The escape behavior of non-gaseous fission products recoiled into graphite

Abstract

The escape behavior of non-gaseous fission products recoiled into graphite was studied experimentally. The annealing was done isothermally at temperatures from 200 to 1450 °C for 10 min to 24 h, and the fission products within the graphite crystal and on its surface were determined by leaching the annealed graphite sample with nitric acid. The fractions of fission products remaining within the crystal, existing on the surface, and lost during the annealing were obtained for 131I, 140Ba, 89Sr, 137Cs, 141Ce, 103Ru, 95Zr- 95Nb and for the gross fission products as a function of temperature and time of annealing. A fast and a slow step of migration in the crystal were observed, and the migration behavior was found to be complicated. The accumulation of 131I on the surface was negligible, and that of the other fission products was governed by whether vaporization from the surface occurred or not. The experimental results were interpreted by a proposed escape mechanism, consisting of three successive processes: release from the trapping sites, diffusion of the released fission products, and desorption from the surface. It was postulated that the fission products formed surface compounds immediately after they reached the surface of the graphite crystal and these compounds desorbed. The mathematical expressions for the release, diffusion and desorption processes are presented. The trapping state of each fission product in the graphite crystal was represented by an initial activation energy spectrum, and special defects having the activation energies for release of 1.3 ± 0.1, 2.0 ± 0.1 and 2.9 ± 0.1 eV atom were found to be effective in trapping fission products. The diffusion resistance for released fission products could be regarded as negligible in the slow step of migration in the graphite. The desorption behavior was satisfactorily explained by the model that fission products desorbed from a heterogeneous surface.