50.原子炉圧力容器の照射脆性と亀裂寸法との関係

Abstract

Unstable fracture is discussed for welds in reactor vessel belt-line region for light water reactors up to 1, 000 MWe class.Critical surface crack sizes on welds correspond to reactor operating conditions including a loss of coolant accident are calculated using linear elastic fracture mechanics. The highest neutron fluence level is assumed to be 5×1019n/cm2.The result of calculations shows that the depth of surface crack required to protect against the unstable fracture during design lifetime of reactor pressure vessel, α, is approximately (1.15×10-2) B exp {2.19(α/l)}, where l is length of crack and B weld thickness.Due to neutron attenuation by the vessel wall material itself, the NDT temperature of welds in reactor vessel belt-line region becomes lower from the inner surface to the outer surface. Gradient of radiation induced NDT temperature through the wall is analogous to that of water quenched 12 in. thick A533-B class 1 steel plate.By applying this analogy, the effect of the gradient of radiation-induced toughness on unstable fracture is discussed in relation to the following average NDT temperature, NDTθ.NDTθ=NDT-(FTE-FTEθ)NDT stands for the highest NDT temperature through thickness, FTE stands for the Fracture Transition Elastic temperature of material having uniform NDT temperature with thickness position, and FTEθ stands for the Fracture Transition Elastic temperature of material having uneven NDT temperature gradient with thickness position.