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1.4.1 Susceptibility of Materials to Irradiation Embrittlement

Typically, irradiation embrittlement of the reactor pressure vessel (RPV) is to be considered in the areas near the core where neutron irradiation exceeds 1017 n/cm2 (E > 1 MeV). At lower neutron irradiation levels, irradiation embrittlement can be neglected. This limit is hence included in national regulations, codes and standards, such as CFR (10 CFR Part 50, Appendix G [10CFR50G] and Appendix H [10CFR50H]) or KTA [KTA3201.2] and defines the RPV "beltline" as the welds and forgings or plates surrounding the core suffering significant irradiation embrittlement. In the context of long term operation, more areas of the RPV may exceed the screening limit of 1017 n/cm2 (E > 1 MeV).

The neutron fluence limit of 1017 n/cm2 (E > 1 MeV) reduces the scope of relevant materials to low-alloy RPV steels in light-water reactor (LWR), on which the majority of research is concentrated. Besides, irradiation embrittlement of zirconium alloys used for CANDU pressure tubes is relevant for CANDU reactors. Austenitic stainless steel as RPV cladding is also exposed to neutron irradiation. As the cladding is not considered as structurally relevant part of the reactor coolant pressure boundary, there is much less research focus on irradiation embrittlement of austenitic stainless steels.

1.4.2 Low-Alloy Steels

The RPV is the centrepiece of an LWR and its lifetime determines the lifetime of the nuclear power plant (NPP). The structural material used for RPVs is always a low-alloy steel (LAS). Therefore low-alloy steels are most often subject of irradiation studies to investigate RPV base and weld material properties as a function of irradiation parameters. Focus was laid on WWER-440 and WWER-1000 base and weld materials (e.g., 15Kh2NMFAA, 15Kh2MFA, Sv-10KhMFT), but Western RPV materials (e.g., A533-B class 1, A508Cl3) were also subject of investigations [JRC30141]. In this context, JRQ steel is the International Atomic Energy Authority (IAEA) reference material for pressurized water reactor (PWR) RPV [English2003]. For WWER-1000, a reference material was also manufactured and characterised in a similar way [JRC30613]. A set of model alloys representing PWR RPV steel with systematic variation of chemical composition was manufactured to study fluence effects together with the influence of certain alloying or accompanying elements like copper, phosphorous, or nickel [JRC24564].

Examples of LAS used in NPP construction include 12KhMFA, 12Kh2N2MAA, 15Kh2MFA, 15Kh2N2MAA, 15Kh2NMFAA, 15Cr2NiMoVA, 16MND5, 18MND5, A508Cl3, A533-B, JRQ, JWP, JWQ, Magnox steel, and Sv-10KhMFT.

1.4.3 Austenitic Stainless Steels

Austenitic stainless steel (ASS) is usually used at two relevant points where irradiation embrittlement is of concern: on the one hand as structural material in RPV internals and core internals, on the other hand as RPV cladding. As ASS does not have such a pronounced ductile-to-brittle transition temperature (DBTT), an analysis of the dependence of the DBTT on surrounding parameters is not feasible [IAEA09]. Thus, tensile properties such as yield strength or elongation at rupture are used to monitor the embrittlement [JRC34527] [Keim2012]. In the PRIS project, mechanical property data of the irradiated ASS AISI304(L) and AISI316(L) as a function of neutron dose was collected and generated, respectively [PRIS2004].

Examples of ASS used in NPP construction include Sv-07Cr25Ni13, Sv-8Cr19Ni10Mn2Nb, AISI304, AISI304L, AISI316, and AISI316L.

1.4.4 Zirconium Alloys

Zr alloys are typically used for fuel cladding on the one hand and CANDU pressure tubes [IAEA98] on the other hand. Research projects investigating irradiation embrittlement of Zr alloys, however, could not be identified in FP4, FP5, or FP6 or JRC Direct Actions in the same period. Typical materials would have been, e.g., Zircaloy-4 or Zr-2.5Nb [JRC31980] [Keim2012].

Examples of Zr alloy used in NPP construction include Zircaloy-2, Zircaloy-4, Zr-2.5Nb, and Zr-1Nb.