The Retention of Fission Products in Dispersed Fuels. Dragon Project Report

Recent experimental results on the release of fission products from coated particle fuels dispersed in graphite can be interpreted as a combination of recoil and diffusion. Whereas the coating retention for noble gases is good, many metals and halogens escape more readily and migrate to cooler regions of the fuel element where surface absorption and low diffusion coefficients can support a high concentration. Purge arrangements have little control on this release. Future fuel concepts may dispense with purge systems and employ fission product sinks as part of the fuel element structure to complement the retention of the coating. An outline of the design criteria for such fuels is suggested here. The concentration dependence of diffusion and desorption from the surfaces are pointed out and the time dependence of fission product concentration is discussed as a possible limitation to fuel element life. (author)