Chemical form consideration of released fission products from irradiated fast reactor fuels during overheating

Abstract

Experiments simulating overheating conditions of fast reactor severe accidents have been previously carried out with irradiated fuels. For the present study, the chemical forms of the fission products (FPs) included in the irradiated fuels were evaluated by thermochemical equilibrium calculations. At temperatures of 2773 K and 2973 K, the most stable forms of Cs, I, Te, Sb, Pd and Ag are gaseous compounds, and their fractions are about 100% in an oxygen partial pressure range between 10−5 and 103 Pa. Cs and Sb detected in the thermal gradient tube (TGT) in the experiments can take gaseous chemical forms of elemental Cs, CsI, Cs2MoO4, CsO and elemental Sb, SbO, SbTe, respectively. The chemical forms of Cs and Mo are sensitive to the oxygen partial pressure. By comparing experimental results and the estimations, it is seen CsI thermochemically behaves in a manner that traps it in the TGT, while elemental Cs trends to move as fine particles. The moving behavior of the gaseous FPs will obey not only thermochemical principles, but also those of particle dynamics.