Radionuclide transport in a long-term operation supercritical CO₂-cooled direct-cycle small nuclear reactor

In recent years, to overcome the challenges of nuclear power plants due to their large scale, numerous types of small modular reactors are being designed worldwide. Small modular reactors are required to have a capability to operate in environmentally challenging regions with long refueling time. Supercritical CO₂ (S-CO₂)-cooled direct-cycle reactor is one of the candidates that can meet these requirements. In order to evaluate if the design achieves this goal, transport of generated radionuclides from the long-term operation has to be evaluated in order to estimate the radioactivity accumulation in the system. However, existing radionuclide transport evaluation method does not reflect the characteristics of supercritical fluids properly. In this paper, the fission product plate-out behavior of Korea Advanced Institute of Science and Technology Micro Modular Reactor (KAIST-MMR), a 10-MWe class S-CO₂-cooled fast reactor with direct S-CO₂ power conversion system, is studied. The evaluation methodology was developed via integrating suitable models while considering the shape of the nuclear fuel, component geometries, characteristics of a working fluid in supercritical state, and the fast reactor design. As a result of the analysis, the degree of plate-out of KAIST-MMR is not expected to have serious radioactivity accumulation within the components. Most of the sorption occurred at the turbine and recuperator hot-side inlet region. In particular, the dose rate of the turbine was quite low, because the size of the turbine is very small.