Ceramic composite moderators as replacements for graphite in high temperature microreactors

Fabrication of neutron absorbing metal hydride entrained ceramic matrix shield composites

“…Frontiers in Nuclear Engineering frontiersin.org plateau pressure of <900 °C (Vetrano, 1971) and the list of ceramics that sinter to full density under 1,000 °C is extremely limited, forming such a composite is challenging. In previous work by the authors (Cheng et al, 2022), we systematically investigated the effects a range of lithium halides additives on the sintering kinetics of MgO and identified LiF as the most effective sintering aid (Benecke et al, 1967;Carnall, 1967;Hart et al, 1970;Jiang et al, 2017). As shown in Figure 5, the sintering onset temperature of pure submicron MgO powder was reduced from nominally >1,000 °C to <700 °C by adding 1 wt.% of LiF (~300 °C reduction in sintering temperature).…”

“…These centers reduce the free flow of phonons. Moreover, as seen from literature, HfH 2 (Ito et al, 2009) has lower thermal conductivity than MgO (Cheng et al, 2022), it can act as an insulating phase that hinders heat transfer within the composite, thereby reducing the thermal diffusivity and thermal conductivity. Frontiers in Nuclear Engineering frontiersin.org…”

“…In our previous work (Cheng et al, 2022;Snead et al, 2022;, composite sintering temperature was significantly suppressed through the use of LiF sintering aid in the monolithic MgO and MgO (LiF): ZrH 2 systems. This initial 350 °C reduction in MgO sintering temperature, and the 250 °C increased processing temperature for ZrH 2 through processing in a hydrogen atmosphere allowed the first demonstration of a ceramic-matrix metal-hydride composite.…”

“…We note that the thermal conductivity of the powder mixture is a critical factor in the sintering process (Guillon et al, 2014). As MgO has a relatively high thermal conductivity of ~44 (W/ m•K) (Cheng et al, 2022) as compared to HfH 2 at ~10 (W/m•K) (Ito et al, 2009) the higher onset sintering temperature can, at least in part, be attributed to a processing thermal gradient from sample periphery of as large as 100 °C (Anselmi-Tamburini et al, 2005). The delay in sintering from Figure 8A Frontiers in Nuclear Engineering frontiersin.org primary lower H-phase has been identified as HfH 1.4 , consistent with the known phase diagram (Dottor et al, 2023).…”