Quantifying Stochastic Geometric Effects in the HTR-PROTEUS Benchmark

This work systematically quantifies geometric uncertainty in the HTR-PROTEUS Core 4.2 benchmark to support the DOE/NRC Criticality Safety for Commercial-Scale HALEU Fuel Cycle and Transportation (DNCSH) project. Using a discrete element method-based software packaged called Project Chrono::GPU, 110 realistic random pebble packings were generated for Monte Carlo transport calculations using SHIFT, Serpent, and MCNP. Results demonstrate that random pebble positioning introduces approximately 30 pcm uncertainty in keff, significantly exceeding individual Monte Carlo statistical uncertainties of 5 pcm. Analysis of TRISO particle positioning methods reveals that the legacy modeling approach of using an ordered lattice representation of TRISO particles underestimate keff by approximately 10 pcm compared to true random positioning, with semi-random methods showing 2-3 pcm differences. These findings establish the importance of high-fidelity geometric modeling for HALEU criticality safety applications, particularly for fuel transportation and storage where reactivity control is paramount. This work directly supports the HALEU Availability Program by providing analyses and computational methods for systems with random pebble packings and TRISO particle positions, relevant for modern pebble bed reactors and scenarios where ordered pebble arrangements cannot be guaranteed.