Rapid aqueous release of fission products from high burn-up LWR fuel: Experimental results and correlations with fission gas release

Abstract Studies of the rapid aqueous release of fission products from UO 2 and MOX fuel are of interest for the assessment of the safety of geological disposal of spent fuel, because of the associated potential contribution to dose in radiological safety assessment. Studies have shown that correlations between fission gas release (FGR) and the fraction rapidly leached of various long-lived fission products can provide a useful method to obtain some of this information. Previously, these studies have been limited largely to fuel with burn-up values below 50 MWd/kg U. Collaborative studies involving SKB, Studsvik, Nagra and PSI have provided new data on short-term release of 137 Cs and 129 I for a number of fuels irradiated to burn-ups of 50–75 MWd/kgU. In addition a method for analysis of leaching solutions for 79 Se was developed. The results of the studies show that the fractional release of 137 Cs is usually much lower than the FGR covering the entire range of burn-ups studied. Fractional 129 I releases are somewhat larger, but only in cases in which the fuel was forcibly extracted from the cladding. Despite the expected high degree of segregation of fission gas (and by association 137 Cs and 129 I) in the high burn-up rim, no evidence was found for a significant contribution to release from the rim region. The method for 79 Se analysis developed did not permit its detection. Nonetheless, based on the detection limit, the results suggest that 79 Se is not preferentially leached from spent fuel.