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.