Combating corrosion in the world’s aging nuclear reactors

Forty years of hard labor in punishing conditions sounds like an interminable sentence. Imagine finding out near the end of that period that the sentence has been extended for another 20 years and maybe another 20 beyond that.

That’s the plight of nuclear reactors. These giant metal contraptions were typically designed to generate electricity for about 40 years, day after day resisting damage from a corrosive, watery world of extreme temperatures, pressures, and ionizing radiation. Now many are being asked to soldier on for at least another 20 years.

To extend reactors’ lifetimes, scientists and engineers are continually improving methods for monitoring and predicting the integrity and strength of these multibillion-dollar metal structures. And they are developing corrosion-resistant replacement materials to keep nuclear reactors operating safely and reliably for 60 years or longer.

“The US nuclear fleet is getting up there in years,” says Gary S. Was, a materials scientist and nuclear engineer at the University of Michigan. Was, a specialist in metal corrosion, is referring to commercial nuclear power reactors that generate electricity. According to the US Energy Information Administration (EIA), 96 reactors operate in 29 US states. In 2019, they supplied roughly 20% of the electricity in the US and on average were about 38 years old.

Most utility companies that initially held licenses to operate nuclear reactors for 40 years have already received 20-year extensions on their plants, Was says.

Roger Hannah and Neil Sheehan, public affairs officers for US Nuclear Regulatory Commission operations in the Southeast and Northeast US, respectively, confirm that the Turkey Point Nuclear Plant, 40 km south of Miami, and Peach Bottom Atomic Power Station, 80 km southeast of Harrisburg, recently received additional extensions that would allow them to continue operating for a total of 80 years each. Hannah and Sheehan add that other plants are seeking to do the same.

“There’s even talk in the industry of going to 100 years,” Was says.

The trend to keep reactors running longer and longer is not limited to the US. The 440 or so reactors located throughout the world are 30 years old, on average, and getting older. And although some of them are scheduled to be shut down and decommissioned, many are being upgraded with new parts to greatly extend their years of operation.

And more nuclear facilities are on the horizon. In the US, for example, two nuclear reactors are close to completion at the Vogtle Electric Generating Plant near Waynesboro, Georgia.

Meanwhile, according to the World Nuclear Association, another 50 nuclear reactors are being constructed in 15 countries, mainly China, India, Russia, and the United Arab Emirates. And to help keep pace with a projected 28% increase in world energy use by 2040, another 100 reactors with anticipated service lives spanning many decades are on order or planned.

When it comes to ensuring these reactors continue to deliver reliable power safely, “it’s all about aging management,” says Mark Nutt, using the industry catchphrase for the program of inspections and mitigation steps that enable aging reactor vessels, piping, and other power-plant components to continue operating for many decades. Nutt, a manager of nuclear technology research at Pacific Northwest National Laboratory (PNNL), says ongoing advances in materials and corrosion science play a major role in reactor longevity.

A look inside reactors

Nuclear power systems harness the intense heat released in nuclear reactions and use it indirectly to generate electricity. The principle of operation is much the same as that in power plants that burn coal, natural gas, and other fossil fuels, which together accounted for roughly 63% of electricity production in the US in 2019, according to the EIA. In all these plants, heat is used to produce steam, and the steam drives the rotary motion of a turbine to generate electricity. In hydroelectric plants and wind turbines, flowing water or wind directly spins a turbine.

Unlike power plants that generate heat by burning fossil fuels, commercial nuclear reactors typically use pellets of uranium dioxide with diameters and lengths on the order of 10 mm. Fuel manufacturers load these bean-sized pellets into 4.5 m long slender metal tubes made of a neutron-permeable zirconium cladding material. Then they bundle the tubes together, forming assemblies of roughly 200 or more fuel rods, depending on the design and size of the reactor. Hundreds of these assemblies containing millions of pellets weighing a total of roughly 100 metric tons are submerged in a giant vessel filled with a heat-carrying medium, or coolant. In the great majority of commercial reactors, the coolant is water.

The heat that drives the reactor comes from ²³⁵U nuclei in the fuel pellets. As these nuclei undergo spontaneous fission, they split into two smaller nuclei and liberate heat and neutrons. The neutrons can collide with other ²³⁵U nuclei in the pellets and cause them to fission, liberating more heat and additional neutrons. Those neutrons can strike other nuclei, thereby setting off a chain reaction that heats the fuel-rod assemblies and in turn heats the water. Plant operators regulate the chain reaction and reactor temperature by inserting neutron-absorbing control rods into the core and withdrawing them.

The most common type of nuclear power reactor is a pressurized water reactor. These reactors keep the water under high pressures—greater than 15 MPa (150 atm)—so that it remains a hot liquid as it flows over the fuel rods. The water can reach temperatures of about 320 °C. To prevent the spread of radioactive material, water in this section circulates through a closed cycle: a system of pipes known as the primary loop, making no direct contact with the outside world. This circulation of superhot, pressurized water transfers heat to a secondary loop, causing another, nonradioactive collection of water to boil, turn to steam, and drive the turbine.

Constant contact with high-temperature, high-pressure water can take a toll on metal no matter how tough it is. And because of the intensity of the heat, pressure, and radiation in the reactor environment, a small defect can quickly grow into a large one. For that reason, inspectors regularly use high-resolution ultrasound, electromagnetic imaging, and other methods to search for tiny flaws, cracks, and signs of fatigue in 25 cm thick reactor walls, safety systems, and other reactor components, says PNNL’s Aaron Diaz, a project manager who develops analytical techniques for inspecting reactors.

Although the stainless steels and nickel-based alloys from which many reactor components are made have largely resisted damage and corrosion, some have not done as well as others. In a couple of well-known cases, a type of chemically induced damage known as stress corrosion cracking (SCC) majorly weakened key parts.

According to PNNL materials scientist Mychailo Toloczko, this type of damage can occur at weld joints between dissimilar metals because of mechanical stress that develops as weld seams cool and contract. If those spots start to corrode, they are especially prone to microscopic cracking, which can lead to failure.

That mode of damage, which Toloczko says is generally considered “the main life-limiting degradation mechanism” for commercial reactors, wreaked havoc at the Davis-Besse Nuclear Power Station near Toledo, Ohio. The corrosion problems detected there and at other plants and the analyses that followed led to a better understanding of damage mechanisms in nickel-based alloys and the implementation of more corrosion-resistant materials.

Close call at Davis-Besse

In 2002, during a routine shutdown for refueling and safety inspections, plant operators at the Davis-Besse plant noticed corrosion and cracking on the head of the reactor. Refueling shutdowns are standard. Engineers power down the reactor typically every 12–18 months, remove some of the used fuel, and replace it with fresh fuel. The 2002 inspection revealed damage in a part of the mechanism that lowers control rods into the reactor core—the device that puts the brakes on the chain reaction and regulates the reactor temperature and power output.

After removing some of the upper components to take a look below them, the inspectors discovered boric acid deposits and “a football-sized hole in the reactor head,” according to Bogdan Alexandreanu of Argonne National Laboratory, an expert in corrosion who analyzed pieces of the damaged head.

Alexandreanu explains that because boron efficiently captures neutrons, engineers commonly add minute concentrations (parts-per-million levels) of boric acid to the coolant water to help control the fission rate. They balance the pH by adding lithium hydroxide.

SCC had caused tiny cracks to form in a part of the control-rod mechanism where welding joined various alloys, including a nickel-chromium-iron material known as alloy 600. The cracks allowed hot borated water to slowly escape from the interior of the reactor through that site. As the leaked water pooled and evaporated, the concentration of boric acid increased, gradually forming a corrosive solution that ate through much of the thickness of the reactor head, which was not made of alloy 600. The progressing corrosion did not penetrate a lower layer, made of yet another alloy, which held the pressurized coolant in check, averting possible disaster.

Plant operators replaced the damaged head with a very similar one that happened to be available from a canceled power-plant construction job. In 2010, that head also failed in much the same way as the first replacement: at weld sites containing alloy 600.

The failures implicated the Ni-Cr-Fe alloy as highly susceptible to SCC. But why did these alloy 600 parts fail while so many others—made of the same material—held up just fine? The short answer is that the sheets of alloy 600 used to make the failed Davis-Besse reactor head parts were faulty.

Researchers, including Alexandreanu, came to this conclusion from tests in which samples of the materials were controllably stressed and monitored electronically for crack growth—typically over months—while held in a test chamber. Conditions in the chamber reproduced those found in pressurized water reactors in terms of water temperature and pressure and chemical composition.

From these tests, the researchers learned how the alloy’s structure influenced its resistance to cracking. Alexandreanu notes that in well-made samples of alloy 600, the interfaces between microscopic crystallites, or grain boundaries, as metallurgists call them, are decorated with nanoparticles of chromium carbide. The particles, known as precipitates, function as roadblocks, he explains, preventing cracks from quickly propagating along the grain-boundary route.

In samples taken from the troubled Davis-Besse reactor heads, which Alexandreanu says were probably made from alloy 600 stock that was not processed at a high enough temperature, the precipitates were scattered throughout the metal, not clustered along grain boundaries, where they could protect the material.

A key outcome of these kinds of tests was that engineers in the US and other countries have taken to replacing reactor parts made from alloy 600 with ones made of alloy 690. Both materials are based on nickel, chromium, and iron. But alloy 690 is 28% chromium by weight, compared with 15% for alloy 600.

Similar large-scale studies conducted on alloy 182, which was long used as a weld material to join dissimilar metals in pressurized water reactors, showed that it, too, is highly susceptible to a type of SCC. Now alloy 152 has generally replaced alloy 182. Both alloys consist mainly of nickel, chromium, iron, and vanadium, but 152 is 30% chromium by weight, while 182 is 16%.

The higher chromium content in alloys 690 and 152 aids these materials in forming a thin but robust passivating oxide film that resists further oxidation. The layer serves as a barrier, protecting the metal from corroding on contact with water in the reactor.

“The replacement materials are vastly more resistant to stress corrosion cracking than the earlier ones,” Alexandreanu says. “But how well they will perform after 60 or 80 years, especially at weld seams and interfaces, has not yet been established.”

Accident-tolerant fuels

Replacement materials may also be in the offing for fuel-rod cladding—the zirconium alloy from which the hollow fuel rods are made. The cladding needs to be permeable to neutrons so they can diffuse through the fuel assemblies, causing fission and sustaining the nuclear chain reaction, but robust enough to survive in a reactor.

Researchers have been driven to make rapid progress in that area since 2011, when an earthquake-triggered tsunami caused an electrical blackout at the Fukushima Daiichi power station in Japan. Although the plant’s safety systems quickly stopped the chain reactions, the loss of power meant that cooling water could not circulate through three reactors, causing residual heat produced there to raise the temperature in the cores to well over 1,000 °C, destroying the fuel and eventually leading to explosions caused by built-up hydrogen gas.

The problem was exacerbated by the zirconium-alloy fuel-rod cladding, says Steven J. Zinkle, a nuclear engineer at the University of Tennessee, Knoxville, who previously served as director of the Materials Science and Technology Division at Oak Ridge National Laboratory (ORNL). “What were we thinking when we built reactors out of these things?” Zinkle asks.

He explains that despite “heroic alloying efforts” to improve zirconium’s oxidation resistance, the metal is fundamentally incompatible with hot water and steam. As the temperature rises above 1,000 °C, the rate of zirconium oxidation in water—which is highly exothermic—climbs exponentially. The reaction dumps additional heat into the reactor and generates the hydrogen gas that caused the Fukushima explosions. “A big reduction in the oxidation kinetics compared with zirconium can make a night-and-day difference in a loss-of-cooling-water accident,” Zinkle says.

A number of approaches to make such accident-tolerant fuels (ATFs) are now in various stages of development. Global Nuclear Fuel, a General Electric–led joint venture with Hitachi, announced earlier this year that fuel-rod assemblies made with two types of its ATF products—IronClad and Armor-coated zirconium cladding—were installed in reactors at the Clinton Power Station in Clinton, Illinois. The ongoing test of the new materials follows a smaller one that began in 2018 in a reactor at Southern Nuclear’s Edwin I. Hatch Nuclear Plant in Georgia.

Raul B. Rebak, a corrosion engineer and research group leader at GE, says that IronClad is a cladding material made of an iron-chromium-aluminum-molybdenum alloy “with extreme resistance to attack” by steam and hot water. His team is evaluating the performance of two versions of these alloys—ranging in chromium content from 12 to 21%—in commercial reactors. The researchers still need to assess embrittlement and other factors that can affect long-term performance. “We are not settled yet on the final composition,” Rebak says.

Regarding the Armor product, Rebak says that for now, he has to remain tight lipped. He offers that it’s a thin proprietary protective coating applied on top of the standard zirconium alloy. The coating enables fuel-rod assemblies to withstand temperatures of up to 1,000 °C without oxidizing and corroding, he says.

Another approach to ATFs—currently in the R&D stage—is making the cladding from a silicon carbide composite. Silicon carbide is hard and inert and can withstand extreme temperatures. But ceramics tend to be brittle—an unattractive property for reactor operations. This composite, however, is flexible. It’s made from thin silicon carbide fibers that are woven into a textile and infiltrated with additional silicon carbide to fill in the voids. Zinkle says a number of ongoing studies indicate that the ceramic composite is “a great candidate” for fuel-rod cladding in pressurized water reactors.

Molten salt reactors

Corrosion resistance is key to the success of today’s nuclear reactors. But it may play an even greater role in future types of reactors that are being designed to be more thermodynamically efficient, more economical, and safer. One example, which is still under development, is the molten salt reactor (MSR), a type of reactor in which the nuclear fuel is contained in a molten salt—a high-temperature liquid that also serves as the coolant. The concept has been studied on and off since the 1960s.

Compared with today’s high-pressure water reactors, which run at about 320 °C, MSRs run at higher temperatures, such as 750 °C or higher. This higher temperature is central to boosting thermodynamic efficiency. And they operate at atmospheric pressure, which bypasses the need for exceptionally strong materials and expensive safety systems. But the molten salt can be highly corrosive to reactor materials.

Stephen S. Raiman, an R&D associate at ORNL and a corrosion specialist, aimed to develop a broad mechanistic understanding of the way reactor materials degrade in molten salts. But by doing an extensive literature search, he found it was difficult to compare studies and identify meaningful trends because there was a lack of standardization in corrosion testing methods and a wide variety of experimental variables.

So he compiled decades’ worth of molten salt corrosion data and teamed up with ORNL’s Sangkeun “Matt” Lee, a data scientist, to comb through the compilation with statistical algorithms. Raiman says the one factor that stood out above all others in predicting a material’s corrosion resistance was the molten salt’s purity (J. Nucl. Mater. 2018, DOI: 10.1016/j.jnucmat.2018.07.036).

Molten salts are hygroscopic: they tend to attract oxygen, water, sulfur, metal halides, and other impurities that can corrode materials, Raiman says. “Salt purity really matters. Pure salts cause much lower corrosion rates than impure salts.”

While the timeline for MSRs’ reaching operation is fairly long, anticorrosion work in conventional reactors has moved relatively rapidly toward implementation. That’s unusual for the field. Historically, the nuclear industry has been slow to implement new materials, Zinkle says. Every aspect of plant operation is highly regulated because of safety concerns, leading to a costly qualification process that can drag on for many years before new materials are approved by the Nuclear Regulatory Commission and used commercially.

Recent events in the area of corrosion resistance show that change can come relatively quickly, especially when it comes to identifying safer fuels. “Before Fukushima, no one was thinking about replacing zirconium cladding,” Zinkle says. Now, in under a decade, research programs were developed and funded, best candidates for ATFs have been identified and tested, and industry got on board, ramping up production of the new materials to conduct large-scale tests. With safety regulators’ ongoing input, fuel-rod assemblies made from new materials are now sitting in commercial reactors, generating electricity and providing real-world experience.

The work, PNNL’s Nutt says, demonstrates that “the R&D doesn’t stop when the plant comes on line.”