Porosity in nuclear graphite and its impact on nuclear reactor science and criticality safety applications

K Ramic and T Greene and I Al-Qasir and A Campbell and LM Debeer-Schmitt and C Celik and M Krzystyniak and A Marsicano and K Grammer and JIM Damian and F Bostelmann and YQ Cheng and CW Chapman and G Arbanas and Z Karriem and M Baird and D Wiarda and JM Brown and ML Martins and M Senanayake and L Leal and K Guber and J McDonnell and D DiJulio and G Procop and WA Wieselquist, CARBON, 244, 120619 (2025).

DOI: 10.1016/j.carbon.2025.120619

Porosity in nuclear-grade graphite significantly influences its low- energy neutron scattering, yet its effect on underlying phonon properties remains debated. This work integrates inelastic and small- angle neutron scattering (INS/SANS) experiments, advanced atomistic simulations with a novel machine-learned potential (DeepMD), total cross-section measurements, and neutronics calculations (SCALE, MCNP, OpenMC) to investigate porosity's impact on neutron thermalization. INS measurements on diverse graphite grades reveal no discernible porosity effect on phonon spectra, which align with crystalline graphite. Conversely, total cross-section data below approximate to 10 meV show increased scattering attributable to SANS. Our DeepMD simulations demonstrate that realistic micropores do not distort phonon spectra, challenging the assumptions in current ENDF/B-VIII.1 porosity thermal scattering laws (TSLs). These TSLs, based on random atom removal, produce unphysical phonon spectra and inflate inelastic cross-sections. Augmenting a crystalline TSL with an SANS component accurately captures experimental total cross-sections. Neutronics benchmarks (ICSBEP/IRPhE) show ENDF porosity TSLs unphysically increase neutron multiplication factor, keff. Crucially, incorporating SANS physics (NCrystal/OpenMC) indicates accurately modeled porosity negligibly affects keff, reactor physics, or criticality safety.

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