Neutron embrittlement of VVER reactor pressure vessels—recent results, open issues and new developments

Decisions regarding the verification of design plant lifetime involve a determination of the component and circuit condition. Neutron embrittlement of reactor pressure vessels (RPV) becomes a crucial consideration for continued safe plant operation. Since 1991, the European Commission (EC) has financed a significant number of projects in this area, in particular through the TACIS and PHARE programmes. In Nuclear Safety support programmes the countries mainly concerned are Russia, Ukraine, Armenia, and Kazakhstan for the TACIS programme, and Bulgaria, Czech Republic, Hungary, Slovak Republic, Lithuania, Romania and Slovenia for the PHARE programme. The corresponding results of these projects on neutron embrittlement have contributed to a better understanding of the available data and their uncertainties. Recent confirmations of the irradiation temperatures of the surveillance specimens in the operating VVER nuclear power plants show that the surveillance specimens can be used further for the validation of the current and the expected neutron embrittlement. Taking into account the latest results as a baseline 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, whose scope is:

• Upgrading the surveillance databases.

• Acquisition of new impact test and toughness results on reconstituted surveillance specimens, including the evaluation of the Mater Curve Approach.

• Further validation tests of the shape of the fracture toughness curve and the base and weld metal and characterization of the cladding.

• Preparation of some selected upgraded RPV integrity assessments, with insights on the latest approved methodology.

The paper summarizes the major conclusions of the recent completed EC projects, reviews the remaining major open issues in the field of reliable determination of fracture toughness properties of the operating VVER RPVs, and details the scope of the new projects.