Toxicity of irradiated advanced heavy water reactor fuels (2)

The good neutron economy and online refuelling capability of heavy water moderated reactors enable them to use many different fuels types such as: low enriched uranium, plutonium, or thorium, in addition to their traditional natural uranium fuel. The toxicity and radiological protection methods for these proposed fuels, unlike those for natural uranium, are not well established. This study uses software to evaluate the composition and toxicity of three irradiated advanced heavy water reactor oxide fuels as a function of post-irradiation time. All three fuels use plutonium assumed to be recovered from light water reactor (LWR) fuel via reprocessing. The first heavy water reactor fuel investigated is a homogeneous thorium-plutonium fuel designed for a once-through fuel cycle. The second fuel is a heterogeneous thorium-plutonium- ²³³U bundle, with graded enrichments of ²³³U in different parts of a single fuel assembly, assumed to be part of a recycling fuel cycle in which ²³³U from previous cycles is recovered. The third fuel is one in which plutonium and ²⁴¹Am is mixed with natural uranium. Each of these fuels turns out to be considerably more radiotoxic, for several years after reactor shutdown, than standard natural uranium reactor fuel would be, and it is shown that initial plutonium content is the most important factor affecting final radiotoxicity. For natural uranium, the isotope ²³⁹Pu is a significant contributor to the internal dose from exposure, and evaluating its presence in urine using mass spectrometry is sufficient to estimate internal doses as low as 1 mSv - the level required by regulation. If this method is extended so that ²⁴⁰Pu is also measured, then the combined amount of ²³⁹Pu and ²⁴⁰Pu is sufficiently high in the thorium-plutonium fuel that internal exposure to this fuel can be monitored using this method, but the fraction of these isotopes in the other two fuels is sufficiently low that they would remain below the detection limit. Thus, new techniques such as faecal measurements of ²³⁹Pu (or other alpha emitters) will be required for these fuels. (author)