Purpose: this paper concentrates on the development and validation of an approach enabling to identify radiologically relevant radionuclides in radioactive waste at different stages of their management, as well as to implement their direct instrumental control.
Material and methods: The research was carried out by comparing relevant radiological effects produced by radionuclides contained in activation RW from VVER-440 unit (reactor vessel, vessel internals and fuel rod cladding) on human at different stages of RW management. AKDAM-2.0 complex was used to evaluate the changes in the radionuclide inventory of materials during their irradiation and cooling.
Results: The study enabled to calculate relevant contribution of different radionuclides to the overall activity and dose rate from external and internal human exposure at different stages of RW management. This paper proposes a novel approach to identify radiologically relevant radionuclides based on a comparative evaluation of their contribution to the dose rate their activity, half-lives and migration properties. Relevant lists of radionuclides were developed for the studied materials. These radionuclides should be considered in the safety assessments of RW management practices and the post-closure safety demonstration for deep geological disposal facilities (DGDF). The study suggests that direct instrumental control is feasible for a number of radionuclides along with the use of an integrated approach enabling to evaluate the content of other radionuclides.
Conclusions: Further validation proved the viability of the proposed approach in the development of lists of radiologically relevant radionuclides. It has been found that the generated list is consistent with international practice used to demonstrate DGDF safety. It should be noted that only a small number of radionuclides should be considered radiologically relevant notwithstanding a variety of radionuclides generated due to material activation. In some cases, certain radionuclides are required to be added to the lists specified in the appendices to Radiation Safety Standards (RSS). However, the direct instrumental control covering the entire list of radiologically relevant radionuclides (for each RW management stage) is considered not feasible and hard to implement. The study shows that an integrated approach combining direct instrumental control for certain radionuclides, periodic destructive tests and inventory calculations is a preferred option. At the stage of periodic radiation monitoring in the disposal facility, such instrumental control should be focused on the evaluation of disposal system properties affecting its safety (hydrogeochemical, hydrogeological characteristics and etc.).
REPORTING THE RADIONUCLIDE INVENTORY IN THE DWPF PRODUCT (U)
