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2025 ANS Winter Conference & Expo
November 9–12, 2025
Washington, DC|Washington Hilton
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Latest News
Senate EPW Committee to hold Nieh nomination hearing
Nieh
The Senate Environment and Public Works Committee will hold a nomination hearing Wednesday for Ho Nieh, President Donald Trump’s nominee to serve as commission at the Nuclear Regulatory Commission.
Trump nominated Nieh on July 30 to serve as NRC commissioner the remainder of a term that will expire June 30, 2029, as Nuclear NewsWire previously reported.
Nieh has been vice president of regulatory affairs at Southern Nuclear since 2021, though since June 2024 he has been at the Institute of Nuclear Power Operations as a loaned executive.
A return to the NRC: If confirmed by the Senate, Nieh would be returning to the NRC after three previous stints totaling nearly 20 years.
Yuxuan Liu, Kyle Vaughn, Brendan Kochunas, Thomas Downar
Nuclear Science and Engineering | Volume 195 | Number 1 | January 2021 | Pages 50-68
Technical Paper | doi.org/10.1080/00295639.2020.1780853
Articles are hosted by Taylor and Francis Online.
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.
