Release of Fission Products During In-Pile Melting of UO₂

A series of experiments to study the amounts and forms of fission products released during simulated reactor accidents is described. These experiments consisted of melting miniature stainless-steel-clad UO₂ fuel elements in a helium atmosphere in the Oak Ridge Research Reactor and measuring the fission products released. Fission and gamma heat in the reactor raised the temperature of the miniature fuel element sufficiently high to melt the UO₂ without the use of external heat. In these experiments with UO₂, nearly all of the iodine, tellurium, and cesium, and more than half of the strontium, zirconium, ruthenium, barium, and cerium were released from the fuel. Release of the latter group of fission products and uranium from a zone including the fuel and surrounding heat insulators was generally less than 3%. The minimum temperature of this zone during fuel melting was 1000 C. The retention of fission products within the high-temperature zone is considered to be significant since, during an actual reactor accident, temperatures corresponding to those of the high-temperature zone would probably occur only within the immediate area in which the fuel is overheated. Analysis of the observed distributions of deposited fission products yields information about their behavior and form. Ruthenium follows the stainless-steel cladding as it melts and vaporizes. Certain fission products are associated with millimicrometer-size particles of two size groups, one centered around 22 angstroms and the second around 30 angstroms in diameter. Comparisons of the fission-product-release values from in-pile and various out-of-pile experiments indicate that the in-pile releases are greater, probably because of more extreme temperatures.