The project "VVER Cladded Reactor Pressure Vessel Integrity Evaluation with Respect to Pressurised Thermal Shock Events" (Project) was funded by the European Union as part of a Horizontal Programme for Community support in the field of Nuclear Safety for 2002 for the Czech Republic.
 
One of the main activities of the Project was the performance of the large-scale specimen tests with specimens which consisted of both the base metal and cladding materials. This type of test configuration was considered necessary to determine the fracture behaviour of cracks po­tentially occurring in the RPV with cladding. After these tests, an extensive finite element simulation programme was accomplished for modelling numerically the fracture behaviour of the specimens during loading. A hypothesis on the failure mode of the large-scale specimens and the potential failure modes of the RPV during a PTS event as well as a proposal of critical failure parameters and failure criteria were outlined from the results of the tests and their analyses.
The Project was performed in ten technical tasks as follows:
 
Task 2: Experimental determination of the physical properties of the cladding materials.
Task 4: Application of non-destructive test methods.
Task 6: Experimental verification and calibration of "Automated Ball Indentation Method - ABIT".
Task 8: Preparation of failure criteria for PTS calculations.
Task 10: Evaluation of the cladding role in reactor pressure vessel lifetime.
The results of the analyses indicate that postulating an under-clad crack brings significant benefits for the RPV integrity assessment expressed in maximum allowable critical temperature of brittleness (TKa) which can contribute to possible RPV lifetime extension. This benefit is higher for VVER 440 RPVs than for VVER 1000 RPVs, which is possibly due to thicker cladding and smaller postulated crack. The benefit can be also affected by the type of the analysed transient.
 
One of the main conclusions of this project is that the cladding integrity assessment procedure defined in Task 9 [15] which is based on the J-integral evaluation (crack growth initiation toughness) can be integrated into the RPV integrity assessment regarding PTS events. It is also found that the crack driving force in PTS scenarios is expectedly lower than the toughness of the cladding in the end-of-life condition (Task 1).
Project Aims
 

• To prepare and validate procedures for integrity evaluation of reactor pressure vessels with the presence of austenitic cladding with/without defects found during in-service inspections. These procedures had to include a realistic behaviour of austenitic cladding of reactor pressure vessels especially during Pressurised Thermal Shock events (PTS). These regimes were found to represent the most severe ones in reactors operation and thus they were considered as leading ones in vessel integrity and lifetime assessment;
• To implement second general recommendation of type I of the Council Report on Nuclear Safety in the Context of the Enlargement, ensuring the implementation of good safety practices within EU;
• More specifically to harmonise RPV integrity evaluation with requirements of PWR and IAEA codes and rules taking into account specifics of VVER cladded RPVs.

Project Results
Three main tasks were considered within the project:

• Validation of the procedure with experimental results from large scale specimens with cladding and different type defects. An unique archive material from a decommissioned RPV was used for these tests;
• Incorporation of a method for an assessment of allowability of defects found during in –service inspection, using qualified non-destructive procedures (NDT) and equipments;
• Identification of close interconnection between postulated defect sizes and location with results from NDT qualification.

Finally, experimentally validated procedure for RPV integrity evaluation were to be submitted to the SUJB for approval and to NPPs for the use within Periodic Safety Review.
The budget for this project is: 960 000 euro.
Further Project Results
Further information on the project could be sought from the beneficiary organizations.