Determination of Cl Impurities and 36Cl Instant Release from Used Candu Fuels

Abstract

ABSTRACTChlorine-36 has been identified as a potential source of radiological risk in the disposal of nuclear fuel waste. The radioisotope 36Cl (t1/2 = 3 × 1O5 a) is produced by neutron activation of Cl impurities in UO2 fuel. The total average Cl impurity level in four unirradiated CANDU UO2 fuel samples was 2.3 ± 1.1 ppm. ORIGEN-S calculations using a 5 ppm Cl impurity in a CANDU fuel resulted in a 36Cl activity comparable to the activity of 129I and 14C produced in the fuel thus requiring 36Cl to be considered in disposal risk assessments. The “instant release” of 36Cl from the gap and grain boundary regions of the fuel to solution was measured by leaching both clad fuel and fuel samples crushed to grain-sized particles. The 36Cl concentration was measured by Accelerator Mass Spectrometry. The 36Cl releases from fuel samples taken from 8 different fuel bundles ranged from 0.5% to 20.4% of the total 3 Cl inventory over a leaching period of 32 days. The 36Cl released was found to correlate with the stable Xe gas release, the fuel burnup and the linear power rating (LPR). For a typical CANDU fuel with an LPR of -42 kW/m, the “instant release” of 36Cl would be about 5% of the total inventory.