Fuel Loading Pattern for APR1400 Reactor Core

by Mark Gino Aliperio

Introduction

In the pressurized water reactor like APR1400, annually about one third of the fuel assemblies are vacated 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. An optimal nuclear reload design can be defined as a configuration which has the maximum cycle length for the given fuel inventory or uses the minimum amount of fissionable materials for the given cycle length while satisfying safety constraints such as limitation on power peaking factor.

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 addition, the fact that this is a nonlinear and discrete problem creates complications in the use of conventional optimization techniques. Several techniques have been developed in order to optimize this distribution for the fuel assemblies in the core. A better choice in the configuration of the fuel assemblies loading is essential to guarantee the adequate use of the fissile elements, and also to guarantee safety during the operation.

A number of different loading patterns have been considered, with the general conclusion that more energy is extracted from the fuel when the power distribution in the core is as flat as possible. The usual approach to loading pattern optimization involves high degree of engineering judgment, a set of heuristic rules, an optimization algorithm and a computer code used for evaluating proposed loading patterns.

In this report, a loading pattern for APR1400 nuclear reactor core was proposed. The proposed loading pattern was analyzed to check if it satisfies the design criteria. The approach used to develop and design the loading pattern was based on engineering judgment, previous results, check-and-balance method. As a result, the optimized number of assemblies for corresponding types were determined.

Experimental Design and Methodology

The simulations in this report were done using CASMO3 and MASTER3. In this report, nine (9) 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. The configurations of the 16×16 fuel assembly such as the position and distribution of fuel rods and gadolinia burnable absorbers are shown in Figure 1. Note that in designing the assembly, it must be octant symmetric. As depicted in the results of Homework 2, the location of the burnable absorbers in the assembly is reasonably chosen. In addition, 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.

To design a loading pattern for the reactor core (in this case for APR1400), initial conditions such as fuel rod enrichment and number of fuel rods assembly were set. For assembly types with gadolinia burnable absorbers – B1, B2, B3, C1, C2, and C3, there are 20 BA rods (8.0 w/o) in each assembly. Moreover, the average fuel enrichment, εavg, per assembly in w/o U-235 is calculated.

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) was then designed. The reactor core is analyzed using MASTER3. To have an acceptable loading pattern, it must satisfy the design criteria as follows:

                                  Burn – up:                              17,500 MWD/MTU

                                  Maximum pin peaking;          less than 1.55

                                  Maximum pin burnup:           60,000 MWD/MTU

                                  Boron Concentration:            less than 800 ppm

Results and Discussions

After several simulations of core analysis in MASTER3, the proposed loading pattern as presented, satisfies the design criteria such as the burn-up, maximum pin peaking and burn-up, and boron concentration. The specific numbers of each assembly types are presented and its location in the core is shown:

The proposed loading pattern satisfies the criteria for burn-up. At 17,414 MWD/MTU burn-up, the boron concentration is at 10ppm. Moreover, the summary of results for maximum pin peaking is shown below, where it also satisfies the criteria of such value to be less than 1.55. Previous trials resulted in pin peaking higher than 1.55 due to the arrangement and configuration of fuel assemblies. Thus, 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. Thus, satisfying the design criteria.

Summary and Conclusion

In this report, a loading pattern was designed for APR1400 reactor core that satisfies the design criteria. Boron concentration at the beginning of cycle was 745.67 ppm (<800ppm); maximum pin peaking values were all less that 1.55; and the burn-up at 10 ppm was 17,414 MWD/MTU (~17,500 MWD/MTU). Although there are several methods to optimize the loading pattern, the results of the previous projects were used as reference for this specific loading pattern.

This work has been presented during the Philippine Nuclear Research and Development Conference 2020 organized by the Philippine Nuclear Research Institute.

References:

[1] J. Lamarsh & A. Baratta, “Introduction to Nuclear Engineering 3rd ed”, Prentice-Hall Inc (2001) ; [2] K. Trontl, et. al, “Machine Learning of the Reactor Core Loading Pattern Critical Parameters”, Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 10-13, 2007; [3] “PLUS7 Fuel Design for the APR1400” Document; APR1400-F-M-TR-12001-NP, KEPCO 2012