RPV integrity reassessment

Two TACIS regional projects have commenced, which shall be considered twin projects, in the frame of the most recent programme launched by the European Commission on this particularly sensitive safety issue. They will be implemented simultaneously in the Russian Federation and Ukraine and have to be developed in very close cooperation, since the results of the second project shall be integrated in the final assessment done in the frame of the first project.

The first project has the aim to generate the conditions for an extensive understanding of the situation regarding the RPV integrity assessment, with a

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179 particular concern about materials embrittlement aspects. This project includes the validation of the global programme on the basis of a consistent state of the art evaluation of current knowledge, including a comprehensive identification of the most critical and urgent remaining open safety issues. This task has been carried out within an international expert group (Senior Advisory Group), specifically set up for the purpose. Furthermore, this project defines the conditions for improving the results of the VVER 1000 and 440/213 RPV surveillance programmes, the corresponding experimental programme being implemented in the twin project and made available later. The evaluation of these results and their consistency with others shall be performed with the aim to conclude on specific aspects, such as validation or re-assessment of the neutron embrittlement prediction laws, the ‘quality’ of the surveillance programmes, further assessment of spectrum and flux effects on neutron embrittlement, and the direct measurement of fracture toughness in comparison with the application of the codified Charpy V/Ic K correlation.

This project also includes the preparation of the technical syntheses needed for performing End of Life RPV integrity assessments, aiming to assess, at least, the most sensitive cases among all. The JRC is the main contractor for this project. A Russian consortium involving the RRC Kurchatov Institute, CRISM Prometey and EDO Gidropress, as well as a Ukrainian one involving the Institutes for Problems of 4/7 Strength and Nuclear Research of the National Academy of Science are involved as local subcontractors.

The second project shall be seen as an experimental ‘support project’. It includes the performance of in-depth analyses, as well as complementary inves-tigations and tests, which are being considered as necessary for upgrading the available surveillance results. A significant number of reconstituted standard and pre-cracked Charpy V surveillance specimens will be prepared according to the needs defined in the first project. The impact tests and the fracture toughness measurements, according to the ‘Master curve’ approach are also being performed in that frame. Specific consideration is given for the imple-mentation of the specimen reconstitution technique in Ukraine and the qualifi-cation of Ukrainian specialists for the corresponding techniques. Further tests for underpinning advanced methods for the evaluation of the fracture toughness are also proposed. They are partly dedicated to further validation of the ‘local approach’, but they also provide for the complementary assessment of the shape of the temperature dependent fracture toughness curve. No additional reference irradiation is proposed at that stage of programming, since it has been considered more efficient to rely on upgraded surveillance results.

The detailed programme will not be in force until the Senior Advisory Group will have agreed. The results of this project will later be included in the final

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stage of the first project. A tender is foreseen for that project, which is intended to identify the most appropriate industrial Western main contractor.

Conclusions on the evidence of acceptable safety margins and the expected remaining reactor lifetime shall be given to the operators. Past experience on the implementation of mitigating measures (thermal annealing of the RPV core zone, heating of the emergency core cooling system water storage tank) shall be taken into account for identifying the relevant mitigation measures to be evaluated. The possible benefit which could be expected from implementation of optimized in-service inspection programmes and/or the impact of further R&D results, should be carefully identified. Recommenda-tions for immediate or later implementation of mitigation measures will be given to the utilities. WWER 1000 RPVs, with particular insights on those having a high nickel content for the core weld, and some sensitive WWER 440/

213 RPVs are of most concern.

3.2. ‘Leak before break’ project

The JRC is ensuring for AIDCO the technical follow-up of TACIS projects. As an example, for the project ‘leak before break’ (LBB) applicability review and basic implementation for WWER-1000/320, the JRC has evaluated the experimental testing programme performed by the Russian institutes ZNIITMASH, ZKTI and Izorsky factories. LBB calculations performed by the consortium (Ansaldo, Empresarios Agrupados and EDO Gidropress) were also assessed, as well as the in-service inspection programme and leak detection system at Balakovo NPP Unit 2.

In this LBB project, plant-specific data on material properties and manufacturing procedures of the main coolant piping and surge line were collected and ‘as-built’ layout drawings for the selected reference Balakovo NPP Unit 2 was elaborated. Using these data as input, the first LBB assessment was carried out by the local subcontractor following current Russian guidelines.

Then a second LBB assessment was carried out using both Russian and Western fracture mechanics approaches. A materials testing programme was formulated and implemented in order to elaborate more specific material parameters for advanced fracture mechanical LBB studies. Finally, a third LBB assessment was performed by using upgraded material data collection, as well as the more advanced material parameters (fracture toughness) obtained in the completed materials testing programme. Simultaneously, a review and evaluation of the existing leak detection system and in-service inspection systems was carried out at Balakovo NPP Unit 2 and recommendations were made for upgrading the systems accordingly.

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181 It was shown that LBB criteria could be met at Balakovo Unit 2, providing that an adequate leak detection system was installed and the periodic in-service inspection will be upgraded. The LDS should be able to detect and localize a leak of 1.9 l/min in the surge line and 3.8 l/min in the main coolant piping within one hour after occurrence. The leak detection system should be based on, at least, three detection methods based on different principles (acoustic, humidity and radiation). An acceptable leak detection system system has already been planned and created for new WWER 1000 plants under construction, but it needs to be qualified and benchmarked before approval and implementation. Furthermore, the periodic in-service inspection programme must be completed by an automatic inspection method in addition to the manual method presently used. For the surge line, the scope of inspection must also be increased to cover 100% of the piping welds instead of presently 5/7 50%. As a main result and deliverable of this project, the following reference methodology report was elaborated: Description of the Reference Methodology for the LBB Assessment of Main Coolant Piping, Summary of Application to Balakovo NPP Unit 2 and Recommendations for Implementation at Other WWER 1000 NPPs. This report could serve as a handbook for conducting LBB assessments at all WWER 1000 as well as other PWR plants.