Addendum to ORNL CF-61-2-46, Molten-Salt Reactor Experiment Preliminary Hazards Report

A New Look at Licensing Basis Events for the Molten Salt Reactor Experiment

“…This case study of the LBE selection process was performed on a design that had already been constructed and operated, so the safety approach is well documented. The hazards and accident sequences discussed in the preliminary hazards report [6] and the MSRE Safety Analysis [7] were used to develop an initial list of LBEs for the proposed design. These references provided sufficient details on some IEs and phenomena of concern for the system, as well as important MSRE safety system actions.…”

“…Based on the MSRE's double barrier safety approach, the authors of the "Molten Salt Reactor Experiment Preliminary Hazards Report" [6] and the "MSRE Design and Operations Report Part V: Reactor Safety Analysis Report" [7] did not evaluate accidents in which the second barrier to release was ruptured and a large fraction of the fission product activity escaped. Since these types of accidents would require two independent failures as well as a mechanism to transport this activity offsite, such accidents are deemed to be incredible.…”

“…The preliminary hazards report discusses hazards that posed a threat to the first barrier for the fuel salt, including (1) melting of pipe/component walls due to reactivity excursion or decay heat, (2) failure from excessive stresses such as thermal or pressure, and (3) failure due to corrosion. Some IEs that were assumed to be capable of producing these hazards and that could possibly lead to a failure of this first barrier if unmitigated include [6]:…”

“…The hazards report discussed six conditions that presented the risk of a breach of the first barrier and thus could have posed a threat of a release through the second barrier, a seal-welded containment described in more detail in Section 3.2 of this report. 3 A maximum credible accident is evaluated in the preliminary hazards report [6] and the design and operations report [7]. This accident consists of the release of fuel salt accompanied by the in-leakage of enough water to generate steam without consequent cooling.…”

“…The plant was also provided with a simple processing facility for treating batches of fuel salt with (1) hydrogen fluoride to remove oxide contamination and (2) fluorine to remove uranium. ORNL's MSRE designers minimized the risk of a release of radioactive material from the MSRE by providing at least two independent barriers for each component that contained or could contain multicurie amounts of radioactive material [6]. This report describes how existing MSRE hazard reports and safety analyses were used to investigate how the early steps of the process for selecting LBEs would be performed for an advanced non-LWR reactor design.…”