Molten Salt Reactor Fundamental Safety Function PIRT

Abstract

The phenomenon identification and ranking table (PIRT) process was employed to evaluate the current capability to assess the ability of liquid-salt-fueled molten salt reactors to achieve their fundamental safety functions (FSFs). The PIRT process provides a structured mechanism to elicit and document expert opinions on the most important phenomena and the corresponding level of knowledge with regard to achieving the FSFs. The PIRT panel included reactor developers, accident progression evaluation tool developers, US Department of Energy (DOE) national laboratory technical staff, university researchers, and US Nuclear Regulatory Commission (NRC) staff. The information-gathering process concentrated on phenomena related to accidents in which the fuel salt-including any in the cover gas-has been released from the first barrier layer. This information-gathering process included identifying all potential accidents that could result in the release of a substantial amount of radionuclides that could breach or bypass the first barrier. The PIRT did not identify any previously unrecognized systemic vulnerabilities. However, the elicitation process identified several areas with low levels of knowledge and significant potential impact on accident progression. These areas either require additional research to improve the state of knowledge or additional design conservatism to accommodate the remaining unknowns.

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

“…A Phenomena Identification and Ranking Table (PIRT) evaluation for MSRs was recently conducted to rank the accident scenario phenomena by their impact on achieving the fundamental safety functions and to identify the corresponding knowledge gaps [104]. The goal of the PIRT is to prioritize the identified knowledge gaps so that research is targeted to support MSR licensing efforts.…”

“…Mechanical processes that form aerosols are largely omitted from nuclear accident analyses for LWRs because they tend to not produce the fine particles that are of concern for aerosol transport and radionuclide release [6]. However, the mechanical formation of molten salt aerosols is not well understood and merits experimental investigation to determine its significance in aerosol production for MSRs [104]. Experiments that test each possible mechanical aerosol formation mechanism are suggested to determine their relative importance.…”

“…The bubbles may be formed from noble gas fission products present in the fuel salt or from gas that has been entrained in spilled or agitated molten salt. The formation of aerosols from molten salt bubble bursting is not well studied [107] and received a high priority ranking pertaining to the release of radionuclides in the recent MSR campaign PIRT [104]. The rupture of molten salt surface bubbles can be simulated experimentally by bubbling a hot non-condensable gas into a static pool of molten salt.…”

“…This aerosol formation mechanism was demonstrated in early MSRE tests in which mixing a small amount of fuel salt with water resulted in droplet formation and dispersal all over the container [108]. Understanding aerosol formation due to energetic interactions is currently a low priority because this type of event is considering highly unlikely for the MSR designs that have been proposed to date [104] A limited number of experiments have been conducted to study aerosol formation by vapor nucleation. In one test, air was drawn by suction over a molten salt surface in a furnace, and aerosol particles were formed by the condensation and nucleation of vapors in the cool air stream [109].…”

“…• Effects of irradiated salts: A comparison of results from hot cell volatilization tests and follow up tests with non-irradiated surrogate fuel salt indicate that radioactivity is responsible for aerosol formation [41]. One hypothesis is that beta-decay that occurs near the molten salt surface can free and project a molten salt droplet from the salt surface due to the recoiled ion momentum [104]. During the MSRE hot cell mist experiments described in ORNL-4254 (pages 107-114), the smallest observed droplets (35 -180 Å) could be due to a beta recoil effect but are also in the size range expected for bubble bursting.…”