Development of in-service inspection rules for liquid-metal cooled reactors using the system based code concept

システム化規格概念を用いた液体金属炉用供用期間中検査規定の開発

高屋 茂  ; 浅山 泰 ; 矢田 浩基 ; Roberts, A. T.*; Schaaf, F.*

Takaya, Shigeru; Asayama, Tai; Yada, Hiroki; Roberts, A. T.*; Schaaf, F.*

液体金属炉の供用期間中検査に関する要求は、ASME規格Section XI, Division 3に定められているものの、一部規定については準備中の状態である。1980年の初版発刊以降、大きな改訂は行われてこなかったが、今回、新しく事例規格(Code Case) N-875が発刊された。本事例規格は、目標信頼性に基づく規格・基準類の合理化を目指すシステム化規格概念を活用して開発されたもので、Section XI, Division 3に対する代替検査要求を定めている。本論文では、Code Case N-875の概要を説明するとともに、高速増殖原型炉「もんじゅ」の一次主冷却系配管を対象とした例題検討を通して、Code Case N-875の具体的な評価手順についても解説する。

Inservice inspection rules for liquid-metal cooled plants were historically provided by Section XI, Division 3 of the ASME Boiler and Pressure Vessel Code. However, some parts of the Code remained as being in the course of preparation. Although no major revisions were made to Division 3 since the first issue in 1980, a newly developed and published Code Case N-875, now provides alternative examinations to the methods previously contained in Division 3. The Code Case was developed using the System Based Code concept pursuing rationalization of codes and standards based on reliability targets throughout a plant's service life. In this paper, an overview of the Code Case is presented. The technical foundation to establish the applicability of these alternative examinations as delineated in the Code Case, consists of Stage I and II evaluations with compensating individual considerations. Stage I is a structural integrity evaluation without the contribution of inservice inspections, while Stage II is evaluation of the detectability of a postulated flaw. Not only conventional direct detection methods, but also indirect detection methods are permitted to be employed through the Stage II evaluation. Furthermore, the detailed evaluation procedures are illustrated through the application of the Code Case's evaluation criteria to the primary heat transport piping system of a prototype sodium-cooled fast breeder reactor in Japan, specifically Monju.