Corrosion Science Keeps Nuclear Power in the Nation’s Energy Mixmkw3a@virginia.edu
Nuclear energy produces more carbon-free electricity in the United States than all other sources combined, according to the U.S. Energy Information Administration.
Achieving a net-zero carbon future depends on nuclear energy, and nuclear energy’s viability depends on regulatory action and public acceptance. Storing and containing nuclear waste, and preserving materials in generation-IV reactors’ anticipated extreme environments, are two ongoing concerns. Advances in corrosion science can help overcome these challenges.
Materials scientists and engineers at the University of Virginia have joined multi-institutional teams involving universities, national labs and their private sector partners to understand and mitigate corrosion that occurs in storage containment, and to guide the selection of anti-corrosive materials for novel reactor designs. Their research advances U.S. Department of Energy initiatives, including two Nuclear Energy University Program grants and two Energy Frontier Research Centers funded by the Office of Basic Energy Sciences.
Identifying waste sites most and least vulnerable to container corrosion
Two decades ago, the nuclear industry and its regulators envisioned interim storage of spent fuel as a safe, flexible and cost-effective approach to keeping plants operating during the siting of a permanent, consolidated repository.
Because political, legal and regulatory issues have delayed approval of a proposed repository deep within Yucca Mountain, the volume of waste contained in interim storage facilities has continued to expand. The United States has 80,000 metric tons of spent nuclear fuel. Spent fuel rods are housed in 3,000 canisters in 70 different sites, principally owned and managed by power producers. The Nuclear Regulatory Commission anticipates the need to relicense up to 20 of these sites within the next 10 years.
UVA’s Robert G. Kelly, AT&T Professor of Engineering and professor of materials science and engineering, and James T. Burns, associate professor of materials science and engineering, lead an effort to help the Nuclear Regulatory Commission recertify these private- and public-sector sites for interim storage of spent nuclear fuel.
The encasements for the spent fuel rods present technical questions that Kelly and his team want to answer. Here’s how the process works: Spent rods that are thermally and reactively hot are first submerged in pools; this step is called active cooling. Spent fuel rods are next placed inside a stainless steel canister that is 7 to 8 feet in diameter and 25 feet long and welded shut. The stainless steel canister is then encased in a concrete tube commonly called a cask. The cask is cleverly designed to allow passive cooling. Holes drilled into each end of the cask allow cooler air to enter at the bottom; the air rises as it is heated and escapes at the top.
The holes make the stainless steel canisters vulnerable to localized corrosion and environmental cracking—Kelly’s and Burns’ area of expertise. Salt aerosols, which can travel 100 miles inland, are especially concerning. At higher temperatures, the salt deposited on the canister can form a saturated chloride solution on the canister surface, potentially weakening the weld that keeps the canister shut. If the canister remains immobile, a weakened weld is not a safety issue; the amount of radiation released would be inconsequential. If a canister with a weakened weld is moved, however, the top sheet could slide off and expose the rods.
The Nuclear Regulatory Commission wants to know which of the 3,000 canisters are most and least likely to undergo corrosion. Kelly’s team earned a $718,000, three-year grant from the Nuclear Energy University Program to develop a modeling tool that assesses risk by location, taking into account exposure to aerosols and other contaminants and temperature dynamics within the concrete cask.
“If we can show that, for a given site, it is not feasible for a crack to form in the container to a critical size, then the NRC can narrow the list of sites that require a physical inspection,” Kelly said.
The model focuses on the formation and growth of pits in the stainless steel caused by salt deposits and other corrosives. Leveraging research conducted by Liat Bell, a Ph.D. student advised by Kelly, and Danyil Kovalov, a post-doc in Kelly’s group, the team has a general ability to predict the largest size a pit can become given the environmental conditions.
Burns uses the pit as a starting point for the growth of a stress-corrosion crack, taking into account the physics of passive cooling within the cask and the material properties of the canisters themselves. His Ph.D. advisee Sarah Blust and post-doc Zach Harris aid in this research.
VEXTEC, a small technology business near Nashville, Tennessee, leads the probability modeling, with the task of running multiple simulations to identify variables that are most important in determining canister life. The model assigns a probability that under specified conditions a crack will grow to a certain length. Researchers at the Sandia National Laboratories, including UVA Engineering alumna Rebecca Schaller and Ph.D. student Ryan Katona, will apply the model in ongoing storage work in order to validate or refine the model with a real canister.
Working toward a permanent solution to nuclear waste storage and containment
The need for long-term storage and containment of radioactive liquid and sludge is a legacy of the nation’s Cold War nuclear weapons program. Because of these materials’ composition and rate of decay, the time scale for long-term storage is thousands of years.
A permanent solution, therefore, depends on where and how these radioactive materials are contained. Turning the nuclear waste into a solid form, such as a ceramic or glass, is a promising solution that gives the radioactive matter time to decay to safer levels as long as the glass or ceramic doesn’t dissolve, leaking into soil and groundwater.
The Center for Performance and Design of Nuclear Waste Forms and Containers, or WastePD, is an Energy Frontier Research Center established at Ohio State University in 2016 to generate a fundamental understanding of waste forms’ degradation mechanisms.
Gerald Frankel, Distinguished Professor of Engineering, professor of materials science and engineering, and director of the Fontana Corrosion Center at Ohio State University, has served as WastePD director since its founding.
WastePD aims to understand nuclear waste forms’ ability to withstand corrosion in a repository over very long periods of time—how the materials dissolve or otherwise degrade. Both the glasses and ceramics, as well as metal that would contain the glasses, are considered in the research.
John R. Scully, Charles Henderson Chaired Professor of Materials Science and Engineering at UVA, leads the metals research thrust in WastePD, supported by a multi-million-dollar, six-year award. This research focuses on extremely corrosion-resistant alloys.
Two fundamental strategies of mitigating corrosion are thermodynamic immunity or very high kinetic resistance. Corrosion-resistant alloys fall with the latter strategy. They are protected by the spontaneous formation of a nanometer-thick surface film that regulates oxidation. Whether these alloys’ corrosion-resistant properties come from the film’s ability to resist breakdown, or its ability to reform quickly when breakdown occurs, remains an open question. Regardless, the capacity of the thin film to maintain its self-healing ability over time is a subject of Scully’s ongoing research.
These materials were designed not by trial and error or lessons learned, but by integrated computational materials engineering, known by its acronym ICME in practice. This approach uses scientific principles and models of fundamental aspects of materials science to predict material properties and accurately regulate some of the details of controlling mechanisms and chemical/physical interactions in the degradation process. The team quickly identified a nickel-rich high-entropy alloy predicted to form a protective oxide thin-film and exhibit exceedingly high corrosion resistance, which was subsequently demonstrated in electrochemical testing.
These studies usher in new era in the science-based design of corrosion resistant materials.
Preventing emissions of radioactive gases
The spent fuel rods are not the only waste product of nuclear fission. Radioactive gases within the fuel rod also pose a concern. When the U.S. Department of Energy was created and gained stewardship of the U.S. nuclear enterprise, it required nuclear plant managers and operators to ensure these gases are not released into the atmosphere.
The capture-and-storage method approved at that time is a USB-sized canister constructed with a carbon-steel alloy. When the Department of Energy prompted a second look at the capture process in the early 2000s, officials discovered that a number of the canisters had corroded all the way through, not at the welds but on their sides.
Scully earned a grant of just under $1 million in 2017 from the Nuclear Energy University Program to find out what happened. He turned to Sean Agnew, UVA professor of materials science and engineering and an expert in materials’ mechanical properties and time-dependent phenomena, to help explain why the canisters corroded within the space of two decades, which is unusually fast.
The initial answer was within easy reach. The material selection was made in the late 1970s, a time when scientific knowledge of long-term degradation processes with various stages was in its infancy. Officials and plant operators appear to have been unaware of contemporaneous studies that Agnew’s literature review brought to light, which indicated that carbon-steel was vulnerable to corrosion in the intended application.
Materials scientists and engineers are never satisfied with the easy answer, however. Scully continued to explore the corrosion problem with his advisee, Charles Demarest. Demarest hypothesized that the accelerated corrosion resulted when a decay product of the radioactive gas called rubidium interacted with chloride salts. Agnew looked into liquid metal embrittlement as a possible cause of the accelerated corrosion. Both Demarest’s and Agnew’s experiments returned a null result, deepening the mystery.
Through this process of elimination, Scully and his colleagues hypothesized that the decay product—rubidium—was reacting with the chromium in the stainless steel canister. Chromium is a beneficial element, forming a protective oxide layer on the surface of steels, which contain Cr as an alloying element. If rubidium enters the mix, forming a chromium-rubidium oxide, it could render the oxide layer less effective as an anti-corrosive.
To test this hypothesis, Scully asked Bi-Cheng Zhou, UVA assistant professor of materials science and engineering, to identify and assign probabilities to reactions between fission gas decay products such as rubidium and corrosion-inhibiting oxides on a steel surface. Zhou and Kang Wang, a post-doctoral fellow in his computational thermodynamics and kinetics research group, sought to predict the states of a material at specific conditions. Their model correlates material properties and a material state to temperature, pressure and other real-world conditions.
Their modeling results suggest that rubidium-bearing products do react with chromium oxide to form a new oxide, possibly rendering the stainless steel vulnerable to accelerated corrosion. Efforts to validate the model results with experimental proofs are underway in Scully’s research group.
Realizing the promise of generation-IV reactor designs
Whether nuclear waste is placed in casks above ground or containers underground, the process of retrieving, cooling, encasing, moving and ultimately storing nuclear waste poses safety and environmental risks. Generation-IV nuclear reactor designs aim to reduce, if not eliminate, waste associated with nuclear fission. Within the past five years, the U.S. Department of Energy ramped up its support for advanced reactor research, with a keen eye toward molten salt and liquid metal-cooled reactors.
One challenge is that these new reactor designs must operate continuously at temperatures as high as 800 degrees Celsius. Consequently, the design requires a pressure vessel material that performs well under extreme conditions that are hostile to materials for long periods of time.
Scully addresses the viability of next-gen reactor materials as a member of an Energy Frontier Research Center dedicated to understanding how the extreme conditions encountered by materials in nuclear reactors—radiation damage, corrosive environments, high stresses and high temperatures—couple to affect the corrosion properties of the material. Fundamental Understanding of Transport Under Reactor Extremes (FUTURE), launched in August 2018, is co-led by scientists at the Los Alamos National Laboratory and the University of California, Berkeley.
Scully earned a $600,000 grant to explore how these extreme conditions affect transport and reactions that control corrosion of high-performance materials at the solid/liquid interface. Scully’s research focuses on understanding the concurrent effects of radiation and corrosion on these materials.
A Virtuous Cycle
The Department of Energy grants build on UVA’s research strength in corrosion and electrochemistry. UVA Engineering faculty are known for their work on environmental cracking, localized corrosion, and service life prediction. They also contribute to understanding next-generation materials, including high-entropy alloys and additively manufactured components, devising methods to make corrosion resistance a critical part of materials’ fundamental design.
Scully credits these Department of Energy programs with sustaining a virtuous cycle that inspires new talent.
“Students are offered unique opportunities to do great work,” he said. “Some may go on to join the national laboratories and become thought leaders in materials science and engineering. The support provided by national laboratories and the Department of Energy makes it possible for the next generation of researchers to do the same.
“It is an honor to join these efforts. Our partnership with the national laboratories contributes to new discoveries and knowledge in corrosion science and electrochemistry that would be otherwise unobtainable.”