Published online by Cambridge University Press: 03 September 2012
Chlorine-36 has been identified as a potential source of radiological riskin the disposal of nuclear fuel waste. The radioisotope 36Cl (t1/2 = 3 × 1O5 a) is produced by neutronactivation of Cl impurities in UO2 fuel. The total average Climpurity level in four unirradiated CANDU UO2 fuel samples was2.3 ± 1.1 ppm. ORIGEN-S calculations using a 5 ppm Cl impurity in a CANDUfuel 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 regionsof the fuel to solution was measured by leaching both clad fuel and fuelsamples crushed to grain-sized particles. The 36Cl concentrationwas measured by Accelerator Mass Spectrometry. The 36Cl releasesfrom fuel samples taken from 8 different fuel bundles ranged from 0.5% to20.4% of the total 3 Cl inventory over a leaching period of 32days. The 36Cl released was found to correlate with the stable Xegas release, the fuel burnup and the linear power rating (LPR). For atypical CANDU fuel with an LPR of -42 kW/m, the “instant release” of 36Cl would be about 5% of the total inventory.