加速器駆動未臨界システムの安全性検討;異常事象の検討と事故事象の安全解析

Investigation of safety for accelerator driven system; Investigation for abnormal event and safety analysis for accident event

菅原 隆徳  ; 西原 健司  ; 辻本 和文 ; 倉田 有司; 大井川 宏之 

Sugawara, Takanori; Nishihara, Kenji; Tsujimoto, Kazufumi; Kurata, Yuji; Oigawa, Hiroyuki

加速器駆動未臨界システム(ADS)は、未臨界状態で外部中性子源により運転されることから、一般的な臨界炉に比べて安全性が高いとされている。本検討では、ADSで起こりうる異常な事象を系統的に整理し、炉心損傷の可能性が考えられる事象について詳細な安全解析を実施することで、ADSが炉心損傷事故の可能性を包含していないかどうか検討した。レベル1 PSAの結果に基づいて異常な事象を整理し、その結果を踏まえて安全解析を行った結果、対象としたすべての事象で炉心損傷は起こらず、再臨界事故も起こらない結果が得られた。基準外事象においては、クリープ破断による炉心損傷の可能性が考えられるものの、その発生頻度は極めて低く、対象としたADSは炉心損傷及び損傷に伴う再臨界の可能性が非常に低いシステムであるといえる。

It is supposed that the Accelerator Driven System (ADS) is safer than other critical systems such as Fast Reactors since the ADS is driven by the external neutron source in the subcritical state. In this study, the investigation to confirm the possibility of Core Disruptive Accident (CDA) for the ADS was performed by Level 1 PSA and detailed safety analyses. Abnormal events for the ADS were sorted by Level 1 PSA and calculation cases for the safety analysis were discussed. Based on these results, safety analyses for Unprotected Transient Over Power (UTOP) and Unprotected Loss of Flow (ULOF) were carried out. These results showed that there were hardly possibilities of CDA for the ADS. In the beyond design basis accidents (UTOP and ULOF), it was considered that the creep rupture of the clad tube would happen. However, the frequency of the accidents was exceedingly small. It was concluded that the ADS had very little possibility of CDA or the re-criticality accident.