ABSTRACT

Boiling water reactors have internal components manufactured with stainless-steel type 304. Some relevant cracks have been detected through the periodic inspections of their internal components. Regarding the core shroud, cracks have been observed along the circumferential welds. The causes of such cracks are: radiation, vibration, stress corrosion cracking (SCC) and residual stresses, acting alone or simultaneously. Initially, the material was ductile. However, it becomes brittle as hours of operation are accumulated. In the case of this paper, three asymmetric cracks, at the circumferential horizontal weld H7, have been postulated after 25 years of operation. The loading case considered was the combination of an earthquake with a loss of coolant accident (LOCA). The structural integrity was evaluated after 30 and 60 years of operation. In the last case, such evaluation was carried out during an extended period of operation of 30 years. In all cases, a fracture mechanics approach was followed. The results showed that one of such cracks was below the safety factor. In order to get an accurate evaluation, an elastoplastic and limit load analysis is required. This has been discussed in a companion paper.

摘要

沸水反应堆内部部件采用304不锈钢制造。通过定期检查内部部件，发现了一些相关裂纹。关于核心护罩，沿圆周焊缝观察到裂纹。产生此类裂纹的原因有：辐射、振动、应力腐蚀开裂（SCC）和残余应力，单独或同时作用。最初，该材料具有延展性。然而，随着工作时间的累积，它变得脆弱。在本文的案例中，在运行 25 年后，假设在圆周水平焊缝 H7 处出现三个不对称裂纹。所考虑的载荷情况是地震与冷却剂损失事故 (LOCA) 的组合。在运行 30 年和 60 年后对结构完整性进行了评估。在最后一种情况下，此类评估是在 30 年的长期运营期间进行的。在所有情况下，都遵循断裂力学方法。结果表明，其中一处裂缝低于安全系数。为了获得准确的评估，需要进行弹塑性和极限载荷分析。这已在一篇配套论文中讨论过。