Behaviour of Fission Products in Sodium

The techniques being employed to study the behaviour of mixed fission products in sodium are described and experimental results for 137Cs are presented. The experiments are designed to examine possible methods for controlling fission product activity levels in sodium-cooled fast reactors. Specimens of a Pu-Co-Ce fuel alloy, irradiated to 1019 fissions and cooled for two years, are the source of fission products for behavioural studies in a sodium forced convection loop. Liquid fuel and sodium are contacted at 600°C in one portion of the loop and the remainder of the loop is cycled between 550 and 110°C. The release and distribution of gamma-active species are detected by scanning loop components with a scintillation detector and 400-channel pulse-height analyser. The behaviour of betaemitters is determined through radiochemical analyses of sodium, metal foils and trapping materials removed at intervals from the loop. Of the long-lived gamma-emitters in the fuel, only 137Cs has been released in a quantity sufficient for detection by gamma scanning. Partition of 137Cs between sodium and cover gas, and between sodium and traps containing stainless steel mesh, carbon and sodium oxide, has been detected, Trapping techniques that appear promising in the loop experiments are examined in more detail by encapsulating individual gamma-active species with sodium and a collection material. A capsule is arranged so that the fission product source and sink are separated by a static sodium column several inches long. The capsule is heated in the range 150-500°C and the approach to equilibrium is followed using a movable scintillation detector and single channel analyser. In this manner the adsorption of 137Cs on stainless steel, nickel and graphite has been studied. Estimates have been made of the diffusibility of caesium in sodium and the adsorption rates of caesium on stainless steel and graphite. (author)