Chlorine-36 in nuclear waste disposal—1. Assessment results for used fuel with comparison to 129I and 14C

Chlorine-36 in used fuel arises from the activation of 35CI. Unlike 129I and 14C, it was not immediately clear that 36Cl would be present in significant quantities in used fuel, because 35Cl would be present only as a contaminant and little was expected because of the volatility of Cl during the high-temperature sintering used in fuel production. With more intensive elemental analysis of reactor metals, Cl was observed, and the analyses were extended to include used fuel. Chlorine-36 was found. It was expected to behave much like 129I and 14C; rapidly released from the wasteform, mobile in geological materials and biologically important. This study assessed the impact of 36Cl in nuclear fuel waste and results are presented here relative to the other radionuclides. For radiological dose to humans and for chemical toxicity effects, 36Cl has 20-fold less impact than 129I. For radiological dose to non-human biota, 36Cl exceeds 129I for plants because of the markedly higher soil-to-plant transfer. Large isotopic dilution in the surface environments is the most important feature of dose estimation for 36Cl. All the predicted impacts, however, are very small for the disposal concept for Canadian nuclear fuel waste.