Hydrogen control approach in the S-PRIME moderated thermionic reactor

A preliminary analysis was carried out to estimate they hydrogen partial pressure in a NaK cooled 40 kWe thermionic reactor using zirconium hydride moderator elements. This analysis evaluated the potential problem of hydrogen pressure build-up within the Cs cavity of the thermionic fuel elements. To avoid a significant degradation in the performance of the TFEs, a maximum limit of 1.33 PA (0.01 torr) of hydrogen pressure was selected. Hydrogen permeation through the heat rejection walls of the NaK loop was not sufficient to keep the hydrogen pressure down to this desired level if stainless steel or another material with similar permeation properties was used in the NaK containment boundary. The use of a niobium diffusion window or a yttrium H2 getter bed offers two possible solutions to this problem