INL has been given a leadership role in international revitalization of nuclear power. To enable the Department of Energy's (DOE) nuclear mission for the Laboratory, INL must have exceptional capability in research, development, and deployment of the next generation of advanced materials and nuclear fuels that will deliver increased performance and reliability for increasingly demanding energy generation environments. Developing nuclear reactors with a 60-year life and space nuclear reactors that perform indefinitely without maintenance or refueling requires integrating knowledge of materials and component degradation phenomena and mechanisms, structure and properties relationships, real-time performance assessment and validation, and multi-scale lifetime prediction models. The nature of challenges to materials and fuel in advanced nuclear systems and the engineering and scientific developments necessary to increase performance and reliability are illustrated in the figure below. The scientific capabilities that underpin enhanced materials performance and reliability for nuclear systems also support the multi-program and national security missions of the Laboratory.
Current Approach to Reliable Systems
The current generation of commercial nuclear plants was designed several decades ago. Designs were based on knowledge of fuel performance and material properties derived from laboratory-scale testing, reduced by appropriate safety factors specified by design rules in the American Society of Mechanical Engineers (ASME) Code. Operating conditions in the U.S. plants are limited to temperatures below about 350°C with water as the coolant. Validation of the actual performance of fuel and structural materials is based primarily on inspections during shut-downs or destructive examination of surveillance coupons. Accumulation of radiation damage to fuel and materials is managed by scheduled replacement of components or by major plant overhauls. Commercial reactors currently operate on a once-through fuel cycle, with spent fuel designated to go to a geological repository for final disposition.
Major operating surprises continue to occur in existing commercial nuclear plants around the world. Although there have been notable successes using the current design approach, such as naval reactors that do not require refueling during the lifetime of the vessel, the limitations of this approach are highlighted in recent failures in service. For example, there is severe corrosion in U.S. light water plants (including both structural materials and fuel cladding in boiling water reactors), premature decommissioning of Canadian heavy water reactors due to corrosion, and failure of major secondary side steam piping in a Japanese plant.
Service failures typically occur because laboratory testing did not identify or adequately address material degradation mechanisms such as corrosion and embrittlement or because in-service environments changed to conditions that had not been investigated. Entirely new degradation mechanisms, including irradiation-assisted stress corrosion cracking, were observed in service that were not predicted by laboratory testing. Plants were not designed to be readily inspected and inspection schedules do not accurately reflect risk and consequences of failures. Online monitoring is limited to plant operating parameters such as pressure and temperature.
Evolutionary Developments
Concepts for next generation nuclear power plants include designs that operate at significantly higher temperatures or with different coolants than today's reactors. Higher temperatures are dictated by the desire to use nuclear heat to produce hydrogen from water using a thermo-chemical reaction or the need to improve plant economics by incorporating greater thermal efficiency. Some designs call for structural materials to operate at 1000°C with fuel temperatures as high as 1200°C. Hydrogen plants will have materials in operating conditions that are extremely corrosive (potentially including pressurized sulfuric acid at 850°C) and will require that very compact heat exchangers be integrated with the nuclear plant. Fuel for this type of reactor will be composed of billions of millimeter-sized composite particles embedded in carbon balls or blocks. New fuel types may be developed to recycle long-lifetime fission products from commercial fuel to reduce the need for geological repositories. Some components of these reactor systems will experience significantly greater radiation doses compared to those in current generation plants because the desired license period for the plant will be 60 years compared to the 40-year expectation for existing plants.
Degradation mechanisms for materials and fuel for these increasingly aggressive operating conditions need to be thoroughly understood so that rates of in-service damage and component lifetime can be predicted. Determining degradation rates will require a combination of experimental characterization in environments that are representative of both temperature and corrosion conditions expected in the reactor, as well as modeling and simulation. A materials test reactor, like the Advanced Test Reactor (ATR) and the associated ability to quantitatively examine material damage in hot cells are critical tools in conducting accelerated tests to assess fuel behavior and radiation damage at the larger doses expected in these plants.
New design rules will need to be incorporated into the ASME Code to allow materials to be used under conditions where small amounts of deformation will occur after long exposure at high temperature (creep) and under the combined influence of stress and corrosion. Detailed understanding of degradation of fuel and structural materials, combined with simulations of heat and mass transfer and structural mechanics models, will allow targeted surveillance and inspection to be performed in critical locations where both the probability and risk associated with failure are high. This concept is being developed currently by the Nuclear Regulatory Commission under an embryonic project known as Proactive Management of Degradation Mechanisms; a similar scheme has been called for by the Materials Technology Institute, a chemical industry group, under the title Risk Based Inspection.
Revolutionary Developments
Further in the future, more advanced nuclear power plants are envisioned operating at high temperatures, with more damaging fast neutron spectra and using potentially corrosive liquid metal coolants. Fuel is expected to operate to higher burn-up, with the consequent greater chemical changes and radiation damage. Space nuclear plants are in the conceptual design stage that will operate autonomously and intermittently at temperatures well in excess of 1,000°C.
To operate in these extreme service conditions, new materials and fuel compositions will be required. The relationships among structure, properties, and performance of these materials in harsh operating conditions will be determined largely by modeling and simulation, with experimental validation. It will be particularly important to accurately model radiation damage in fuel and materials since materials test reactors are being shut down because of cost and proliferation concerns (for example, the Fast Flux Test Facility and the University of Michigan Ford Reactor). Furthermore, the large radiation doses anticipated would be difficult to obtain in tests of reasonable length. Reliable operation of these plants will be based on a combination of the detailed understanding of material and fuel behavior across dimensional scales from atoms to bulk properties, in situ sensors that continuously monitor the environment and material response to the environment, and sophisticated lifetime prediction models for specific components that use the material degradation mechanisms and sensor data as inputs. For example, the National Aeronautics and Space Administration has explicitly identified the need for such a philosophy for remote operation of space nuclear power.
Related Links:
- Center for Advanced Energy Studies (CAES)
- Center for Advanced Modeling and Simulation (CAMS)
- Chemistry
- Contacts:
- Richard Wright, (208) 526-6127, Send E-mail