[en] This paper outlines the concept of the leak before break (LBB) assessment applied for 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 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 fracture mechanics technique up to a through-wall-penetration. The eventual crack length at a 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. Sodium leak rate from a circular opening equivalent to the postulated maximum leak path area was evaluated and that 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)