Abstract
Japan Atomic Energy Agency (JAEA) has been developing the Japan Sodium-cooled Fast Reactor (JSFR) in the Fast reactor Cycle Technology development (FaCT) project from JFY2006. Risk targets were set out as part of the safety-related design requirement: i.e., the quantitative safety design requirements on the Core Damage Frequency (CDF) and the Containment Failure Frequency (CFF). This paper describes a preliminary evaluation of achievement level of JSFR to the risk targets at the FaCT project phase-I: JFY2006 to JFY2010. A Level-1 Probabilistic Safety Assessment (PSA) has been implemented preliminarily to evaluate the CDF related to internal initiators in power operation. In the FaCT phase-I, there was a design modification that increases the frequency of core damage resulting from loss of decay heat removal function. This design modification is a change in a steam generator operation sequence after reactor scram to mitigate thermal stress by a severe thermal transient. After this modification, the steam generator is not utilized for decay heat removal. To compensate this risk increase, the Decay Heat Removal System (DHRS) was enhanced: i.e., non-safety-related blowers were added to the DHRS air coolers. The risk reduction effect of this design enhancement was confirmed by the level-1 PSA. The calculated CDF became lower than the both requirements on CDF and CFF. On one hand, the seismic load was considered as an important external initiator of the core damage. After the Niigata-ken Chuetsu-oki Earthquake in 2007, the postulated seismic load for the JSFR seismic design was reconsidered, and it became severer. In addition, it was decided to develop newly a horizontal seismic isolation system for the JSFR. In the FaCT phase-I, the site location of JSFR has not been determined yet, and so the seismic hazard cannot be quantified. Rather, the seismic margin of the JSFR should be evaluated by conducting a seismic fragility evaluation. So, we conducted the seismic fragility evaluation of principal structures and components in terms of core damage prevention. This evaluation was based on the seismic response analysis, which considered the seismic isolation effect and the hardening effect of the laminated rubber bearing in the seismic isolation system. In this analysis, we assumed the seismic ground motion 1 to 5 times as large as the postulated seismic load. As a result, we confirmed that the principal structures and components of JSFR have sufficient seismic margin. Based on this, we judged the risk targets could be achieved against the seismic event.
