In recent years, cracks due to stress corrosion cracking (here-after it is called SCC) have been found in many pipes of the primary loop recirculation systems(here-after PLR) of boiling water reactor(here-after BWR).

Their pipes were made of low carbonized stainless steel (SUS316(LC)). Formerly, it was thought that SCC due to sensitizing never took place, if the material was carbonized lower. However, afterwards from investigations of actually used materials and experiments it became clear that if the metal surface became hard, inner-granular SCC sensitivity increased and the crack propagates as inter-granular SCC in the simulated BWR environmental water.

According to the above-mentioned investigations and experiments, the SCC mechanism for low carbonized stainless steel is as follows:

As the first step, inner-granular SCC initiates and propagates to the depth of surface processed zone in the section where stress level is high and cracking sensitivity increases due to hardness increase by surface processing. As the second step, the crack can propagate as inter-granular SCC, depending on stress and chemical conditions at the crack tip.

It is well known that SCC initiation depends on combination of three factors such as material, environment and stress. Accordingly, it is effective to reduce the influence of these three factors in order to prevent SCC.

Repair and preventive techniques of mitigation for materials and residual stress of PLR piping against SCC are mainly as follows:

1. CRC : Corrosion Resistant Cladding
2. Internal Polishing
3. IHSI : Induction Heating Stress Improvement
4. SHT : Solution Heat Treatment
5. HSW : Heat Sink Welding
6. Weld Overlay