Direct Measurement of Fracture Toughness for Life Extension

摘要

The Electric Power Research Institute Materials Reliability Project (MRP) is pursuing the resolution of selectedrnexisting and emerging pressurized water reactor materials performance, safety, reliability, operational and regulatory issues.rnA focus of the MRP Reactor Pressure Vessel Imegrity Issue Task Group is to resolve technical issues associated withrnapplication of fracture toughness properties for reactor vessel imegrity assessments. The direct measurement of fracturerntoughness utilizes the Master Curve approach for testing in the brittle-to-ductile transition region. It has been accepted as anrnalternate method for determining a materials reference temperature in the ASME Code (Code Case N-629). However,rnseveral issues have been identified that will require resolution prior to successful application of this technology for reactorrnpressure vessel life attainment and life extension. An objective of the EPRI MRP is to develop a strategy using directrnmeasurement of fracture toughness that is consistent with the current approach based on Charpy V-notch measurements.rnA comparison is provided using both the direct measurement approach and the Charpy based approach for resultsrnfrom two reactor pressure vessel welds. The single edge beam [SE(B)] specimens were tested in three-point bending inrnaccordance with ASTM E 1921-97, "Standard Test Method for Determination of ReferenCe Temperature, To, for Ferritic Steelrnin the Transition range". Testing was performed on specimens obtained from two welds that had been irradiated in a reactorrnvessel surveillance capsule and on specimens in the unirradiated condition.rnAn assessment was performed of the fracture tougtmess transition temperature shifts (i.e., K~c shifts) in comparisonrnto the Charpy transition temperature shifts using the data generated in an earlier evaluation of the same weld materials. (ThernCharpy V-notch data were from specimens irradiated in the same surveillance capsule as the specimens used to determine thernKxcshifts.) The irradiated reference temperature can be computed using three differem approaches: 1) computed directlyrnfrom the irradiated value of To in accordance with ASME Code Case N-629; 2) computed using the initial (unirradiated)rnvalue of To in accordance with ASME Code Case N-629 plus the irradiation induced transition temperature shift measuredrnusing the Charpy impact specimenmeasurements; and 3) computed using the initial (unirradiated) value of To in accordancernwith ASME Code Case N-629 plus the irradiation induced transition temperature shift predicted using empirical correlationsrnbased on the copper and nickel content of the welds and neutron fluence for the surveillance capsule. The first approach wasrnused to illustrate the potential benefits of the direct measurement approach.rnThe results of this evaluation were then used to assess the various strategies imended for maintaining adequaternmargins for cominued vessel operation. The strategies considered were for application of the fracture toughnessrnmeasurements that preserved the conservatisms inherent in the Charpy based approach but credited the increased accuracy ofrnthe direct measurement approach. The benefits realized from application of the strategies based on this approach werernassessed in terms of maintaining vessel imegrity throughout an extended lifetime. Successful demonstration and regulatoryrnacceptance of the direct measuremem methodology will allow owners of operating nuclear power plants to use lower materialrnreference temperatures of the reactor pressure vessel in the evaluation of pressurized thermal shock and in establishing heatuprnand cool-down limits for normal operation. This will result in greater plant operating flexibility and will provide a viablernstrategy for demonstrating adequacy of the reactor pressure vessel for life extension while retaining appropriate margins ofrnsafety.