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Conference Article: Philosophy of leak before break assessment for Japanese prototype FBR

Wakai, T.; Aoto, K.; Ichimiya, M. (Japan Nuclear Cycle Development Institute (JNC), O-arai Engineering Center, Ibaraki (Japan)); Ito, K. (Japan Nuclear Cycle Development Institute (JNC), Monju Construction Office, Ibaraki (Japan))

Abstract

This paper outlines the concept of the leak before break (LBB) assessment applied to the primary coolant pipe of the Japanese prototype liquid metal cooled fast breeder reactor 'MONJU'. Reviewing possible failure modes for the primary coolant pipe made of an austenitic stainless steel, it could be concluded that the fatigue (creep-fatigue, for the hot leg piping) failure is most likely mode of pipe failure. Although through wall-penetration would not be expected to occur under design duty loading conditions, the maximum leak path area produced by fatigue crack growth was evaluated for the elbow of the reactor inlet down comer piping to consider a postulated sodium leak accident. In the estimation, an initial flaw was assumed at the beginning of service and fatigue crack growth from the initial flaw was simulated using a fracture mechanics technique up to a through-wall-penetration. The eventual crack length at through-wall-penetration was obtained under conditions beyond design duty cycles. Crack opening displacement for the penetrated crack in the pipe subjected to design duty internal pressure was also evaluated, which led to the postulated maximum leak path area. The sodium leak rate from a circular opening equivalent to the postulated maximum leak path area was evaluated and was much larger than detectable one by the leak detection system employed in MONJU. It was shown that there was much room between the detectable crack size and the size corresponding to the unstable fast fracture under design duty loading conditions and that LBB was ensured. (author)

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key words: crack propagation; failures; fracture mechanics; leak detectors; leaks; monju reactor; pipes; primary coolant circuits; sodium; breeder reactors;epithermal reactors; fast reactors; fbr type reactors; liquid metal cooled reactors; lmfbr type reactors; power reactors; sodium cooled reactors;
Reference:
Proceedings of a technical meeting held in Kalpakkam, India 13-17 January 2003
International Atomic Energy Agency, Vienna (Austria)
IAEA-TECDOC--1406, pp:25-33