Toughness behaviour of reactor pressure vessel steels
|Dr. Hans-Werner Viehrig
The strategic importance of permanent assurance of the integrity of nuclear Reactor Pressure Vessels (RPVs) has always been acknowledged. A basic safety criterion is that the RPV steels remain sufficiently tough. The fracture toughness is defined as the material’s resistance to either onset of crack initiaton or crack growth by deforming plastically. Regulations require very low RPV failure probabilities both for normal operation and postulated accident events. Vessel designs and integrity assessment assume the presence of large cracks and rare, but severe, loading conditions, such as pressurized thermal shock. This combination could conceivably result in catastrophic fast fracture if the vessel steel is sufficiently brittle. In nuclear power plants (NPPs), exposure to neutrons degrades the fracture toughness of RPV steels in region surrounding the reactor core. Therefore, this NPP life-time limiting steel embrittlement has to be supervised continuously. Sampling and testing need to resemble the power plants’ loading conditions realistically while at the same time being reasonable in terms of testing efforts at lowest possible specimen demand.
- Determination of quasi-static toughness properties from three-point bending tests
- Determination of dynamic toughness properties from instrumented impact testing
- Characterisation of mixed-mode fracture behaviour in the brittle-ductile transition region (Fig. 1) by Wallin’s Master Curve (MC) concept (Fig. 2) according to ASTM E 1921 at quasistatic and elevated testing velocities
- Re-evaluation of the Master Curve concept for a possible implementation in German regulations and Codes as a replacement for outdated procedures
- MC concept is applicable to extremely irradiated steels of both Western origin (JRQ, JFL) and WWER-type steels
- Routinely determination of fracture toughness properties from slow and impact testing, in all failure regions (brittle, brittle-to-ductile transition region, ductile), on unirradiated and also highly irradiated material
- Reusal of broken specimen halves to machine new one (reconstituted specimens) minimises waste.
- Application of the Master Curve concept in the lifetime extension assessments for German NPPs