Objective

The objective of the present project was to perform a limited integrity assessment of the VVER 1000 Reactor Pressure Vessel (RPV) taking into account the irradiation embrittlement effect.

The project covered four major areas:

1. Collection of the material data on Russian VVER 1000 RPVs.

2. Evaluation of baseline material properties.

3. Evaluation of the surveillance material data.

4. Limited RPV integrity assessment for one unit.

In addition, some experimental results from test reactor programmes have been analysed to complete the limited surveillance data.

Result

The surveillance test results of the 3 Russian VVER 1000 units, i.e. Balakovo 1, Kalinin 1 and Novovoronezh 5, show that the embrittlement rates of the high-Ni welds (No. 4) of the RPVs could be higher than those specified by the Russian Guide, but supplementary material data for higher neutron doses is needed for assessing the end-of-life material properties.
Besides the low specimen neutron doses, uncertainties associated with the irradiation conditions (neutron dose determination, irradiation temperature) as well as the small number of test results reduce the applicability of the present surveillance test results.

As a general conclusion from the material data and the results of the PTS assessment, showing no or a small margin in respect to the predicted fracture toughness, mitigation measures may be needed for most of the Russian VVER 1000 RPVs against the embrittlement of the beltline weld (No. 4).
According to the preliminary evaluation performed, heating the ECCS water from 20?C to 55?C could be a sufficient measure.
More relevant test data, at least on the high-nickel welds, as well as plant specific integrity assessments are necessary for addressing the need of further mitigation measures against irradiation embrittlement.