A Wide-Range Embrittlement Trend Curve for Western Reactor Pressure Vessel Steels

Abstract

Embrittlement trend curves (ETCs) are used to estimate the magnitude of neutron irradiation embrittlement as a function of both exposure (fluence, flux, temperature, …) and composition (copper, nickel, manganese, phosphorus, silicon, …) variables. ETCs provide information needed to assess the structural integrity of operating nuclear reactors, and to determine their suitability for continued safe operation. ETCs may use any of a number of different metrics (e.g., ΔT41J, ΔYS, ΔTo) to quantify the magnitude of embrittlement, and may use data from a number of different sources (e.g., commercial power reactors, material test reactors). Past efforts on ETC development in the United States have used data drawn from domestic licensees. For example, the ETCs in Regulatory Guide 1.99 Revision 2 and 10 CFR 50.61a were based on ΔT41J data from Charpy specimens tested as part of licensee surveillance programs [RG199R2 Regulatory Guide 1.99, “Radiation Embrittlement of Reactor Vessel Materials,” Rev. 2, U.S. Nuclear Regulatory Commission, Washington, DC, May 1988 and 10CFR5061a Title 10, Section 50.61a, of the Code of Federal Regulations, “Alternative Fracture Toughness Requirements for Protection against Pressurized Thermal Shock Events,” promulgated Jan 4, 2010.]. While this approach has addressed past needs well, future needs such as power uprates, license extensions to 60 years and beyond, and the use of low copper materials in new reactors produce future operating conditions for the U.S. reactor fleet that may differ from past experience, suggesting that data from sources other than licensee surveillance programs may be needed. While data for these conditions is currently available, it is mostly quantified using embrittlement metrics other than ΔT41J, and it arises mostly from sources other than the U.S. surveillance program. This paper draws together embrittlement data expressed in terms of ΔT41J and ΔYS from a wide variety of data sources as a first step in examining future embrittlement trends. This evidence supports development of a “wide range” ETC based on a collection of over 2500 data. The paper includes an assessment of how well this ETC models the whole database, as well as significant data subsets. Comparisons presented herein indicate that a single algebraic model, denoted WR-C(5), represents reasonably well both the trends evident in the data overall as well as trends exhibited by the following four data subsets: ΔT30 data from the U.S. surveillance program, ΔT30 data from non-U.S. surveillance programs, ΔT30 data from test reactor irradiations, and ΔYS data from test reactor irradiations. Of particular importance, the WR-C(5) model indicates the existence of trends in high fluence data (Φ > 2−3 × 1019 n/cm2, E > 1 MeV) that are not as apparent in the U.S. surveillance data due to the limited quantity of ΔT30 data measured at high fluence in that dataset. Additionally, WR-C(5) models well the trends in both test and power reactor data despite the fact it has not term to account for flux. While the appropriateness of using data from such a variety of sources to inform a trend curve is debated by the technical community, it is suggested that one appropriate use of the WR-C(5) trend curve may include the design irradiation studies to validate or refute the findings presented herein. Additionally, WR-C(5) could be used, along with other information (e.g., other trend curves, theoretical expectations, plant-specific data, etc.), as one tool to predict irradiation trends pending the availability of confirmatory data in the high fluence régime.

Current understanding of radiation-induced degradation in light water reactor structural materials

“…In this curve, the Cu content for CRPs, and Ni content and temperature for MD are included. Modification of the prediction equations or trend curves is being discussed in other European countries based on an empirical formulation using fluence and chemical factors [134][135][136][137].…”

“…On the other hand, long-term operation requires a reliable embrittlement prediction at fluences much higher than the fluence ranges of current surveillance databases. Thus, there have been discussions on extending the materials' embrittlement databases, using MTR data and constructing an international database [137]. Prediction using MTR data requires careful considerations on high-flux effects.…”

Roadmap for Nondestructive Evaluation of Reactor Pressure Vessel Research and Development by the Light Water Reactor Sustainability Program

“…The TTS implications of LBP and transformation of CRPs to Mn-Ni-Si dominated features are increasing TTS with increasing neutron fluence, results not predicted by models based on surveillance data at relatively low fluences. References [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] were used variously for much of the preceding discussion.…”

In situ and tomographic characterization of damage and dislocation processes in irradiated metallic alloys by transmission electron microscopy

“…[9][10][11] This degradation of mechanical properties has important implications in the monitoring of current nuclear reactors as well as in the development of the next generation of reactor structural materials. 12,13 Advances in both areas require a fundamental understanding between the short-and long-term microstructural changes induced by irradiation damage and their effects on the mechanical properties.…”

“…14,15 The degradation of mechanical properties, however, can continue to evolve over the course of decades and involves interactions spanning multiple grains. 13 As the conditions encountered in application cannot be replicated in a single experiment, the development of accurate simulations is vital. This includes computer simulations as well as experiments capable of simulating the effects experienced by materials in application.…”