As number of aged nuclear power plants and their service years are growing, the importance of proactive consideration about potential degradation modes of reactor materials after long term operation is also growing more than ever.One possible materials degradation mechanism we want to pay attention is long term thermal ageing of alloys at LWR coolant temperatures around 300C.While massive diffusion of either alloying elements or impurities is not possible in such a temperature range, ageing degradation caused by compositional separation at very short distance or by mono-layer segregation along fast diffusion paths (high energy boundaries)might be possible even at the coolant temperatures if service time is long enough and activation energy of the phenomenon is low enough.