Results of a Phenomena Identification and Ranking Table (Pirt) Exercise for a Severe Accident in a Small Modular High-Temperature Gas-Cooled Reactor

Abstract

Canada has attracted specific interest from developers of nonwater-cooled small modular reactor (SMR) technologies, including concepts based on high-temperature gas-cooled reactors (HTGRs). It is anticipated that some research and development (R&D) will be necessary to support safety analysis and licensing of these reactors in Canada. The Phenomena Identification and Ranking Table (PIRT) process is a formalized method in which a panel of experts identifies which physical phenomena are most relevant to the reactor safety analysis and how well understood these phenomena are. The PIRT process is thus a tool to assess current knowledge levels and (or) predictive capabilities of models, thus providing direction to a focused R&D program. This paper summarizes the results of a PIRT process performed by a panel of experts at Canadian Nuclear Laboratories for a limiting or “worst-case” accident scenario at a generic HTGR-type SMR. Suggestions are given regarding the highest priority R&D items to support severe accidents analysis of these reactors.

Regulatory Considerations Regarding Potential High Temperature Fluid Releases in Advanced Reactor Designs

“…Analyses indicate that graphite dust combustion is unlikely, given the expected form and concentration of the dust [125]. Deflagration or detonation of CO/H2 could present an additional challenge to reactor building integrity [126], but their production levels are dependent on the graphite form and the conditions associated with oxidation [127].…”

State of Knowledge on Aerosols and Bubble Transport for Mechanistic Source Term Analysis of Molten Salt Reactors

“…In addition, aerosols suspended in an air atmosphere may behave differently than those in an inert gas atmosphere. Air ingress was included in the maximum credible accident scenario of the Canadian Nuclear Laboratory PIRT for a small MSR [116]. • The effect of humidity: Like the effect of air, exposure of salts to humidity can increase salt redox potential and produce highly volatile radionuclide species.…”