICD | Indirect Coupling Design |

IHTS | Intermediate Heat Transfer System |

IHX | Intermediate Heat Exchanger |

KDII | Key Design Integration Issue |

LP | Low Pressure |

MS | Molten Salt |

OTSG | Once Through Steam Generator |

PCS | Power Conversion System |

PFU | Plasma Facing Unit |

PHTS | Primary Heat Transport System |

PL | Pipeline |

RH | Re-Heater |

SG | Steam Generator |

SRC | Source |

VV | Vacuum Vessel |

WCLL | Water-Cooled Lithium Lead |

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Parameter | BZ | FW | DIV PFU | DIV CAS | VV |
---|---|---|---|---|---|

Source power (MW_{th}) | 1483.2 | 439.8 | 136.0 | 115.2 | 86.0 |

Mass flow rate (kg/s) | 7660.0 | 2272.0 | 5317.8 | 860.8 | 1928.0 |

PHTS inlet temperature (°C) | 328.0 | 328.0 | 136.0 | 210.0 | 200.0 |

PHTS outlet temperature (°C) | 295.0 | 295.0 | 130.0 | 180.0 | 190.0 |

Pressure (MPa) | 15.5 | 15.5 | 3.8 | 3.5 | 3.2 |

Pumping power (MW) | 13.6 | 4.2 | 6.0 | 0.2 | 3.1 |

Parameter | Value |
---|---|

Pump 1 outlet pressure (kPa) | 6500 |

HP turbine outlet pressure (kPa) | 980 |

Deaerator pressure (kPa) | 330 |

Condenser pressure (kPa) | 5 |

Superheating temperature (°C) | 299 |

Inlet or Bleed | Value |
---|---|

HP turbine: inlet (kPa) | 6400 (at 299 °C) |

HP turbine: first bleed (kPa) | 3600 |

HP turbine: second bleed (kPa) | 2700 |

HP turbine: to deaerator (kPa) | 1100 |

LP turbine: inlet (kPa) | 1020 (at 265 °C) |

LP turbine: first bleed (kPa) | 86 |

LP turbine: second bleed (kPa) | 34 |

Parameter | Pulse | Dwell |
---|---|---|

Primary pumps (MW) | 17.8 | 17.8 |

DIV and VV pumps (MW) | 9.3 | 9.3 |

Salt pumps (MW) | 7.0 | 14.1 |

Water circulation pumps (MW) | 13.9 | 13.9 |

Total Rankine pumps (MW) | 9.0 | 10.9 |

Total consumption (MW) | 57.0 | 66.0 |

Heat Exchanger | Pulse | Dwell |
---|---|---|

Condenser (MW) | 1307.6 | 1377.4 |

FWH1 (MW) | 102.4 | 110.6 |

FWH2 (MW) | 68.4 | 66.8 |

DIV PFU (MW) | 136.0 | 1.3 |

FWH DW1 (MW) | 5.4 | 132.4 |

VV (MW) | 86.0 | 2.2 |

FWH DW2 (MW) | 1.2 | 95.7 |

DIV CAS (MW) | 115.2 | 0.5 |

FWH DW3 (MW) | 1.2 | 90.9 |

FWH1 (MW) | 67.0 | 88.5 |

FWH2 (MW) | 88.9 | 72.7 |

Parameter | Pulse | Dwell |
---|---|---|

Gross power (MW) | 757.7 | 764.2 |

Net electric power (MW) | 700.7 | 698.1 |

Average gross power (MW) | 758.2 | |

Average net electric power (MW) | 700.5 | |

Isentropic efficiency (HP turbine) | 89.6% | 89.8% |

Isentropic efficiency (LP turbine) | 91.8% | 91.8% |

Gross efficiency | 33.5% | 3381.3% |

Net efficiency | 31.0% | 3089.2% |

Average gross efficiency | 36.3% | |

Average net electric efficiency | 33.6% |

Parameter | Pulse | Dwell |
---|---|---|

Primary pumps (MW) | 17.8 | 17.8 |

DIV and VV pumps (MW) | 9.3 | 9.3 |

Water circulation pumps (MW) | 12.3 | 6.1 |

Total Rankine pumps (MW) | 10.0 | 1.2 |

Total consumption (MW) | 50.1 | 34.7 |

Heat Exchanger | Pulse | Dwell |
---|---|---|

Condenser (MW) | 1573.1 | 204.4 |

FWH1 (MW) | 110.7 | 3.2 |

FWH2 (MW) | 92.6 | 17.8 |

DIV PFU (MW) | 136.2 | 1.0 |

FWH DW1 (MW) | 13.1 | 31.1 |

VV (MW) | 86.0 | 0.5 |

FWH DW2 (MW) | 3.8 | 12.1 |

DIV CAS (MW) | 115.2 | 0.7 |

FWH DW3 (MW) | 4.0 | 10.2 |

FWH1 (MW) | 105.2 | 14.1 |

FWH2 (MW) | 251.5 | 3.0 |

Parameter | Pulse | Dwell |
---|---|---|

Gross power (MW) | 892.9 | 40.3 |

Net electric power (MW) | 842.7 | 5.9 |

Average gross power (MW) | 827.1 | |

Average net electric power (MW) | 778.3 | |

Isentropic efficiency (HP turbine) | 86.6% | 87.0% |

Isentropic efficiency (LP turbine) | 90.5% | 42.3% |

Gross efficiency | 35.8% | 14.2% |

Net efficiency | 33.8% | 1.5% |

Average gross efficiency | 35.7% | |

Average net electric efficiency | 33.6% |

Parameter | WCLL DCD [6] | WCLL DCD AUXB | WCLL ICD |
---|---|---|---|

Gross power Pulse (MW) | 791.6 | 892.9 | 757.7 |

Gross power Dwell (MW) | 62.4 | 40.3 | 764.2 |

Average Gross efficiency | 34.9% | 35.7% | 36.3% |

Average net electric efficiency | 31.0% | 33.6% | 33.6% |

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**MDPI and ACS Style**

Narcisi, V.; Ciurluini, C.; Padula, G.; Giannetti, F.
Analysis of EU-DEMO WCLL Power Conversion System in Two Relevant Balance of Plant Configurations: Direct Coupling with Auxiliary Boiler and Indirect Coupling. *Sustainability* **2022**, *14*, 5779.
https://doi.org/10.3390/su14105779

**AMA Style**

Narcisi V, Ciurluini C, Padula G, Giannetti F.
Analysis of EU-DEMO WCLL Power Conversion System in Two Relevant Balance of Plant Configurations: Direct Coupling with Auxiliary Boiler and Indirect Coupling. *Sustainability*. 2022; 14(10):5779.
https://doi.org/10.3390/su14105779

**Chicago/Turabian Style**

Narcisi, Vincenzo, Cristiano Ciurluini, Giovanni Padula, and Fabio Giannetti.
2022. "Analysis of EU-DEMO WCLL Power Conversion System in Two Relevant Balance of Plant Configurations: Direct Coupling with Auxiliary Boiler and Indirect Coupling" *Sustainability* 14, no. 10: 5779.
https://doi.org/10.3390/su14105779