Annealing of a VVER-440 reactor pressure vessel in Loviisa

Loviisa 1 NPS is a VVER-440 unit of model B-213 of Soviet design. It was the 1. VVER-440 plant with a surveillance program to follow the neutron irradiation embrittlement of the RPV. The 1. surveillance test results in 1980 indicated that the embrittlement of the core weld was much faster than expected. Several mitigations were carried out during the years in Loviisa to ensure the integrity of the RPV and safe operation of the plant. The critical circumferential core weld of Loviisa 1 RPV was annealed during the refuelling and maintenance outage in August 1996. The annealing was carried out by Skoda Nuclear Machinery LTD in a joint consortium with Bohunice Nuclear Power Plant. Loviisa 1 was the 1. VVER-440 RPV of model B-213 that was annealed. VVER-440 RPV's that were earlier annealed in Eastern European Countries were of the older model B-230. The annealing parameters were established based on extensive material investigations and stress analysis. The target annealing temperature was 465 - 505 deg C and the heating/cooling rate less than 20 deg C/h. According to the results of the material investigations about 80% recovery could be expected. The re-embrittlement of the core weld is monitored by a new surveillance program. The life time of the RPV will be estimated later based on the results from the new surveillance program. (authors)