Brittle Fracture Resistance of the Reflection Shield of a WWER-1000 Reactorunder Normal Operating and Emergency Conditions

To date, most WWER-1000 nuclear power units in Ukraine have reached the end of their design life (30 years). In view of this, work is under way to extend the service life of critical equipment elements beyond the design life (to up to 60 years). In addition to reactor vessels, these critical elements include vessel internals (VI). One of the key approaches in such works is a predictive assessment of the structural integrity of VI structures using mathematical modeling of physical processes typical for power unit operation under intense radiation exposure of structural material. The service life prediction methodology constantly requires refining and taking into account more factors affecting structural integrity. The paper considers the kinetics of changes in the stress intensity factor for postulated cracks in the reflection shield of WWER-1000 VIs during long-term operation and at the time of reactor emergency during the rupture of primary coolant circuit pipelines with a nominal diameter of 100 to 850 mm in the most dangerous areas in terms of brittle fracture resistance. A significant impact of taking into account the residual stresses formed as a result of welding and heat treatment of the reflection shield during manufacturing on the results of calculating the brittle fracture resistance was revealed, which may affect the conservatism of assessment methods when justifying long-term operation.