Verification of MPACT for the APR1400 Benchmark

Abstract

This report describes several benchmark calculations performed using the transport code MPACT, that were compared to reference solutions generated by the Monte Carlo code McCARD in order to support continuous improvements to MPACT, increasing reliability of reactor modeling software, and test the capability of MPACT to model advanced reactors. The benchmarks are based on the Advanced Power Reactor 1400 MWe designed by the Korea Electric Power Corporation.

The reactor core is composed of 241 fuel assemblies. Nine assembly types are specified utilizing different configurations of 1.71 wt% UO2, 2.00 wt% UO2, 2.64 wt% UO2, 3.14 wt% UO2, 3.64 wt% UO2, and gadolinia burnable absorbers. The reactor is controlled by seven control rod assembly banks. The banks are either 4-fingered or 12-fingered. The benchmark problems completed include single fuel pin, single 2-D fuel assembly, 2-D core, 3-D core, control rod worth, and 3-D core depletion. Nine temperature and boron conditions were considered for the various geometries. Additionally, MOC and spatial mesh parametric studies were performed using single fuel pins and single 2-D fuel assemblies to determine if default meshing parameters were sufficiently accurate.

For the calculations in this report, the MPACT 51-group cross section library based on ENDF-B/VII.1 was used. The calculations are performed with P2 scattering. For the MOC discretization, the Chebyshev-Yamamoto quadrature type was used with a ray spacing of 0.05 cm, 16 azimuthal angles per octant, and 2 polar angles per octant.

Overall, MPACT shows excellent agreement compared to the Monte Carlo reference solutions generated by McCARD. MPACT effectively predicts the reactivity for different geometries as well as several temperature and boron conditions. The largest deviation from McCARD occurs for cold zero conditions in which the fuel, moderator, and cladding are all 300 K. This is likely due to an incorrect hydrogen scattering matrix used by MPACT. Excluding these cases, the rho reactivity difference from McCARD is consistently below 100 pcm. For single fuel pin problems, the highest error occurs for the lowest fuel enrichment of 1.71 wt% UO2, indicating possible, albeit small, enrichment bias in MPACT’s cross section library. Furthermore, MOC and spatial mesh parametric studies indicate that default meshing parameters and options yield results comparable to finely meshed cases, thus verifying that default MOC and spatial discretization parameters generate accurate results for the benchmark problems.

Additionally, there is very good agreement of the radial and axial power distributions. Using P2 scattering instead of the default TCP0 scattering method corrected an in-out radial power tilt for the 2-D core, 3-D core, control rod worth, and 3-D core depletion problems. With P2 scattering, the RMS pin and assembly power differences for all cases are below 1%, and all RMS axial power differences are below 1.65%. These results are comparable to previous results from the VERA progression problems benchmark.

Regarding the hot full power 3-D core depletion, there was some variation in the critical boron concentration calculated by MPACT compared to nTracer and DeCART. Future work entails investigating the reasons for these differences.