Fission Gas Behavior in Oxide Fuel Elements of Fast Breeder Reactors

Abstract

The fission gas behavior in oxide fuel elements irradiated within the irradiation program of the Fast Breeder Project was determined. The amount of fission gas was measured in three steps as released fission gas, fission gas retained in bubbles and pores, and fission gas retained in the fuel matrix. This was done by the puncturing of the fuel rods, by the grinding of the fuel, and by the chemical solution of the powdered fuel, respectively. Under the conditions prevailing in fast breeder reactors, the fractional fission gas release is ∼90% or more after medium and high burnups. The release rate is not constant, but there are periods with particularly high release rates (breakaway release). The retained fission gas reaches concentrations of ∼1.5 × 10−2 gas atoms per uranium and plutonium atoms in fuel regions with temperatures below 1100°C. The concentration decreases with increasing temperatures. At temperatures above 1500°C the concentration of the retained gas is ∼2 × 10−4 gas atoms per initial metal atom. Up to five times more fission gas is retained in bubbles and pores than in the matrix. There is a relation between plastic deformation of the cladding by mechanical interaction with the fuel and the concentration of fission gas in bubbles.