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Treatment of long term irradiation embrittlement effects in RPV safety assessment

Final Report Summary - LONGLIFE (Treatment of long term irradiation embrittlement effects in RPV safety assessment)

The irradiation conditions of pressurized water reactors (PWR) and properties of reactor pressure vessel (RPV) steels relevant for long term operation (LTO) are summarized. The known data of the RPV steels and the maximum EOL fluence values provided the basic information for the development of the test matrix used in LONGLIFE.
In total, the test matrix contained 15 different RPV steels (8 weld materials, and 7 base materials) in various irradiation conditions which were systematically investigated in the microstructural examination. The applied microstructural techniques are: atom probe tomography (APT), transmission electron microscopy (TEM), small angle neutron scattering (SANS), positron annihilation spectroscopy (PAS) and Auger electron spectroscopy (AES). Indications of late blooming effects (LBE) were found in some cases, but clear criteria for the occurrence/exclusion in terms of irradiation conditions and chemical composition have still to be developed. The commonly accepted trend, that low flux and low irradiation temperature promotes LBE, is supported. A significant flux effect on the microstructure was observed for Cu-bearing RPV steels. Except for low fluences, this microstructural changes do not give rise to significant flux effects on the mechanical properties. Low-Cu RPV steels irradiated at different fluxes up to the same fluence were not included in LONGLIFE database. Such flux pairs have to be identified in the future in order to extend the conclusions on flux effects. New insight into the effect of the composition of RPV steels, in particular Mn, Ni, Si, Cr and P, on the microstructure was accumulated.
With respect to mechanical properties and testing, the focus was put on LTO relevant issues, namely: (i) saturation of radiation embrittlement for high neutron fluences, (ii) late blooming effects, (iii) change of master curve shape, (iv) flux effect, (v) non-hardening embrittlement and intergranular fracture. A phosphorus effect on hardening was observed in the low-Cu materials. The embrittlement and hardening curves do not show saturation at high fluences. In real welds, the effect of the irradiation fluence can be obscured by the inhomogeneity of the initial structure. Late blooming effects on mechanical properties were found in two materials supported by LBE on the microstructure. Most of the materials did not show an LBE (change of slope in the fluence dependence of hardening or embrittlement).
Knowledge gaps in internationally-accepted procedures for assessing the effects of irradiation embrittlement in RPV steels under LTO conditions have been identified. None of the existing trend curve models describes the LONGLIFE database in its entirety satisfactorily. Recommendations for addressing the issue of trend curve development have been made. Guidelines for monitoring radiation embrittlement during life extension periods were developed. These include for example: re-use of tested surveillance specimens, the use of miniature specimens and the use of advanced fracture toughness approaches as well as withdrawal schemes.
The international training course SOTERIA was held in Seville in 2012, jointly organised with PERFORM60. The LONGLIFE Final Workshop was held in Dresden in January 2014. A double objective was reached: to present and discuss the results of the LONGLIFE project and to identify research needs beyond LONGLIFE. The co-operation with Russia was part of the project. A coordination agreement was signed.