Innovative Accident Tolerant Fuel Materials May Help Extending the Life of Light Water Reactors: A Perspective Open Access

The electricity in the civilian grid comes from many sources of energy such as fossil fuels (coal, natural gas, and liquid oil), renewables (wind, solar, and hydro), and nuclear. The burning of fossil fuels produces pollutants and greenhouse gases, which are associated with climate change. The renewable sources are in the category of clean or green energy as they do not release carbon dioxide into the atmosphere. Recently, more and more countries are also categorizing nuclear energy as clean or green energy. Every year, nuclear-generated electricity in the U.S. saves our atmosphere from more than 470 million metric tons of carbon dioxide emissions that would otherwise come from fossil fuels. That is the same as taking annually off the road nearly 100 million passenger vehicles. Clean sources of electricity are also called low-carbon or carbon-free. The U.S. Department of Energy and the Nuclear Energy Institute (NEI) report three reasons why nuclear energy is desirable as a source of clean energy over more conventional green sources such as wind or solar.^1-2  (a) Nuclear energy’s land footprint is small since a typical 1 GW nuclear facility in the United States needs less than 300 ha to operate. Meanwhile, wind farms require 360 times more land area to produce the same amount of electricity and solar photovoltaic plants require 75 times more space than nuclear. (b) Nuclear fuel is extremely dense, and therefore produces a very small amount of waste regarding used fuel. Some of the used fuel can also be reprocessed but the U.S. does not currently engage in reprocessing.^1-2  In a future of affordable and reliable carbon-free electricity, nuclear energy should represent a significant share, complementing wind power and solar energy. The third advantage of nuclear power as clean energy is that it is weather-independent as it can operate 24 h every day of the year. In the last few decades, the power “availability” from nuclear stations has also increased because of fewer shutdowns prompted by corrosion-related failures as engineers understood better the mechanisms of interactions between alloys and the environments in the reactor.

In March 2011, there was a major earthquake in the northeast of Honshu Island (Japan) which eventually triggered a 50 ft (15 m) tall coastal tsunami. The force of this tsunami was extremely rare, and however, so highly destructive that killed over 20,000 people and destroyed over 100,000 homes by the coastline. The tsunami wave also overcame the onsite built sea walls and reached the buildings of at least four of the six nuclear power stations (NPS) at the Fukushima Daiichi location. The strength of the wave washed away the emergency diesel generators at the side of NPS 1 to 4. This resulted in NPS black out, that is, there was no electricity to power the pumps to cool the fuel rods in the four reactors. The overheating of the fuel rods led to the Zr alloy cladding reacting with water and steam inside the reactor core to produce several tons of hydrogen gas which escaped the reactor pressure vessel (RPV) into the housing buildings and later ignited. In spite of this severe accident in March 2011, there were no casualties at the Fukushima site either from building explosions, by core meltdown, or by the release of radioactivity into the environment.⁵  Eleven years later in 2022 there are no reports of an increase in cancer deaths near the Fukushima site. That is, in spite of the extensive media coverage of this incident, no direct effect on the increase in deaths can be attributed to the 2011 accident. Eleven years later there is a greater worldwide society acceptance of nuclear power as a source of clean energy (e.g., outcome of the COP26 meeting in Glasgow in 2021 and the COP27 meeting in Sharm El-Sheikh in 2022).²  Eleven years after Fukushima, the Western world (including North America and Europe) may have lost its leadership in providing technical support to countries that want to join the nations that rely on nuclear power for the civilian grid. After a short post-Fukushima period or reassessment, China has continued its accelerated pace to build newer NPS (Figure 2). Japan has not re-started most of the reactors that shut down in March 2011. And as this manuscript is being prepared, Germany is still committed to rid itself of all nuclear-generated power by the end of 2022 despite the uncertain reliable supply of natural gas from Russia. A recent appreciation of nuclear power benefits made Germany and California delay their intended shutdown of nuclear power stations.

Since October 2012, the U.S. Department of Energy (Office of Nuclear Energy) is working with fuel vendors to develop a family of fuels called accident tolerant fuels (ATF).^5-7  The objective of ATF is to make light water reactors (LWRs) NPS more resilient to the lack of coolant accidents.^5-8  It is understood that an NPS loaded with ATF concepts would not suffer consequences similar to hydrogen explosions at the Fukushima Daiichi site in March 2011.^5,7 

In the Fukushima situation, the UO₂ fuel itself was not the problem as the weakest link was the Zr-based housing or cladding of the fuel. Figure 4 shows that currently there are three main ATF candidate concepts to replace the existing Zr alloys, including (a) coated Zr alloy tubes,¹⁰  (b) monolithic FeCrAl (IronClad) alloys,^11-12  and (c) silicon carbide-based composites.⁵  These three concepts are innovative materials that have never been used in a nuclear reactor core before. It is likely that coated concepts will be the first generation of ATF cladding installed in commercial reactors because they may need a shorter time or an easier path for regulatory approval. The three ATF cladding concepts (Figure 4) were identified as materials that can better tolerate accident conditions (e.g., endure higher temperatures for longer times) than the current Zr alloys by maintaining their bundle architecture while generating less ignitable hydrogen gas in reaction with water. The characterization of the three main ATF concepts for cladding needs to be assessed in the entire fuel cycle, starting with the demonstration that they can be economically fabricated in an industrial setting followed by an acceptable performance in a commercial LWR and having nondifficult issues in the back end (used fuel rods disposition). The characterization of the ATF concepts in each stage of the fuel cycle is to retire their specific risks.

A coated Zr-based tube is being pursued by the three major fuel vendors in the U.S. The coated concept is considered evolutionary as it will imply the application of a less than 20 µm thick coating on the outside diameter of the Zr-based cladding. This coating will not only retard the reaction of the substrate with steam in the case of a loss-of-coolant accident, but it can also act as a fretting resistance barrier, for example, between the grid separators and the fuel rods. The chemistry of the coating may change depending on the type of reactor application. For example, a coating may perform well in a hydrogen or reducing ∼300°C water environment but it may not tolerate an environment that may have a more elevated redox potential. A few micrometers of a dense, pure chromium coating seem to perform well for fuel rods in hydrogen-containing pressurized water reactor (PWR) environments after exposure for up to two fuel cycles in commercial reactors (Byron #2 and Vogtle #2). The use of coatings to protect current Zr alloy tubing is the nearest-term implementation of an ATF concept to retrofit existing LWRs.

The concept of replacing the current cladding made of Zr-based alloy for a monolithic yet thinner FeCrAl alloy is the midterm ATF concept implementation into an LWR, mainly because this family of ferritic alloys has never been used before in a water environment under irradiation. It has been demonstrated that full-length tubes of approximately 10 mm outer diameter and 0.3 mm wall thickness can be fabricated and welded to hermeticity. The corrosion performance in the ∼300°C water of an LWR depends on the amount of Cr in the alloy. Data is still needed on the performance of fueled FeCrAl rods in commercial reactor environments. The rod performance in a commercial reactor must include the satisfactory fuel-to-cladding interaction in the fuel cavity.

The third ATF cladding candidate in Figure 4 of a ceramic-based material may take a longer time to be implemented in commercial LWRs. Studies are currently being performed to define the architecture of an SiC cladding (mainly concerning hermeticity) and their performance in ∼300°C water environments under irradiation (including fuel-to-cladding interaction). Additionally, a full-length cladding fabrication demonstration needs to be implemented to meet the specifications and performance requirements. One of the earliest applications of the SiC-based approach could be for the channels in boiling water reactors (BWR) as there is no requirement of hermeticity and there is no risk of releasing activated elements into the coolant.

That is, what failed in the Fukushima scenario was that the cladding of the fuel or barrier (2) reacted with the following barrier (3). The plant black-out caused barrier (3) not to respond as designed (i.e., being liquid and at temperatures below 300°C). Not only barrier (3) was not effective anymore but also it attacked barrier (2) (letter “R” in Figure 5), causing fuel (1) to be dispersed. ATF concepts are aimed at producing or fabricating more robust barriers (2) (Figures 4 and 5). In other words, a safer plant operation can be accomplished by minimizing or retiring risks using ATF claddings that would be less reactive with a barrier (3).

In the case of Fukushima, the scenario was the powerful ocean wave, the occurrence of such a large ocean wave had an extremely low frequency, however, the consequences (plant blackout) were catastrophic. One of the manners of reducing risk is using engineering tools.⁸  For example, by using ATF concepts such as IronClad, the outcome of hydrogen gas explosions and core meltdown could have been minimized. The main damage from the Fukushima accident was the perception by the public at large that producing electricity using the heat from nuclear fission was unsafe.

To minimize premature plant shutdowns or decommissioning (Figure 2) ATF engineering concepts could be implemented in the currently operating LWRs before the end of the 2020s. The ATF concepts would reduce the risk of plant operation and allow for plant life extension, probably beyond 80 y. The Fukushima accident was an engineering-related accident (R in Figure 5) triggered by a natural disaster. Using better engineering materials would reduce the risk of accidents and decrease the cost of plant operation making these aging plants more competitive with the burning of natural gas and contributing to a promising share of clean energy.

Commercial nuclear energy organizations assert that the implementation of ATF combined with increased enrichment and extended burnup (up to ∼75 GWd/MTU) (Figure 4) will safely sustain the current LWR fleet through; (1) enhanced fuel performance (economics), (2) enhanced fuel reliability (safety), (3) improved efficiency and operational flexibility, and (4) fuel cycle optimization, while producing less used fuel waste.

• Nuclear energy is currently accepted as a powerful source of clean energy.

• Many light-water power reactors are being decommissioned mainly due to their economically challenged operation.

• For nuclear power to continue to provide clean energy, it would be important to extend the life of existing reactors, probably beyond 80 y.

• The implementation of accident-tolerant materials in LWRs could be a path to power reactors’ life extension.

The financial support of Global Nuclear Fuel and GE Research is gratefully acknowledged. This material is based upon work supported by the Department of Energy under Award No. DE-NE0009047. This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe on privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.