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

The Department of Energy’s (DOE’s) Light Water Reactor Sustainability (LWRS) Program is a five year effort that works to develop the fundamental scientific basis to understand, predict, and measure changes in materials and systems, structure, and components as they age in environments associated with continued long-term operation of existing commercial nuclear power reactors. This year, the Materials Aging and Degradation (MAaD) Pathway of this program has placed emphasis on emerging nondestructive evaluation (NDE) methods that support these objectives. DOE-funded research and development (R&D) on emerging NDE techniques to support commercial nuclear reactor sustainability is expected to begin next year. This summer, the MAaD Pathway invited subject matter experts to participate in a series of workshops that developed the basis for the research plan of these DOE R&D NDE activities. This document presents the results of one of these workshops, the DOE LWRS NDE R&D Roadmap for Reactor Pressure Vessels (RPVs). These workshops made a substantial effort to coordinate the DOE NDE R&D with that already underway or planned by the Electric Power Research Institute (EPRI) and the Nuclear Regulatory Commission (NRC) through their representation at these workshops.

This series of workshops was held in the Oak Ridge, Tennessee, area during the week of July 30 through August 2, 2012. The workshops addressed four areas of NDE interest: (1) Cable Flaw Detection (Monday, July 30); (2) Concrete Aging Monitoring (Tuesday, July 31); (3) Reactor Pressure Vessel Monitoring (Wednesday, August 1); and (4) Fatigue Piping Assessment (Thursday, August 2). The Cable Flaw Detection workshop was held at the Analysis and Measurement Services (AMS) Corporation’s AMS Technology Center at 9119 Cross Park Drive in Knoxville, Tennessee. The other three workshops were held at the Oak Ridge National Laboratory (ORNL) Conference Center at 1 Bethel Valley Road, Oak Ridge, Tennessee.

The purpose of the August 1 workshop was to develop content for this Roadmap for Nondestructive Evaluation of Reactor Pressure Vessel Research and Development by the Light Water Reactor Sustainability Program. The focus was on technical gaps in NDE techniques for detecting embrittlement and weld cracking in reactor pressure vessels. The workshop was attended by 25 people: two representatives from the Nuclear Regulatory Commission (NRC); one representative from the Electric Power Research Institute (EPRI); four representatives from universities (Georgia Tech, Northwestern, and Iowa State); three representatives from industry (Southwest Research Institute, Zetec, and WesDyne); and 15 representatives from national laboratories [ORNL, Pacific Northwest National Laboratory (PNNL), Argonne National Laboratory (ANL), and Idaho National Laboratory (INL)].

The development of a DOE vision for RPV NDE R&D is important because experimental evidence from the LWRS MAaD Pathway indicates that the currently utilized models for RPV lifetime may not be conservative at high fluence. Development of one or more NDE techniques that can assist in the determination of current RPV fracture toughness as well as in prediction of fracture toughness with further aging of the vessel (particularly in the presence of microcracks or other stress concentrators) is essential. The NDE measurements and the corresponding models that can verify their applicability to the problem, sensitivity to embrittlement and microcracking, and accuracy in characterizing physical properties of RPV steel to establish correlations with RPV fracture toughness will provide important information to the LWRS program.

The major emphasis of this workshop discussed the feasibility of using NDE techniques to determine embrittlement of commercial nuclear RPVs. Minor emphasis was placed on emerging NDE techniques that provide better insight into cracking and especially incipient cracking of RPVs.

Three important NDE research areas relative to RPVs were identified during the workshop: 1. NDE measurements towards RPVs embrittlement determination must measure steel properties that can be correlated with the steel’s crack propagation during reactor accident conditions. 2. A comprehensive and well-characterized set of irradiated RPV steel samples needs to be assembled to support NDE technique research. 3. NDE techniques utilized towards RPV embrittlement determination must provide information on embrittlement throughout the thickness of the RPV.

Attempts to use NDE techniques to detect RPV embrittlement have been ongoing since the 1960s without significant success. The traditional approach to determining NDE applicability to detection of embrittlement in steels has been mainly experimental, with measurements on samples with varying degrees of embrittlement to see if there is any correlation to the fracture toughness of the steel. In many cases, these measurements have been made on surrogate specimens (i.e., specimens with varying hardness levels but not necessarily the same sort of microstructure that is indicative of irradiation embrittlement). While this type of research is essential (and still necessary), the DOE vision for NDE research on RPVs needs to expand upon this experimental work to include modeling efforts that assist in first principle understanding of NDE measurements versus changes in steel physical properties, as well as approaches to determine the correlation of that measurement with any crack propagation during accident conditions.

The fracture toughness of steel depends upon several factors, especially lattice defects such as vacancies, dissolved atoms, dislocation loops, solute clusters, precipitates, dislocation, and grain and phase boundaries. Major obstacles to the use of NDE for embrittlement quantification have been (1) the length scales of the features of interest (several nm) relative to the gauge length of typical NDE methods (100s of um and up), and (2) the sensitivity of NDE techniques to multiple factors. The result is the inability (with high confidence), with a single measurement, to distinguish between multiple factors and correlate the measurement to the fracture toughness.

The vision for DOE work in this area centers around efforts to understand how embrittlement (or the presence of microcracking, especially in the vicinity of fabrication flaws in welds) impacts the physical properties of the steel that NDE techniques are capable of measuring, the sensitivity of these NDE techniques, and how those measurements can then be related to the fracture toughness of the steel (or to other related properties). Research into NDE for RPV steels under the LWRS program needs to focus on addressing these gaps.

A consistent theme from all the LWRS NDE workshops and especially from the RPV workshop was that a comprehensive and fully characterized common set of samples for NDE experimentation needs to be assembled. The required RPV sample set is somewhat unique in that it needs to (1) be irradiated over a range of fluence using the appropriate flux and temperature conditions, (2) represent a number of base material and impurity compositions, and (3) be able to allow verification of NDE techniques from 1⁄2-Charpy-sized samples to those representative of full RPV thicknesses. To achieve the DOE vision for NDE research on RPVs, this sample set must be assembled. Statistical design of experimental methods may be applied to reduce the number of specimens that may be necessary for comprehensive evaluation of NDE methods.

The workshop concluded that some emerging NDE techniques appear to show promise for detecting and characterizing microstructural changes in RPV steels. These techniques include nonlinear ultrasonic, micromagnetic measurements and Seebeck-effect-based techniques. These techniques may offer a method for nondestructive characterization of RPV steel which can then possibly be correlated to its fracture toughness.