Evaluate the state of the art, identify the open issues and validate the scope and the content of the experimental programme to be done in TAREG 2.01/03 for further validation of the neutron embrittlement effects and refined PTS analyses.
Define the conditions for improving the VVER 1000 and 440/213 RPV surveillance results.
Take over the results gained under TAREG 2.01/03, evaluate their consistency and conclude on the adequate prediction and surveillance methods.
Establish the necessary updated integrity assessment in order to get a consistent evaluation of the safety margins and of the impact of the possible mitigation measures on these.
Evaluate the integrity of the RPV of selected VVER NPPs with end of life fluence values and material properties.
Implementation of the project
The EC Contractor for the project was EC-DG JRC/IE. The Sub-contractors were:
- In Russia
- RRCKI, Russian Research Centre Kurchatov Institute, Moscow
- CRISM Prometey, Central Research Institute for Structural Materials, ST Petersburg
- EDO Gidropress, Podolsk
- In Ukraine
- IPS, Institute of problems and Strength, Kiev
- KINR, Kiev Institute for Nuclear Research. Kiev
- The beneficiary of the project was Rosenergoatom in Russia and Energoatom in Ukraine.
The project was closely linked to its "twin project" TAREG 2.01/03, which had the aim to provide test results as input for developing trend curves for neutron embrittlement as well as fracture toughness curves for RPV core line material and cladding. Based on the upgraded embrittlement trend curves and fracture toughness curves integrity analyses was carried out for selected VVER 1000 and 440 RPVs for end of life material conditions.
The project was originally scheduled for 4 years, but due to the late start of the a.m. TAREG twin project the project was extended two times. The budget for the project was 2 M€.
Short Tasks review and results
In Task 2 of the project a Senior Advisory Group (SAG) was established in order to evaluate the common knowledge on VVER RPV materials embrittlement aspects related to integrity assessment, and to review open issues and ensure that urgent needs are properly considered in both TAREG projects. The SAG included a limited number of nominated senior experts from various institutes, companies and organisations from CIS and EC countries, which had been involved in projects dealing with the VVER RPV integrity issue earlier. In connection with the SAG activities each member elaborated a topical report on the neutron embrittlement issue and especially on their position regarding the TAREG project plans.
The Russian and Ukrainian organisations prepared a summary report including background, state of the art and open issues regarding the prediction of the changes in fracture toughness properties due to neutron irradiation during operation of VVER NPPs. The SAG group met two times in the spring 2004; in a workshop and a final meeting at JRC in Petten. The main deliverables of the SAG was the Russian-Ukrainian Summary report and the Synthesis Report elaborated as a result of the activities. The SAG concluded that the content of the TAREG project was well defined and the group gave its full support to complete the project as planned in the TOR and PDS. This task was successful and completed according to schedule.
Tasks 3 and 4 included neutron fluence calculations, selection of SS specimens for reconstitution and testing, and development of trend curves for RPV embrittlement due to neutron irradiation. The main activities of these tasks were the neutron fluence re-calculation with modern tools and methods with 3-D modelling and using more detailed geometry of internals around the SS positions. Furthermore more precise data relating to fuel loading and management of the core was used. The neutron fluence calculations took longer time than expected but due to the above delay of the twin project, this had no influence on the general schedule of this project. Based on the results of the neutron fluence calculations and the availability of tested Surveillance Specimens (SS) the selection of tested SS for reconstitution and retesting was decided. The reconstitution and testing was then carried out in the twin project TAREG 2.01/03.
Totally over 1000 test specimens were selected for these activities. Based on the test results received in the twin project new trend curves for neutron irradiation embrittlement of the RPVs was elaborated according to TOR. For the VVER 1000 RPV embrittlement full consensus was achieved regarding embrittlement trend curves for base- and weld metal. For the base metal the embrittlement due to neutron irradiation is the traditional embrittlement equation with an embrittlement factor Af = 1,45 oC and a fluence factor n = 0,8. For the weld metal the embrittlement factor Af is dependent on the Ni, Mn and Si content. The fluence factor n = 0,8 as for the base metal. For the VVER 1000 RPV thermal embrittlement gives a considerable contribution to the total embrittlement trend curve. For the VVER 440/213 RPV the neutron embrittlement is dependent on the impurity contents Cu and P for both the base metal and the weld seam. For the weld metal the trend curve development resulted in 3 different equations depending on the Cu-content of the core weld material. For the VVER 440/213 RPV case there were some controversial opinions regarding embrittlement rate of the core weld between RRCKI and CRISM Prometey.
In Task 5 the new trend curves developed in Tasks 3 and 4 were used for calculation of the end of life (EOL) fracture toughness properties (Klc) of the RPV core region material (base metal and welds). This task included as the main issue the calculation of the Pressurized Thermal Shock (PTS) incidents for selected pilot NPPs in Russia and Ukraine. The PTS calculations were carried out according to the new Russian PTS standard, which allows for assuming only sub-surface postulated cracks in the core region when analysing RPV integrity. Furthermore the cladding shall be proved sound through in-service inspection. In addition it must be shown by calculations that the toughness of the austenitic cladding is good enough at selected neutron fluence (time of operation).
In this project the following pilot NPPs were selected for the RPV PTS studies: Balakovo 2, Kalinin 1, Khmelnitsky 1 and South Ukraine 2 (VVER 1000) and Kola 3 and Rovno 2 (VVER 440/213). The following initiating events leading to PTS were selected: SBLOCA, MBLOCA, steam line break upstream of MSIV, spurious opening of PRZ safety valve and steam generator collector lift off. The results of the PTS calculations showed the last transient was the most dangerous one for all cases studied. It was shown that the RPV integrity is assured to the end of the design life time (40 years) for all above selected NPPs.
This TAREG 2.01/00 project has been rather complicated to complete mainly due to the connection with its twin TAREG 2.01/03 project. The main reasons to the above mentioned delays were the late arrival of input data from the twin project. Nevertheless, the project was very successful and completed fully according to Contract and TOR and maintained a very high scientific level both in Russia and Ukraine. The co-operation between the twin projects was also very good despite the a.m. delays.