PURPOSE OF PERIODIC ISI AND LEAK RATE TESTING
Recognizing the importance of containments, GDCs set up basic requirements for inspecting and testing of containments. However, the detailed requirements have to depend upon the actual experience with the containment structures and containment pressure boundary components. Containment is a vital engineered safety feature of a nuclear power plant. In normal operating conditions, the containment is subjected to operating and environmental stressors, such as ambient pressure fluctuations, temperature variations, earthquakes, ice, and windstorms. In some containment designs, the principal leak-tight barrier is surrounded by another structure, such as a shield wall or a shield building. These structures protect the containment from some of the external influences, and in some cases, collect leakage from the primary containment for processing prior to release to the atmosphere. The mechanical stresses and strains generated by transients under normal conditions and the effects of high-probability (>10-2) external influences are a small fraction of the limiting conditions for which the containment is designed. However, the fatigue life of the containment can be affected by the significant number of cycles of such low-stress transients. The containment is also subjected to various types of internal degradation (aging degradation), depending on the inherent characteristics of the materials, the fabrication processes, and the construction methods. The rate and extent of the degradation are influenced by the sustained environmental conditions such as temperature, humidity, water leakage, and borated water spills.
Thus, the reliability (or the success probability) of a containment to perform its intended function under design basis conditions as well as under higher loads due to severe accidents and beyond design basis earthquakes is influenced by the containment’s inherent capability and the various stresses and degradation mechanisms that act on it. The basic concept in ensuring the reliability of the containment is to track the degradation of the containment components through periodic inspections and check the leak-tight integrity of the containment pressure-retaining components through periodic leak rate testing.