# Optimized Design and Discussion on Middle and Large CANDLE Reactors

^{*}

## Abstract

**:**

**C**onstant

**A**xial shape of

**N**eutron flux, nuclide number densities and power shape

**D**uring

**L**ife of

**E**nergy producing reactor) reactors have been intensively researched in the last decades [1,2,3.4,5,6]. Research shows that this kind of reactor is highly economical, safe and efficiently saves resources, thus extending large scale fission nuclear energy utilization for thousands of years, benefitting the whole of society. For many developing countries with a large population and high energy demands, such as China and India, middle (1000 MWth) and large (2000 MWth) CANDLE fast reactors are obviously more suitable than small reactors [2]. In this paper, the middle and large CANDLE reactors are investigated with U-Pu and combined ThU-UPu fuel cycles, aiming to utilize the abundant thorium resources and optimize the radial power distribution. To achieve these design purposes, the present designs were utilized, simply dividing the core into two fuel regions in the radial direction. The less active fuel, such as thorium or natural uranium, was loaded in the inner core region and the fuel with low-level enrichment, e.g. 2.0% enriched uranium, was loaded in the outer core region. By this simple core configuration and fuel setting, rather than using a complicated method, we can obtain the desired middle and large CANDLE fast cores with reasonable core geometry and thermal hydraulic parameters that perform safely and economically; as is to be expected from CANDLE. To assist in understanding the CANDLE reactor’s attributes, analysis and discussion of the calculation results achieved are provided.

## 1. Introduction

#### 1.1. Super Fuel Utilization Capability

#### 1.2. Highly Stable Operation throughout the Life of the Core

#### 1.3. No Need for the Burnup Control Rod Mechanism

#### 1.4. Radial Power Distribution Can Be Greatly Optimized

#### 1.5. Extremely Long Core Life Can Be Easily Realized

#### 1.6. No Need for Refueling during the Core Life, if Material Integrity Could Be Assured

#### 1.7. Low Infinite Neutron Multiplication Factor of Fresh Fuel

#### 1.8. Advanced Inherent Safety Features

#### 1.9. Attractive Economy

#### 1.10. Efficient Utilization of Thorium Fuel

## 2. Results and Discussion

#### 2.1. Calculation Conditions

_{3}Monte Carlo neutron transport code. It can treat complicated geometry like the hexagonal meshes of actual assemblies and has excellent variance reduction techniques. The number of energy groups in the MCMG code can be varied from a few groups to a multigroup (more than 100 groups), according to individual needs. Either macroscopic or microscopic cross sections can be used and its format is designed in accordance with the ANISN format. The MCMG code has been parallelized in the Parallel Virtual Machine (PVM) and the Message Passing Interface (MPI). The speedup increases linearly with the number of processors. Numerical tests show that the MCMG code can calculate the neutron transport problems accurately.

#### 2.2. The Design and Results of the 1000 MWth CANDLE Reactor

Main Design Parameters | Values/Specifications |
---|---|

Thermal Power/MWth | 1000 |

Core Height/cm | 230 |

Core Radius/cm | 165 |

Radius of Inner Core/cm | 82.5 |

Thickness of Reflector/cm | 50 |

Fuel Cell Radius/mm | 6 |

Thickness of Cladding/mm | 0.5 |

Distance between fuel pins/mm | 0.8 |

Fuel Material of Inner Core | Natural UN |

Fuel Material of Outer Core | Enrichment 2.7% UN |

Cladding Material | HT-9 |

Coolant Material | LBE (44.5%–55.5%) |

Reflector Material | LBE (44.5%–55.5%) |

Core Inlet Temp/K | 600 |

Core Outlet Temp Peak/K | 800 |

Key Results | Values |
---|---|

Keff | 1.0018 |

Burnup Velocity/cm/year | 1.59 |

Max Burnup Depth/% | 57.0 |

Fuel Cell Temp Peak/K | 883.4 |

Cladding Temp Peak/K | 801.2 |

Maximum Coolant Velocity/m/s | 2.2 |

^{−2}s

^{−1}to 1.0e15 cm

^{−2}s

^{−1}at the axial flux peak. According to Figure 5, the burning region materials that are exposed to the highest flux radiation would take about 18.8 years to reach the damage neutron fluence limit, which is about 250 dpa (5.0e23 cm

^{−2}) [9], from new. The counted neutron flux has an energy level higher than 0.1 MeV, which is about 50% of the summation for the present case. In other words, after every performance period of 18.8 years, proper measures such as the “recladding procedure” [10] should be carried out. In such a procedure, all of the fuel pins would be entirely recladded for the longest performance periods. Meanwhile, the spent fuel would be removed and fresh fuel would be added at the two axial core ends, respectively. After this procedure, the state of the core will be rejuvenated and the new operation period will repeat the burnup process of the previous 18.8 years.

**Figure 7.**(

**a**)The 3D radial neutron flux distribution at axial layer 30; (

**b**) layer 47; (

**c**) layer 70 and (

**d**) layer 90.

#### 2.3. The Design and Results of the 2000 MWth CANDLE Reactor

Main Design Parameters | Values/Specifications |
---|---|

Thermal Power/MWth | 2000 |

Core Height/cm | 230 |

Core Radius/cm | 210 |

Radius of Inner Core/cm | 126 |

Thickness of Reflector/cm | 50 |

Fuel Cell Radius/mm | 6 |

Thickness of Cladding/mm | 0.5 |

Distance between fuel pins/mm | 0.8 |

Fuel Material of Inner Core | Natural 82％ UN、18％ ThN |

Fuel Material of Outer Core | Enrichment 2.4% UN |

Cladding Material | HT-9 |

Coolant Material | PbBi (44.5%–55.5%) |

Reflector Material | PbBi (44.5%–55.5%) |

Core Inlet Temp/K | 600 |

Core Outlet Temp Peak/K | 800 |

Key Results | Values |
---|---|

Keff | 1.0019 |

Burnup velocity (cm/year) | 1.84 |

Maximum discharged fuel burn-up (%) | 57.0 |

Fuel temperature peak (K) | 916.7 |

Cladding temperature peak (K) | 801.1 |

Maximum Coolant Velocity/m/s | 3.1 |

**Table 5.**The number densities of the main MAs and the plutonium isotopes in the spent fuel region of outer core.

Nuclide | Number Density (cm^{−3}) | Nuclide | Number Density (cm^{−3}) |
---|---|---|---|

Np236 | 1.21e15 | Am241 | 1.05e20 |

Np237 | 3.52e19 | Am242M | 5.62e18 |

Pu236 | 4.46e13 | Am243 | 8.38e18 |

Pu238 | 4.59e19 | Cm242 | 1.14e17 |

Pu239 | 1.61e21 | Cm243 | 3.33e16 |

Pu240 | 7.76e20 | Cm244 | 1.73e18 |

Pu241 | 3.33e19 | Cm245 | 4.56e17 |

Pu242 | 4.12e19 | Cm246 | 1.41e17 |

^{−3}, which is only 0.57% of that of the uranium isotopes in the fresh fuel. For a fast reactor, the corresponding ratio would be many times higher. Compared with the LWR, the spent fuel amount per produced heat energy is only 10% of LWR in the once-through cycle, according to the burnup depth. Also, if the thermal efficiency is taken into account, the ratio would be reduced to 7% or even less. The CANDLE reactor is suitable for adopting the once-through cycle, considerably reducing the indirect waste generated by the reprocessing, which can usually be thousands of times the amount of the spent fuel. The conclusion should be drawn that the CANDLE reactor is an incredible converter, burner, transmutator and environment saver all in one.

^{−2}s

^{−1}to 1.0e15 cm

^{−2}s

^{−1}at the axial flux peak. Compared with the 1000 MWth case, a much flatter flux distribution covers a larger area, as shown in the 3D figures of Figure 16. Because the core radius is increased, the neutron loss from the inner core region introduced by radial leakage is weakened. This reminds us that a more functional reflector would enhance the flattening of the radial flux distribution. According to Figure 17, the burning region that is exposed to the highest radiation would take about 16.2 years to reach the damage neutron fluence limit, which is about 250 dpa (5.0e23 cm

^{−2}) from the fresh state (the counted neutrons have an energy level higher than 0.1 MeV, which is about 50% of the summation for the present case). This time limit is restricted by the maximum local radiation dose along the fuel assembly with maximum power output and the proportion of the high energy neutrons. So the radial neutron flux distribution is required to be as flat as possible to extend the time limit.

**Figure 16.**(

**a**) The 3D radial neutron flux distribution at axial layer 30; (

**b**) layer 40; (

**c**) layer 60 and (

**d**) layer 80.

^{−2}. Seventy percent of the core will get the fluence beyond 1.0e24 cm

^{−2}and it will take about 37 years for the most radiated fuel assembly to reach this value. If a cladding material supports the 500 dpa (1.0e24 cm

^{−2}) without any devastating integrity crisis, then 30% of the core (174 cm ~ 210 cm in the radial direction) does not need any treatment for the whole life of the core. After 37 years of full power operation without a break, recladding or just simply refueling will be needed for the first and only time in the 60 year lifespan of the core. Additionally, if a suitable material is found to survive the 645 dpa, then the whole core would realize full power operation without a break for 60 years.

- • Firstly, following the CANDLE advancements, a simple refueling mode without any recladding procedure being necessary could be realized. Refueling would be carried out every 15 years conservatively, taking 250 dpa as the limit margin. Thus, in the 60 years core life, the CANDLE will require a very low-levelenriched fuel inventory for four cores and will need to be shut down four times in total. At the same time, research and radiation testing of cladding materialsshould be pushed forward. With the development of suitable materials, the refueling amount and frequency could be reduced and might well be eventually eliminated. A fast reactor with the same power level usually needs refueling every 1.5 years, i.e., in an assumed 60 year core life, it needs a fuel inventory for 13 cores, which is much higher enriched, and shutting down a total of 40 times. It becomes apparent that the CANDLE reactors are the rational substitute forfast reactors.
- • Continue with the SWR (Standing Wave Reactor) development proposed by TerraPower LLC [11,12,13]. Theoretically, a SWR does not need refueling or recladding, but has a complicated shuffling operation of the fuel assemblies every 2 ~ 3 years. Also, the development of cladding materials is also critical for this reactor, because better material could reduce the frequency of the shuffling operations and the amount of the fuel assemblies involved.A SWR does not automatically maintain the steady burnup or maintain the flattened radial flux distribution. Also, the frequent shuffling of the irradiated fuel assemblies poses engineering challenges. On another aspect, the burnup ratio of a SWR is lower than that of a CANDLE, introducing lower irradiation damage to the materials and lower utilization capability of nuclear resources at the same time. We could presume that the SWR is a compromise for the CANDLE for easier engineering approaches and as a support of the CANDLE in the development process for better materia

## 3. Conclusions

## Conflict of Interest

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**MDPI and ACS Style**

Yan, M.; Zhang, Y.; Chai, X.
Optimized Design and Discussion on Middle and Large CANDLE Reactors. *Sustainability* **2012**, *4*, 1888-1907.
https://doi.org/10.3390/su4081888

**AMA Style**

Yan M, Zhang Y, Chai X.
Optimized Design and Discussion on Middle and Large CANDLE Reactors. *Sustainability*. 2012; 4(8):1888-1907.
https://doi.org/10.3390/su4081888

**Chicago/Turabian Style**

Yan, Mingyu, Yong Zhang, and Xiaoming Chai.
2012. "Optimized Design and Discussion on Middle and Large CANDLE Reactors" *Sustainability* 4, no. 8: 1888-1907.
https://doi.org/10.3390/su4081888