By Jeff Hamel, EPRI Program Manager
Although new nuclear plants based on advanced designs such as the AP1000 and EPR are currently being built, there remain opportunities to make design, construction and operational improvements through greater attention to materials management. New nuclear plant development provides a unique opportunity to manage materials degradation throughout the design, construction, and pre-service stages.
Early focus on the materials selected and the fabrication processes used for new plant designs can significantly improve overall plant performance. The Electric Power Research Institute (EPRI) is developing what are called “Materials Management Matrices” for new plant designs to serve this purpose. Using a materials assessment framework previously developed for operating plants, EPRI is working to get ahead of potential materials degradation issues that could impact new plant performance.
Although materials issues for operating and new plants are quite similar, the options to address generic technical issues are much broader for new plants. The opportunity exists to address concerns starting with original materials selection and carrying all the way through optimization of fabrication practices and incorporation of advanced material degradation monitoring features.
Because new plant designs have been effective in incorporating lessons learned from existing plant designs, EPRI’s work primarily emphasizes opportunities for improving long-term resistance to material degradation. The Materials Management Matrices highlight the potential for superior materials performance, optimum component inspectability, and efficient management of material degradation issues throughout the plant design, construction and operation life cycle.
Materials management evaluations focus on the nuclear steam supply system components, including reactor vessel, reactor vessel internals, pressurizer, reactor coolant pumps, steam generators, and ASME Class 1 piping components. The scope is limited to long-lived and passive components. Long-lived components are those that are not periodically replaced and passive components are those that perform their safety function without moving parts or changes in configuration. Examples include vessels, nozzles, penetrations, stationary reactor internals, piping, welds, pump casings, valve bodies, and bolting. Active components such as control rods and drives, pump motors, pump rotating assemblies and internals, and valve stems and internals are not included in the materials management evaluations.
Each materials management assessment consists of three parts:
- 1. Risk priority evaluation
- 2. Industry guidance for managing materials in new plants
- 3. Delineation of “gaps and opportunities” that document actionable activities to improve overall plant performance.
- The risk priority evaluation is conducted with the nuclear steam supply system designer using a simplified failure modes and effects analysis (FMEA) process. The FMEA process evaluates the occurrence, severity, and detection of failure risks for individual components. As applied to advanced reactor designs, these elements are known as likelihood of degradation (LoD), consequences of degradation (CoD), and probability of detection (PoD). The assessment is limited in duration and scope by considering only the most likely and significant degradation mechanisms, such as stress corrosion cracking, fatigue, and corrosion and wear. A team of materials experts is responsible for developing the LoD, CoD, and PoD values based on their technical knowledge, industry experience, and collaborative discussions. A risk priority number, RPN, is then calculated as the product of LoD, CoD, and PoD; the RPN is useful in focusing effort and resources on activities with the greatest impact on reducing component failure risk.
The second phase of the materials management assessment identifies guidance applicable to nuclear power plant design, construction and operations from various industry sources including EPRI, the Nuclear Regulatory Commission, the Nuclear Energy Institute, and the American Society of Mechanical Engineers. Design, construction, and pre-service guidance includes applicable design codes, regulatory requirements, and supplemental guidelines such as the EPRI Utility Requirements Document. Operations guidance includes regulatory requirements (e.g., in-service inspection) and supplemental guidance (e.g., inspection and evaluation guidelines for boiling water reactors and pressurized water reactors). The results of the guidance assessment provide nuclear plant owners, operators, and their vendors with a tool for identifying applicable materials management resources.
Where limited or no guidance is available to mitigate or manage applicable degradation concerns, gaps may be identified, which is the last element of the materials management assessment. These gaps are prioritized considering both the risk the gap is addressing and the time frame in which the gaps need to be addressed in order to have an impact. Unlike materials management for operating plants, where gaps identify issues with an already existing design and set of materials, many of the gaps for new plants can only be addressed early in the plant life cycle, before operation. For example, the decision and opportunity to improve the stress conditions in a weld zone on surfaces wetted by the reactor environment, or to modify the water chemistry conditions at the component surface, must be made early on in the design and fabrication phases of the project.
The results of the materials management assessments are presented in component-based tables that summarize the component configuration, materials of construction, potential degradation modes, risk assessment results, management guidance (for both design/construction/pre-service and for operations), and applicable gaps and opportunities.
Table 1 depicts a materials management table for the upper internals structure of an advanced pressurized water reactor, focusing on the upper support welded assembly and the support column assemblies. Based on the known and potential degradation mechanisms for these components – stress corrosion cracking and reduction in fracture resistance – they are assigned a risk level. Various guidance documents are then listed that address proper design, materials selection, handling during fabrication and installation, and inspection requirements. The right-hand column lists the materials-related technical gaps associated with the upper support welded assembly and the support column assemblies. These include items such as weld process control and optimization (M&F-03), supplemental pre-service nondestructive evaluation for reactor internals (I&M-05), and safety assessment for reactor internals (ASD-01).
The information compiled in the materials management table helps allocate research resources for addressing materials issues for new plants. For example, through its Advanced Nuclear Technology Program, EPRI is studying critical welding and fabrication factors that strongly influence the initiation of materials degradation. EPRI also is developing a risk-informed in-service inspection and pre-service inspection methodology for advanced light water reactor designs that could be incorporated into applicable ASME codes governing nuclear plant inspection.
To date, EPRI has completed comprehensive materials assessments for the GE Hitachi ESBWR, the Westinghouse AP1000, the GE Hitachi ABWR, and the Toshiba ABWR designs. Work is currently underway on the AREVA EPR, the KHNP APR1400, and the Mitsubishi APWR designs.
Author: Jeffrey Hamel is a program manager at EPRI. He oversees research on near-term deployment of advanced light water reactor nuclear plants, development of the Next Generation Nuclear Plant technology, and technical and commercial support for an integrated spent fuel management strategy. Prior to joining EPRI, Hamel worked at General Electric as the manager of specialty projects and was responsible for managing and leading new growth for GE’s nuclear business, particularly in pressurized water reactors and spent fuel services. He can be reached at [email protected].
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