Release of gaseous fission products (Kr, Xe, J) from fuel elements for gas cooled high temperature reactors

First of all a survey is given about existing performance on fission gas release, at which diffusion is dominant as transport phenomenon. The necessity is shown for HTR fuel elements to treat the fission gas release upon a 'two-phase'-diffusion. The general model developed here contains the fission gas release both from direct recoil through the fuel element surface and from possible particle failure besides the two-phase-diffusion in the fuel element graphite. The mathematical description of these release mechanisms is done in the computer code 'STADIF' developed therefore. Numerous experimental results from heavy metal contamination investigations as well as theoretical statements corresponding to uranium diffussion in HTR fuel elements are considered to determine the sources of fission gas release. Guided by the many measurements derived from the Studsvik irradiation experiments with spherical elements the characteristic features of fission gas release, to which also the spontaneous increase of the release rates in single cases of particle failure belongs to, are represented. The model calculations carried out deliver diffusion coefficients of the fission gases Kr, Xe and J in the graphite grains, further some references to the probable number of failed particles, and altogether they show good agreement with the experimental results. (orig.)