WIMS/PANTHER Analysis of UO₂/MOX Cores Using Embedded Supercells

This paper describes a method of analyzing pressurized water reactor UO₂/mixed oxide (MOX) cores with the lattice code WIMS and the reactor code PANTHER. "Embedded supercells," run within the reactor code, are used to correct the standard methodology of using two-group smeared data from single-assembly (SA) lattice calculations. In many other codes the weakness of this standard approach has been improved for MOX by imposing a more realistic environment in the lattice code or by improving the sophistication of the reactor code. In this approach an intermediate set of calculations is introduced, leaving both lattice and reactor calculations broadly unchanged.

The essence of the approach is that the whole core is broken down into a set of embedded supercells, each extending over just four quarter assemblies, with zero leakage imposed at the assembly midlines. Each supercell is solved twice, first with a detailed multigroup pin-by-pin solution and then with the standard SA approach. Correction factors are defined by comparing the two solutions, and these can be applied in whole-core calculations.

The restriction that all such calculations be modeled with zero leakage means that they are independent of each other and of the core-wide flux shape. This allows parallel precalculation for the entire cycle once the loading pattern has been determined, in much the same way that SA lattice calculations can be precalculated once the range of fuel types is known.

Comparisons against a whole-core pin-by-pin reference demonstrates that the embedding process does not introduce a significant error, even after burnup and refueling. Comparisons against a WIMS reference demonstrate that a pin-by-pin multigroup diffusion solution is capable of capturing the main interface effects.

This therefore defines a practical approach for achieving results close to lattice code accuracy but broadly at the cost of a standard reactor calculation.