"Trapped fraction" effect in microfuel with plutonium fuel

Results of the investigation into the thermodynamics of two types of microfuel (with oxygen getter and without it) with plutonium fuel for various degrees of burnup are presented. The behavior of the trapped fraction of Ag, Ce, Cd, Cs, La, Mo, Pu, Pd, Ru, Sr, Te, and Y is investigated. The fraction of any fission product bound into stable chemical compounds with other components of the system and excluded from the diffusion process is called the trapped fraction. An abrupt increase in the trapped fraction of cesium and, correspondingly, a decrease in free cesium during the burnup of ∼26% FIMA (fissions per initial metal atom) or more are found for the microfuel containing no oxygen getter. This leads to a substantial nonlinear burnup dependence of the trapped fraction and should be the cause of an abrupt decrease in the Cs output from the microfuel. It seems likely that the found effect is associated with the formation of carbonate Cs₂CO₃ in the plutonium fuel. In the case of microfuel containing the oxygen getter, no formation of cesium carbonate occurs and the trapped fraction of cesium is almost independent of the burnup.