Fast release from clad and declad spent UOX PWR fuel segments in a bicarbonate solution under anoxic conditions

For the safety assessment of Spent Nuclear Fuel (SNF) in disposal conditions, it is crucial to determine the fast release of the potentially critical radionuclides (e.g. 129I, 137Cs, 99Tc, 93Mo, 90Sr…) upon contact with water. To underpin the quantification of this fast release, leaching experiments were performed at SCK CEN on a PWR UOX fuel from the Belgian Tihange 1 reactor, with a rod average burnup of 50.5 MWd.kg−1HM, an estimated fission gas release of 14% and an average linear power rate of 321 W.cm−1. Tests were carried out with durations up to one year, with clad and declad spent fuel samples, in a NaCl/NaHCO3 solution at room temperature for a period of one year, in a set-up designed to prevent the intrusion of airborne oxygen. In the test with the declad segment, the exposed surface area was much larger than in the test with the clad segment. As a result, in the first days of leaching, the release of all radionuclides was larger for the declad segment than for the clad segment, except for Tc. After one year, the cumulated release from the declad and clad segments are similar, except for Tc, as a result of air intrusion leading to a higher oxidation and concomitant dissolution resumption, more pronounced for the clad than for the declad segment. For the declad segment, the Tc and Mo concentrations decrease, suggesting reduction to less soluble species in spite of the air intrusion. The release of 129I is similar to the fission gas release, while the release of 137Cs is lower, but still accelerated compared to the UO2 matrix. The release of 99Tc, 95-98Mo (representative of 93Mo) and 90Sr is only slightly higher than the matrix dissolution rate. All determined released fractions are in line with the reported values of the literature.