Investigation of the radiolytic stability of a CyMe₄-BTBP based SANEX solvent

Abstract

The radiolytic degradation of the 6,6′-bis(5,5,8,8tetramethyl-5,6,7,8-tetrahydro-benzo[1,2,4]triazin-3-yl)-[2,2′]bipyridine (CyMe₄BTBP) based SANEX (selective actinide extraction) solvent has been investigated. As the solvent used in the extraction process is designed to separate trivalent actinides from lanthanides, the radiolytic degradation is mainly due to alpha decay of extracted minor actinide isotopes. A calculation of dose-rates was done by estimating the concentration of minor actinides in the solvent by fuel burn-up calculations and assumptions on dilutions in the subsequent reprocessing steps. The calculations showed that the main isotopes responsible for the dose-rate are ²⁴²Cm, ²⁴⁴Cm and ²⁴¹Am. ²⁴²Cm is short-lived and has an impact only at short cooling times before reprocessing of the spent fuel. The dose-rates to a SANEX solvent in the reprocessing of standard spent LWR fuels are burn-up dependent and range from at least 0.03–0.2 kGy/h for UO₂ fuels and from 0.4 to 0.8 kGy/h for MOX fuels. Fast reactor fuels yield dose-rates over 1 kGy/h. Based on these results, several radiolysis experiments were carried out in order to compare the effect of low LET external gamma radiation (0.2 kGy/h) and internal alpha radiation with different dose-rates (0.05, 0.2 and 1.0 kGy/h). Significant radiolytic degradation was shown in the gamma radiolysis and in the alpha radiolysis experiment at a dose-rate of 1 kGy/h. These experiments were continued up to an absorbed dose ∼1200 kGy and >300 kGy, respectively. Comparing the alpha radiolysis results for 0.2 kGy/h and 1.0 kGy/h, up to an absorbed dose of ∼120 kGy, no significant difference in the degradation for the different dose rates could be seen. The radiolytic degradation rate for gamma radiation was 40% higher than for alpha radiation.