China Says It's Closing in on Thorium Nuclear Reactor(spectrum.ieee.org)

  >>4.Operator recovery actions raise the possibility of an influx of deborated water into  the core, which may result in recriticality, return to power, and the potential for  core damage.    Staff Response: In the staff’s SER for Chapter 19, dated July 17, 2020,  (ADAMS Accession No. ML20196L734), the staff identified two post-event  recovery scenarios that could pose a challenge to reactivity control. The first  scenario is recovery from a non-LOCA extended decay heat removal system  (DHRS) cooling condition which can occur following initiating events such as  general transients and losses of off-site power. The second scenario is recovery  from a LOCA ECCS cooling condition which can occur following initiating events  such as breaks in the primary coolant lines. The staff notes that either action to  inject would likely be governed by plant-specific procedures; however, such  procedures are not required at the design certification stage and have not been  developed. The staff evaluated the potential core damage risk for the two  scenarios. For reasons discussed in the SER which are supported by two  papers developed by the Office of Nuclear Regulatory Research (RES) (ADAMS  Accession Nos. ML20191A069 and ML20205L317), the staff concludes that  enough margin exists such that these recovery scenarios are unlikely to lead to  core damage based on the physical effects of fluid mixing, reactivity feedback  mechanisms, and associated time constants. Based on the SER and the two  RES papers, the staff found that there is reasonable assurance that there are no  known significant risk contributors that are unaccounted for and that the identified  risk insights are acceptable to support the uses of probabilistic risk assessment  (PRA) at the design stage.