Regulatory Perspective on VHTR Dust Safety Issue

Dust can be generated in VHTR and its impact on VHTR source term has been a long lingering safety issue(1,2). Though the design of tri-structural isotropic(TRISO) fuel is known to prevent large scale failure and fission product release even in accident scenarios, radiologically significant quantities of fission products will be present in the primary system coming from either a small fraction of failed fuel particles or intact fuel because of the diffusion in the fuel(3). The released fission products will be transported in the coolant gas, and plate out onto surfaces in the primary system. But analysis of the fission product distribution, in both normal and accident scenarios, is complicated in VHTR by the likely presence of dust. The FP-laden dust is supposed to be mobilized during the rapid depressurization accident and thus contributes to another source term mechanism. Also in all industrial plants where burnable solid compounds are treated, dust combustion becomes a significant risk and combustion of graphite dust in Chernobyl adds concerns to the dust safety issue. Thus, dust generation, interaction with fission product, transport and combustion constitutes the areas to be resolved for dust safety issue. U.S. NRC is organizing dust working group meeting since 2009 to discuss this dust safety. This paper summarizes the past experiences and current knowledge of dust in section 2 referencing the discussions of the recent 2011 dust working group meeting and suggests a domestic regulatory perspective to resolve the issue in section 3