Environmental Safety Research Group
Research on the Safety of Radioactive Wastes
Various types of radioactive wastes are generated by using nuclear power. Our group has carried out regulatory research to provide technical support for a safety review of both final disposal of various radioactive wastes and decommissioning of nuclear facilities.
The ground surface environment in Fukushima Prefecture and the surrounding area has been contaminated by radionuclides released from Fukushima Daiichi Nuclear Power Plant. To ensure the radiation safety of workers and the public in handling contaminated materials, we have supported the government in devising guidelines and criteria for the treatment and disposal of nuclear waste.
Our group is carrying out research in following three categories of safety to support the safe disposal of radioactive waste and safe decommissioning of nuclear facilities:
I. Safety assessment for disposal of radioactive wastes;
II. Safety assessment for decommissioning of nuclear facilities;
III. Regulatory criteria for treatment of contaminated materials.
I. Study on safety assessment for disposal of radioactive wastes
The safety assessment for geological disposal of high-level radioactive wastes could not be experimentally demonstrated because the duration of safety to be ensured is very long, up to one million years. Therefore, the safety of such disposal will be demonstrated by evaluation of radionuclide migration and exposure doses for the public using calculation codes with models and parameters to quantitatively express the phenomena based on a scenario that describes the future behaviors of the geological repository and surrounding environment.
Uplift is one of the most important phenomena to consider in the long-term evolution of geology. Uplift is of two types: uniform uplift and tilted uplift. Scenarios and geometric models were developed for each type. Calculation codes, such as radionuclide-migration code, were developed.
Radionuclide-migration flux at the outlet of a natural barrier is larger in tilted uplift than in uniform uplift because ground-water velocity is increased by an increase of the hydraulic gradient in tilted uplift. We are developing the evaluation methodology for impact on the public and the environment caused by long-term changes in geological environment and climate.
In addition, it is important to understand the ground-water flow system from a recharged area (where rain infiltrates the ground) to a discharged area (where the ground-water flow out to the surface) for a reasonable assessment of estimating safe doses of public exposure. To this end, evaluation code for regional and long-term ground-water flow, 3D-SEEP, has been developed.
II. Study on safety assessment for decommissioning nuclear facilities
A codified system of safety assessment for decommissioning nuclear facilities is being developed to evaluate the validation of decommissioning plans and confirmations of completion of decommissioning activity that are submitted to regulatory agency. The codified system needs to include a method for evaluating the exposure doses for the public and workers during dismantling of facilities and for estimating the distribution of residual radioactivity on the site after dismantling.
III. Research on regulatory criteria for treatment of contaminated materials
We discuss the regulatory criteria for disposal or recycling of the low-level wastes generated from plant operation and decommissioning of nuclear facilities as well as of the contaminated materials resulting from the accident at the Fukushima Nuclear Power Plant.