New TACIS regional projects on radiation embrittlement and integrity assessment of WWER reactor pressure vessels

Abstract

Neutron embrittlement of the reactor pressure vessel (RPV) core weld of the VVER 440 type of NPP was recognised as a serious problem in the early 1980s, when the first surveillance specimens were tested. A lot of measures were taken in order to ensure the integrity and acceptable safety margins of the RPV in possible pressurised thermal shock transients. In VVER 1000 plants with high Ni content in the RPV core welds, a similar RPV embrittlement problem was also observed, but not so severe since the integrated neutron fluence is significantly smaller than in the VVER 440 plant. Since 1991, the European Commission has financed a significant number of Projects and Shared-Cost Actions in this area. Taking into account, the results of the latest ones as a basis for the development of the necessary material data to be used for upgraded RPV integrity assessment, two new TACIS projects are being launched, jointly with Russia and Ukraine. This paper will present the objective and the scope of these projects, which are basically concentrating on: • Upgrading the surveillance databases by upgrading the neutron dose measurements, • Acquisition of new impact test and toughness results on reconstituted surveillance specimens, including the evaluation of the ‘Master Curve Approach’, • Further validation tests of the shape of the fracture toughness curve and the base and weld metal and characterisation of the cladding, • Preparation of some selected upgraded RPV integrity assessments, with insights on the latest approved methodology.