Dissolution behavior of (U,Zr)O2-based simulated fuel debris in nitric acid

Abstract

To explore the possibility of dissolving fuel debris into nitric acid as a potential pre-treatment for waste treatment in which the U and Pu are removed from the inventory, dissolution tests of U1−xZrxO2 and (U,Pu)1−xZrxO2 were carried out in 6 M HNO3 at 353 K. At the end of the dissolution test (after 4 h), the ratio of dissolved uranium decreased with an increase in the Zr contents, x. While the dissolution of U-rich samples was congruent, a preferential leaching of U was observed with Zr-rich samples. Taking into account these different dissolution phenomena, the dissolution rate analysis was carried out using surface-area model to calculate the instantaneous dissolution rate (IDR). The IDR decreased from 10−5 down to 10−10 mol cm−2 min−1 as x increased from 0 to 0.95. From these findings, dissolution with HNO3 is expected to be only applicable in U-rich part of fuel debris (x < 0.3) if the dissolution in 6 M HNO3 at 353 K is assumed. Application of complexing acids, such as mixture of HNO3 and HF, should be considered to increase the dissolution rate of the Zr-rich part.