Thermochemical considerations in source term evaluation

Abstract

The calculation of the release of fission products from degraded fuel in a light water reactor core uncovery accident is the first step in determining the overall radiological source term. It is the aim of costly experiments to improve our knowledge about the release behavior of the relevant fission products. Since this depends greatly on their chemistry, a thermodynamic evaluation about compound formation and vaporisation in a fuel-fission product-coolant system should precede such tests. It shows how the volatility of these products may change with test conditions. It will need more reduction of the steam atmosphere to get a noticable release of barium and strontium than to have europium show up. It is very unlikely that ruthenium is significantly released even in a nonreduced steam environment. Molybdenum will be released with the cesium in oxidising and slightly reducing atmospheres. Boron has an effect on the iodine and cesium chemistry. This, however, depends greatly on test or accident conditions. It is practically nonexistent at high steam pressures. Low oxygen potential and high boron content of the atmosphere increase the effect.