Insights in the safety analysis of an early microreactor design

Abstract

A safety analysis of a transportable, plug-and-play, heat-pipe-cooled microreactor – or Nuclear Battery (NB)– designed at MIT is presented. The considered design is a semi-autonomous 5 MW (thermal) high-temperature heat-pipe-cooled, yttrium-hydride moderated NB envisioned to be a transportable, flexible, affordable, and distributed low-carbon energy source.The analysis makes use of a recently proposed methodology that integrates i) System-Theoretic Accident Model and Processes (STAMP) to guide a qualitative exploration of the NB threats and hazards, ii) Modeling and Simulation (M&S) to investigate the NB dynamic behavior during accident scenarios, and iii) Goal-Tree Success-Tree Master Logic Diagram (GTST-MLD) to assess risk quantitatively. The methodology has been shown to provide systematic risk insights without the need to rely on prior operating experience, and enables a dynamic evaluation of the risk profile. Moreover, it identifies unknown threats and hazards relevant to the novel microreactor design and explores the related accident scenarios. The key findings of the safety analysis performed on the nuclear microreactor considered are the identification of vulnerabilities related to the use of yttrium hydride as the moderator, the assessment of threats and accident scenarios associated with the loss of sodium from the heat pipes, and, for such scenarios, the quantification of the grace time, i.e., the period during which human operators can intervene to avoid unacceptable consequences.