Fuel Loading Pattern Design for ARP1400 Reactor Core

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Introduction

In a pressurized water reactor like KEPCO's APR1400, about one third of the fuel assemblies are vacated annually and the others are reloaded with new fuel assemblies. In-core fuel management is one of the most challenging areas of nuclear engineering which involves the optimal arrangement of hundreds of fuel assemblies in the core. The optimization of this arrangement is very important from economical point of view to make the nuclear power generating station competitive. The main problem in the fuel assembly position determination is the large number of possible combinations for the fuel loading pattern in the core. In this paper, a loading pattern for APR1400 nuclear reactor core is proposed and evaluated if it satisfies the design criteria. The approach used to design the loading pattern is based on engineering judgment, a set of heuristic rules, optimization algorithm, and check-and-balance method.

Methodology

The simulations in this report are done using CASMO3 and MASTER3. Nine types of fuel assemblies are loaded in the core for this APR1400 reactor core loading pattern – A0, B0, B1, B2, B3, C0, C1, C2, and C3. Note that in designing the assembly, it must be octant symmetric. The reference state condition was also set to where Tf (fuel temperature) = 960.95 K; Tm (moderator temperature) =585.35 K; and Boron Concentration = 500 ppm. After the initial inputs are set and simulated in CASMO3, the cross-sections for each assembly are then generated. A loading pattern for APR1400 reactor core (with 241 fuel assemblies) is then designed using the nine assemblies. The reactor core is the analyzed using MASTER3. To have an acceptable loading pattern, it must satisfy the design criteria: burn – up of 17,500 MWD/MTU; maximum pin peaking less than 1.55; maximum pin burnup of 60,000 MWD/MTU; and boron concentration less than 800 ppm.

Results and Discussion

After several simulations of core analysis in MASTER3, a proposed loading pattern is developed. The numbers of each assembly types and its location in the core is also specified. The loading pattern satisfies the criteria for burn-up (17,414 MWD/MTU) at boron concentration of 10 ppm, and maximum pin peaking of less than 1.55. Previous trials resulted in pin peaking higher than 1.55 due to the arrangement and configuration of fuel assemblies. Hence, logical changes in the loading pattern were employed to balance the power of the core. For example, when the boron concentration was low, fuel enrichments of some fuel rods were increased to achieve such criteria with burn-up. In addition to that, when there is a concentration of high power in some part of the core, the loading pattern were re-arranged and modified such are increasing the number of assemblies with gadolinia burnable absorber, or changing the assembly type with regards to fuel enrichment, to balance the power defect in the core.

Conclusions

In this report, a loading pattern is designed for APR1400 reactor core that satisfies the design criteria. Boron concentration at the beginning of cycle is 745.67 ppm (<800ppm); maximum pin peaking values are all less than 1.55; and the burn-up at 10 ppm is 17,414 MWD/MTU (~17,500 MWD/MTU). Although there are several methods to optimize the loading pattern, the results of this report present an acceptable loading pattern design with an average core enrichment of 2.89%.