Ceramic composite moderators as replacements for graphite in high temperature microreactors

Engineered composites composed of a radiation stable continuous matrix containing a highly moderating entrained phase are attractive candidates for the realization of bulk materials that are structurally and neutronically superior to nuclear graphite. Here, we explore neutronics driven selection of entrained moderating phases in MgO-based ceramic composites with a focus on the MgO-BeO system given its exceptional moderating power and high temperature stability. Using lithium-bearing salts as sintering aids, fully dense MgO-BeO composites with BeO loading up to 40 vol.% are produced through direct current sintering at markedly reduced temperatures relative to phase-pure MgO. Thermophysical properties mapped as a function of the BeO concentration are shown to align with various composite models, thus revealing the influence of underlying defects on the thermophysical property trends. From microreactor neutronics and thermal hydraulic calculations, the MgO-40BeO moderator is shown to increase both cycle length and fuel utilization relative to graphite and with steady-state temperature distributions remaining within specification. The ceramic composite moderators outperform graphite for all metrics considered with significant potential demonstrated for reducing energy costs while enabling novel microreactor designs through the replacement of graphite.(c) 2022 The Author(s). Published by Elsevier B.V.This is an open access article under the CC BY-NC-ND license( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study focuses on the neutronics driven selection of entrained moderating phases in MgO-based ceramic composites, particularly the MgO-BeO system, which exhibits exceptional moderating power and high temperature stability. By using lithium-bearing salts as sintering aids, fully dense MgO-BeO composites with BeO loading up to 40 vol.% were successfully produced through direct current sintering at reduced temperatures. The thermophysical properties of the composites were analyzed, showing trends in alignment with various composite models, indicating the influence of underlying defects. Microreactor neutronics and thermal hydraulic calculations demonstrated that the MgO-40BeO moderator outperforms graphite in terms of cycle length and fuel utilization, while maintaining steady-state temperature distributions within specification. The ceramic composite moderators show significant potential for reducing energy costs and enabling novel microreactor designs.