Validation of Pin-Resolved Reaction Rates, Kinetics Parameters, and Linear Source MOC in MPACT

Over the years, significant validation work for the neutronics code MPACT has been performed against zero-power critical benchmarks and measured data from operating nuclear power plants. Among all of these efforts, however, validation of the pin-resolved capability in MPACT has been limited by the public availability of experimental data and to a lesser degree availability of measurement techniques and facilities that provide such detailed data. Recently, new measurement results to experimentally determine the reaction rate along the pellet radius from the IPEN/MB-01 research reactor facility (IPEN) have been published as a benchmark in the International Reactor Physics Experiment Project handbook. In this paper, we examine MPACT simulation results for several IPEN benchmark experiments with emphasis on the intrapin reaction rate measurements. The IPEN critical experiments with variations in system temperature and gadolinium loadings are modeled first with the latest MPACT cross-section library and linear source (LS) method of characteristics (MOC) capability. The MPACT results of two-dimensional (2-D) models with axial buckling are within 160 pcm from the experimental eigenvalues using the flat source MOC. Using the LS MOC, the errors are no more than 70 pcm, and the temperature trend of various cases is smaller. The MPACT three-dimensional models with LS show slightly worse comparisons than the 2-D models, which may be due to the isotropic transverse leakage and homogenized cross-section approximations of the 2-D/one-dimensional solver. For the reaction rate validation, MPACT produces intrapin reaction rate results within 2σ of the experiment and shows excellent agreement with the Monte Carlo solution. The observed discrepancies between the simulated results and experiment for the fission rate measurements are discussed. The kinetics parameters measured in another IPEN experiment are also compared with MPACT simulations using different kinetics data sources. According to the validation results, JENDL-4.0 and Santamarina et al.’s data are recommended for MPACT transient calculations.