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Light Water Reactor Sustainability Program: Materials Aging and Degradation Technical Program Plan

Components serving in a nuclear reactor plant must withstand a very harsh environment including extended time at temperature, neutron irradiation, stress, and/or corrosive media. The many modes of degradation are complex and vary depending on location and material. However, understanding and managing materials degradation is a key for the continued safe and reliable operation of nuclear power plants.

Extending reactor service to beyond 60 years will increase the demands on materials and components. Therefore, an early evaluation of the possible effects of extended lifetime is critical. The recent NUREG/CR-6923 gives a detailed assessment of many of the key issues in today’s reactor fleet and provides a starting point for evaluating those degradation forms particularly important for consideration in extended lifetimes. While life beyond 60 will add additional time and neutron fluence, the primary impact will be increased susceptibility (although new mechanisms are possible).

For reactor pressure vessels (RPV), a number of significant issues have been identified for recommended attention in future research activities. Large uncertainties for embrittlement predictions can result from sparse or nonexistent data at high fluences, for long times, and for high embrittlement conditions. The use of test reactors at high fluxes to obtain high fluence data is problematic for representation of the low flux conditions in RPVs. Late-blooming phases, especially for high nickel welds, have been observed and additional experimental data are needed in the high fluence regime where they are expected.

For the reactor core and primary systems, several key areas have been identified. Thermo-mechanical considerations such as aging and fatigue must be examined. Irradiation-induced processes must also be considered for higher fluences, particularly the influence of radiation-induced segregation (RIS), swelling, and/or precipitation on embrittlement. Corrosion takes many forms within the reactor core, although irradiation-assisted stress corrosion cracking (IASCC) is of the highest interest in extended life scenarios. Environmentally assisted fatigue is another area where more research is needed. Research in these areas can build upon other ongoing programs in the LWR industry as well as other reactor materials programs (such as fusion and fast reactors) to help resolve these issues for extended LWR life.

In the secondary systems, corrosion is extremely complex. Understanding the various modes of corrosion and identifying mitigation strategies is an important step for long-term service. Primary water stress corrosion cracking (PWSCC) is one key form of degradation in extended service scenarios.

In the area of welding technology, two critical long-standing welding related technical challenges requiring further research and development (both fundamental and applied). The first is the need for an advanced weld simulation tool to support component life extension and reliable lifetime prediction, especially as related to the issue of residual stresses as a primary driving force for SCC. The second challenge is the development of new welding technologies for reactor repair and upgrade.

Concrete structures can also suffer undesirable changes with time because of improper specifications, a violation of specifications, or adverse performance of its cement paste matrix or aggregate constituents under environmental influences (e.g., physical or chemical attack). Changes to embedded steel reinforcement as well as its interaction with concrete can also be detrimental to concrete’s service life. Research is needed in a number of areas to assure the long-term integrity of the reactor concrete structures.

Clearly, materials degradation will impact reactor reliability, availability, and, potentially, safe operation. Routine surveillance and component replacement can mitigate these factors; however, failures still occur. With reactor life extensions up to 60 years or beyond and power uprates, many components must tolerate more demanding reactor environments for even longer times. This may increase susceptibility to degradation for different components and may introduce new degradation modes. While all components (except perhaps the RPVs) can be replaced, it may not be economically favorable. Therefore, understanding, controlling, and mitigating materials degradation processes and a technical basis for long-range planning for necessary replacements are key priorities for reactor operation, power uprate considerations, and life extensions.

Many of the various degradation modes are highly dependent on a number of different variables, creating a complex scenario for evaluating lifetime extensions. In order to resolve these issues for life extension, a science-based approach is critical. Modern materials science tools (e.g., advanced characterization tools, past experience, and computational tools) must be employed. Ultimately, safe and efficient extension of reactor service life will depend on progress in several distinct areas, including mechanisms of degradation, mitigation strategies, modeling and simulations, monitoring, and management

The Materials Aging and Degradation (MAaD) within the LWRS program is charged with R&D to develop the scientific basis for understanding and predicting long-term environmental degradation behavior of materials in nuclear reactorss. The work will provide data and methods to assess performance of SSCs essential to safe and sustained reactor operations. The R&D products will be used by utilities, industry groups and regulators to affirm and define operational and regulatory requirements and limits for materials in reactor SSCs subject to long-term operation conditions, providing key input to both regulators and industry

The objectives of this report are to describe the motivation and organization of the MAaD Pathway within the Light Water Reactor Sustainability Program; to provide detail on the individual research tasks within the MAaD; to describe the outcomes and deliverables of the MAaD, and to list the requirements for performing this research.