Safety performance of the 4S reactor on the ATWS events – statistical estimation of uncertainty

Abstract

4S reactor is a sodium-cooled fast reactor developed as a small-decentralized power supply. The name of “4S” in this reactor stands for Super-Safe, Small and Simple, and they show representative features of the reactor. The purpose of the present work is to evaluate quantitatively the super-safety of 4S reactor, and the safety performance is analyzed with ARGO-3, which is a plant dynamics code of a sodium-cooled fast reactor. In this evaluation, some events, such as Unprotected Loss of Flow (ULOF) and Unprotected Transient Overpower (UTOP), are selected as typical cases from various transients and accidents. After metrics concerned with safety design is defined for each event, it is evaluated with statistical methods whether each metric satisfies acceptance criteria in a given criteria level. Result about ULOF is as follows. The coolant temperature in the nominal hottest assembly outlet, “ T c” is selected as metric, and the upper side value of 95% confidential section in T c is below 900 °C that is acceptance criteria. Also in UTOP, it is shown that the fuel maximum temperature in the nominal hottest assembly, “ T f” satisfies acceptance criteria. This result shows that 4S reactor has margin for safety acceptance criteria.