Fission release from molten fuel

The fraction of various groups of fission products >diich It Is assumed vlU be released from molten fuel Is shown In Table 9 3>... 

In the following description of the reactions occurring during this stage of a severe core damage accident, three different topics will be discussed the release of fission products from the fuel, the release of constituents of the core structural and control rod materials (although these two sources develop almost simultaneously in the reactor pressure vessel so that the volatilized substances can be assumed to enter the gas flow as a mixture) and, finally, volatilization of substances during the molten core - concrete interaction phase. The current state of the art will be discussed with special emphasis on the important chemical phenomena no attempts will be made to establish numerical values of source terms from the results of these experimental and theoretical efforts. 

Although iodine and cesium are readily released from the fuel during core heatup and are expected to be removed by bubble dynamics in molten pools, frequently small fractions (3 to 10%) of these fission products remain in the molten and re-solidified fuel debris. The reason for this presence of volatile fission products in molten fuel debris is not yet fully understood, but the small surface-to-volume ratio of large pools and the possibility of chemical forms of cesium that are relatively stable at high temperatures may be contributing factors. This retention of smaller amounts of iodine and cesium in the molten material might be favored by rapid local heat-up, as would occur if the oxidation of Zircaloy by steam were a significant heat source. 

In Europe a considerable amount of effort has been exerted to derive data in the release of fission products and fuel from the molten core to the sodium, and transport to the cover gas, and to develop codes to model the various phenomena. Future R and D work should focus on the evaluation of the release of fuel and fission products in the cover gas for different accident scenarios, taking into account the above data and codes. 

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