On the oxygen thresholds for the reactions of liquid sodium with urania and urania-plutonia solid solutions

Abstract Some calculations of the oxygen potential thresholds for the reactions of liquid sodium with urania and urania—plutonia solid solutions are presented and compared with experimental data. The reactions relevant to the understanding of the behaviour of breached sodium-cooled fast reactor breeder and fuel pins are those which form a sodium uranate, Na3UO4 (tri-sodium uranium(V) tetroxide) and a sodium urano-plutonate (Na3U1−xPuxO4). The oxygen potential thresholds are thus those for the three-phase fields; sodium, urania and sodium uranate, and sodium, urania—plutonia and sodium urano—plutonate. It has been shown that there is reasonable agreement between the calculated and measured values of oxygen potential in the ternary system. For the quaternary system with plutonium concentrations equivalent to Pu: (U + Pu) ratios of 0.25 there is also reasonable agreement between experimental and calculated values of the oxygen potential of the appropriate phase field. The measured threshold oxygen potentials are very similar for the ternary and quaternary systems as predicted. The difficulty of estimating the threshold oxygen concentration in liquid sodium from oxygen potential thresholds is illustrated by the use of different data for oxygen solubilities in sodium. The use of the vanadium wire equilibration technique to measure the threshold oxygen potentials in these systems is possible in principle; however, existing experimental results suggest that its application is not straightforward in complex systems.