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Brittle failure assessment of RPV considering loss of constraint from transient LOCA conditions

: Siegele, D.; Varfolomeyev, I.; Hohe, J.; Hardenacke, V.; Nagel, G.

Société Française d'Énergie Nucléaire -SFEN-, Paris:
Fontevraud 7 - Contribution of materials investigations to improve the safety and performance of LWRs. Proceedings CD-ROM : International Symposium, 26-30 Sep 2010, Avignon, France,
Paris: SFEN, 2011
9 S.
Contribution of materials investigations to improve the safety and performance of LWRs (FONTEVRAUD) <7, 2010, Avignon>
Fraunhofer IWM ()
brittle failure assessment; fracture toughness; crack size; stress strain

The brittle failure safety of reactor pressure vessels (RPV) has to be demonstrated for all load cases. According to the ASME code and the German KTA standard the crack driving force for defined crack postulates has to be compared with the fracture toughness of the RPV material. Depending on crack size and local stress strain condition the material in the component behaves quite different than the material characterized in standard fracture mechanics tests. Due to the lower constraint in the component, the brittle failure assessment based on these standards can be very conservative. For consideration of the constraint conditions in the component the deterministic quantities as the T-stress or the Q-parameter are often applied. But they consider neither the volume of material under high loading nor the out-ofplane stress state ahead of the postulated crack nor the effect of transient loading. These difficulties can be overcome using local approach models based on the weakest link principle in which the failure probability can be described by quantities based on the local stress state and the affected volume. In order to demonstrate the effect of transient loading on the loss of constraint and to validate the local approach model experiments on bend specimens with shallow cracks under transient loading have been performed. These tests show an enormous shift of the master curve reference temperature of -100 °C compared to the reference temperature from C(T) tests. This shift is caused by the shallow crack effect and even more by the effect of the transient loading. For the assessment of different flaw postulates in the leading location of the RPV the loss of constraint was quantified using the classical Beremin model as well as the enhanced model developed at Fraunhofer IWM. For flaw postulates in the ferritic nozzle corner with flaw depths up to 75 mm (quarter of wall thickness acc. to ASME Code) and with the assumption of broken clad a considerable loss of constraint was determined. In addition, for flaw postulates under the intact cladding the loss of constraint is remarkably higher than with cladding postulated as broken. In summary, with the measured material toughness and the significant loss of constraint a considerablemargin against brittle failure can be demonstrated.