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Letter Report on Metallurgical Examination of the High Fluence RPV Specimens From the Ringhals Nuclear Reactors

Regulations which govern the operation of commercial nuclear power plants require conservative margins of fracture toughness, both during normal operation and under accident scenarios. In the irradiated condition, the fracture toughness of the RPV may be severely degraded, with the degree of toughness loss dependent on the radiation sensitivity of the materials. As stated in previous progress reports, the available embrittlement predictive models, e.g. [1], and our present understanding of radiation damage are not fully quantitative, and do not treat all potentially significant variables and issues, particularly considering extension of operation to 80y.

The major issues regarding irradiation effects are discussed in [2, 3] and have also been discussed in previous progress and milestone reports. As noted previously, of the many significant issues discussed, the issue considered to have the most impact on the current regulatory process is that associated with effects of neutron irradiation on RPV steels at high fluence, for long irradiation times, and as affected by neutron flux. A significant issue associated with such predictive capability is that of RPV materials containing relatively high nickel content and the Ringhals reactors discussed in Section 2 of this report contain weld metals with high nickel contents. There are some U.S. RPVs with relatively high nickel content, e.g., the Palisades Nuclear Plant RPV contains a weld with about 1.3 wt% nickel. The primary objective of the LWRSP RPV task is to develop robust predictions of transition temperature shifts (TTS) at high fluence (φt) to at least 1020 n/cm2 (>1 MeV) pertinent to plant operation of some pressurized water reactors (PWR) for 80 full power years. New and existing databases will be combined to support developing physically based models of TTS for high fluence-low flux (φ < 10 11/n/cm2-s) conditions, beyond the existing surveillance database, to neutron fluences of at least 1×1020 n/cm2 (>1 MeV).

The strong synergistic interactions between copper, nickel and manganese are understood at low to intermediate fluence. However, the interactions at higher fluence, in both low and higher copper steels, needs to be established. Similarly the basic role of phosphorous is established. However, potential interactions with copper and phosphorous effects at high fluence have not been quantified. Similarly, Si-Ni and Si-Mn interactions also result in silicon enrichment in CRPs. Ref. [3] discusses this issue in some detail pointing out that, even in the absence of copper, thermodynamic-kinetic models predicted the formation of Mn-Ni phases, although at low nucleation rates compared to that for CRPs, the effect being that relatively high incubation fluences are required for their formation.[4-8] Figure 1.1 from Ref. [3] schematically illustrates in 1.1(a), however, that once nucleated such late blooming Mn-Ni-Si phases (LBP) rapidly grow to large volume fractions, potentially causing severe embrittlement. Moreover, the models also show that small concentrations of copper may act as a catalyst for such LBP nucleation. None of the current models for embrittlement reflect the potential LBP embrittlement contributions, probably in part because they may require critical combinations of higher nickel and fluence and lower temperature and flux that have yet to be extensively encountered in the surveillance database.[3] The existence of LBPs has been confirmed, as illustrated in Figure 1.1(b), showing an atom probe tomography (APT) map of manganese and nickel atom positions. An enlarged view of a Mn-Ni precipitate in a copper-free, 1.6Ni-1.6Mn wt.% model alloy is also shown.[9] Similar observations have been reported by other researchers around the world.

This report provides the status for the Milestone M3LW-12OR0402012 – “Provide letter report on metallurgical examination of the high fluence RPV specimens from the Ringhals nuclear reactors in Sweden.” This milestone is associated with procurement of material, preparation of specimens, microstructural examinations, and analysis of the results relative to current understanding regarding irradiation-induced microstructural evolution in RPV materials.