Now, 22% of the world’s nuclear power plants are operating beyond 40 years. By 2020, about 30% of the world’s nuclear fleet will be 40 years old. Long time operation (LTO) of existing plant is a key objective in many countries. We need to understand anticipate and mitigate the degradation in nuclear power plants. IAEA have made the guidelines about long time operation and opened PLIM conference once in 5years.In the United States, industry and NRC/DOE have started research program on the subsequent license renewal to maintain NPPs as high performance electrical source, its program is possible to operate beyond 60 years.