Study of solid moderators for the thermal-spectrum supercritical water-cooled reactor (thermo-mechanics and cost)

The feasibility of zirconium hydride as the moderator of a thermal-spectrum supercritical-water cooled nuclear reactor (SCWR) was assessed. Several issues were evaluated including zirconium-hydride / water interaction, hydrogen release, hydrogen redistribution, pressurization of the moderator box at high temperature, phase stability, and compatibility of zirconium hydride with the moderator box material. It was found that: The chemical reaction of zirconium hydride with water is thermodynamically favored but kinetically impaired even at high temperatures. Hydrogen release is low at steady-state, while the use of a hydrogen-impermeable coating on the inner surface of the moderator box might be required at higher temperatures. The moderator-box wall temperature during a LOCA should be limited to 900degC to prevent failure from internal hydrogen pressure. Hydrogen redistribution and release do not threaten the stability of δ-phase zirconium hydride. The use of a solid moderator greatly enhances the thermal capacity of the SCWR core. The issues of hydriding and hydrogen embrittlement of the moderator box material, Alloy 718, appear of little concern, while high-temperature diffusion of zirconium into Alloy 718 might have to be prevented by means of a thin molybdenum protective coating. Finally, fabrication of the SCWR moderator rods appears feasible within the envelope of existing technology. Also, a simple analysis indicated that the use of zirconium-hydride moderator will not result in a significant economic penalty for the SCWR. Several issues key to the neutronic feasibility of zirconium hydride were also assessed and are discussed in a separate paper (no. 36571) presented at this conference. At this point no insurmountable difficulties have been identified in the development of zirconium hydride moderator rods for SCWR thermal-spectrum cores. (author)