Future nuclear power reactors could rely on molten salts—but what about corrosion?

Most discussions of how one can avert local weather change deal with photo voltaic and wind technology as key to the transition to a future carbon-free energy system. However Michael Brief, the Class of ’42 Affiliate Professor of Nuclear Science and Engineering at MIT and affiliate director of the MIT Plasma Science and Fusion Middle (PSFC), is impatient with such discuss.

“We will say we must always have solely wind and photo voltaic sometime. However we do not have the luxurious of ‘sometime’ anymore, so we will not ignore different useful methods to fight local weather change,” he says. “To me, it is an ‘all-hands-on-deck’ factor. Photo voltaic and wind are clearly an enormous a part of the answer. However I believe that nuclear energy additionally has a crucial function to play.”

For many years, researchers have been engaged on designs for each fission and fusion nuclear reactors utilizing molten salts as fuels or coolants. Whereas these designs promise vital security and efficiency benefits, there is a catch: Molten salt and the impurities inside it typically corrode metals, in the end inflicting them to crack, weaken, and fail.

Inside a reactor, key steel elements will probably be uncovered not solely to molten salt but additionally concurrently to radiation, which typically has a detrimental impact on supplies, making them extra brittle and liable to failure. Will irradiation make steel elements inside a molten salt-cooled nuclear reactor corrode much more shortly?

Brief and Weiyue Zhou Ph.D. ’21, a postdoc within the PSFC, has been investigating that query for eight years. Their latest experimental findings present that sure alloys will corrode extra slowly once they’re irradiated—and figuring out them amongst all of the obtainable industrial alloys could be easy.

The primary problem—constructing a take a look at facility

When Brief and Zhou started investigating the impact of radiation on corrosion, virtually no dependable services existed to have a look at the 2 results without delay. The usual strategy was to look at such mechanisms in sequence: first, corrode, then irradiate, then study the influence on the fabric. That strategy tremendously simplifies the duty for the researchers however with a significant trade-off.

“In a reactor, every little thing goes to be occurring on the identical time,” says Brief. “For those who separate the 2 processes, you are not simulating a reactor; you are doing another experiment that is not as related.”

So, Brief and Zhou took on the problem of designing and constructing an experimental setup that might do each without delay. Brief credit a workforce on the College of Michigan for paving the best way by designing a tool that might accomplish that feat in water somewhat than molten salts.

Even so, Zhou notes, it took them three years to provide you with a tool that will work with molten salts. Each researchers recall failure after failure, however the persistent Zhou in the end tried a very new design, and it labored.

Brief provides that it additionally took them three years to exactly replicate the salt combination utilized by trade—one other issue crucial to getting a significant outcome. The toughest half was reaching and guaranteeing that the purity was appropriate by eradicating crucial impurities comparable to moisture, oxygen, and sure different metals.

As they have been growing and testing their setup, Brief and Zhou obtained preliminary outcomes displaying that proton irradiation didn’t all the time speed up corrosion however typically truly decelerated it. They and others had hypothesized that risk, besides, they have been stunned. “We thought we should be doing one thing incorrect,” remembers Brief. “Perhaps we blended up the samples or one thing.”

Nonetheless, they subsequently made related observations for quite a lot of circumstances, rising their confidence that their preliminary observations weren’t outliers.

The profitable setup

Central to their strategy is the usage of accelerated protons to imitate the influence of the neutrons inside a nuclear reactor. Producing neutrons can be each impractical and prohibitively costly, and the neutrons would make every little thing extremely radioactive, posing well being dangers and requiring very lengthy occasions for an irradiated pattern to chill down sufficient to be examined. Utilizing protons would allow Brief and Zhou to look at radiation-altered corrosion each quickly and safely.

Key to their experimental setup is a take a look at chamber that they connect to a proton accelerator. To arrange the take a look at chamber for an experiment, they place inside it a skinny disk of the steel alloy being examined on high of a pellet of salt. Throughout the take a look at, the complete foil disk is uncovered to a shower of molten salt. On the identical time, a beam of protons bombards the pattern from the facet reverse the salt pellet, however the proton beam is restricted to a circle in the course of the foil pattern.

“Nobody can argue with our outcomes then,” says Brief. “In a single experiment, the entire pattern is subjected to corrosion, and solely a circle within the middle of the pattern is concurrently irradiated by protons. We will see the curvature of the proton beam define in our outcomes, so we all know which area is which.”

The outcomes with that association have been unchanged from the preliminary outcomes. They confirmed the researchers’ preliminary findings, supporting their controversial speculation that somewhat than accelerating corrosion, radiation would truly decelerate corrosion in some supplies below some circumstances. Fortuitously, they only occur to be the identical circumstances that metals in molten salt-cooled reactors will expertise.

Why is that final result controversial? An in depth-up take a look at the corrosion course of will probably be defined. When salt corrodes steel, the salt finds atomic-level openings within the stable, seeps in, and dissolves salt-soluble atoms, pulling them out and leaving a niche within the materials—a spot the place the fabric is now weak.

“Radiation provides power to atoms, inflicting them to be ballistically knocked out of their positions and transfer very quick,” explains Brief. So, it is smart that irradiating a cloth would trigger atoms to maneuver into the salt extra shortly, rising the speed of corrosion. But, in a few of their exams, the researchers discovered the other to be true.

Experiments with ‘mannequin’ alloys

The researchers’ first experiments of their novel setup concerned “mannequin” alloys consisting of nickel and chromium, a easy mixture that will give them a primary take a look at the corrosion course of in motion. As well as, they added europium fluoride to the salt, a compound identified to hurry up corrosion.

In our on a regular basis world, we regularly consider corrosion as taking years or many years, however within the extra excessive circumstances of a molten salt reactor, it may noticeably happen in simply hours. The researchers used europium fluoride to hurry up corrosion much more with out altering the corrosion course of. This allowed for extra speedy dedication of which supplies, below which circumstances, skilled roughly corrosion with simultaneous proton irradiation.

The usage of protons to emulate neutron injury to supplies meant that the experimental setup needed to be rigorously designed and the working circumstances rigorously chosen and managed. Protons are hydrogen atoms with {an electrical} cost, and below some circumstances, the hydrogen may chemically react with atoms within the pattern foil, altering the corrosion response, or with ions within the salt, making the salt extra corrosive.

Subsequently, the proton beam needed to penetrate the foil pattern however then cease within the salt as quickly as potential. Below these circumstances, the researchers discovered they may ship a comparatively uniform dose of radiation contained in the foil layer whereas additionally minimizing chemical reactions in each the foil and the salt.

Exams confirmed {that a} proton beam accelerated to three million electron volts mixed with a foil pattern between 25 and 30 microns thick would work effectively for his or her nickel-chromium alloys. The temperature and period of the publicity could possibly be adjusted based mostly on the corrosion susceptibility of the particular supplies being examined.

Optical photographs of samples examined after exams with the mannequin alloys confirmed a transparent boundary between the realm that was uncovered solely to the molten salt and the realm that was additionally uncovered to the proton beam. Electron microscope photographs specializing in that boundary confirmed that the realm that had been uncovered solely to the molten salt included darkish patches the place the molten salt had penetrated all through the foil, whereas the realm that had additionally been uncovered to the proton beam confirmed nearly no such darkish patches.

To substantiate that the darkish patches have been resulting from corrosion, the researchers lower by the foil pattern to create cross sections. In them, they may see tunnels that the salt had dug into the pattern. “For areas not below radiation, we see that the salt tunnels hyperlink the one facet of the pattern to the opposite facet,” says Zhou. “For areas below radiation, we see that the salt tunnels cease roughly midway and infrequently attain the opposite facet. So we verified that they did not penetrate the entire method.”

The outcomes “exceeded our wildest expectations,” says Brief. “In each take a look at we ran, the applying of radiation slowed corrosion by an element of two to 3 occasions.”

Extra experiments, extra insights

In subsequent exams, the researchers extra intently replicated commercially obtainable molten salt by omitting the additive (europium fluoride) that that they had used to hurry up corrosion, and so they tweaked the temperature for much more practical circumstances. “In rigorously monitored exams, we discovered that by elevating the temperature by 100 levels Celsius, we may get corrosion to occur about 1,000 occasions sooner than it could in a reactor,” says Brief.

Pictures from experiments with the nickel-chromium alloy plus the molten salt with out the corrosive additive yielded additional insights. Electron microscope photographs of the facet of the foil pattern dealing with the molten salt confirmed that in sections solely uncovered to the molten salt, the corrosion is clearly centered on the weakest a part of the construction—the boundaries between the grains within the steel.

In sections that have been uncovered to each the molten salt and the proton beam, the corrosion is not restricted to the grain boundaries however is extra unfold out over the floor. Experimental outcomes confirmed that these cracks are shallower and fewer prone to trigger a key part to interrupt.

Brief explains the observations. Metals are made up of particular person grains inside which atoms are lined up in an orderly style. The place the grains come collectively there are areas—known as grain boundaries—the place the atoms do not line up as effectively. Within the corrosion-only photographs, darkish traces observe the grain boundaries.

Molten salt has seeped into the grain boundaries and pulled out salt-soluble atoms. Within the corrosion-plus-irradiation photographs, the injury is extra normal. It is not solely the grain boundaries that get attacked but additionally areas inside the grains.

So, when the fabric is irradiated, the molten salt additionally removes materials from inside the grains. Over time, extra materials comes out of the grains themselves than from the areas between them. The removing is not centered on the grain boundaries; it is unfold out over the entire floor. Consequently, any cracks that kind are shallower and extra unfold out, and the fabric is much less prone to fail.

Testing industrial alloys

The experiments described to this point concerned mannequin alloys—easy mixtures of parts which can be good for learning science however would by no means be utilized in a reactor. Within the next series of experiments, the researchers centered on three commercially obtainable alloys which can be composed of nickel, chromium, iron, molybdenum, and different parts in numerous mixtures.

Outcomes from the experiments with the industrial alloys confirmed a constant sample—one which confirmed an concept that the researchers needed to go in: the upper the focus of salt-soluble parts within the alloy, the more severe the radiation-induced corrosion injury. Radiation will enhance the speed at which salt-soluble atoms comparable to chromium go away the grain boundaries, hastening the corrosion course of.

Nonetheless, if there are extra not-soluble parts, comparable to nickel, current, these atoms will go into the salt extra slowly. Over time, they will accumulate on the grain boundary and kind a protecting coating that blocks the grain boundary—a “self-healing mechanism that decelerates the speed of corrosion,” say the researchers.

Thus, if an alloy consists principally of atoms that do not dissolve in molten salt, irradiation will trigger them to kind a protecting coating that slows the corrosion course of. But when an alloy consists principally of atoms that dissolve in molten salt, irradiation will make them dissolve sooner, dashing up corrosion. As Brief summarizes, “When it comes to corrosion, irradiation makes a great alloy higher and a foul alloy worse.”

Actual-world relevance plus sensible pointers

Brief and Zhou discover their outcomes encouraging. In a nuclear reactor manufactured from “good” alloys, the slowdown in corrosion will most likely be much more pronounced than what they noticed of their proton-based experiments as a result of the neutrons that inflict the injury will not chemically react with the salt to make it extra corrosive.

Consequently, reactor designers may push the envelope extra of their working circumstances, permitting them to get extra energy out of the identical nuclear plant with out compromising on security.

Nonetheless, the researchers stress that there is a lot work to be executed. Many extra initiatives are wanted to discover and perceive the precise corrosion mechanism in particular alloys below completely different irradiation circumstances. As well as, their findings must be replicated by teams at different establishments utilizing their very own services.

“What must occur now’s for different labs to construct their very own services and begin verifying whether or not they get the identical outcomes as we did,” says Brief. To that finish, Brief and Zhou have made the small print of their experimental setup and all of their information freely obtainable on-line. “We have additionally been actively speaking with researchers at different establishments who’ve contacted us,” provides Zhou. “Once they’re planning to go to, we provide to indicate them demonstration experiments whereas they’re right here.”

However already their findings present sensible steering for different researchers and gear designers. For instance, the usual approach to quantify corrosion injury is by “mass loss,” a measure of how a lot weight the fabric has misplaced. However Brief and Zhou think about the mass loss a flawed measure of corrosion in molten salts.

“For those who’re a nuclear plant operator, you often care whether or not your structural elements are going to interrupt,” says Brief. “Our experiments present that radiation can change how deep the cracks are when all different issues are held fixed. The deeper the cracks, the extra probably a structural part is to interrupt, resulting in a reactor failure.”

As well as, the researchers supply a easy rule for figuring out good steel alloys for structural elements in molten salt reactors. Producers present in depth lists of accessible alloys with completely different compositions, microstructures, and components. Confronted with a listing of choices for crucial constructions, the designer of a brand new nuclear fission or fusion reactor can merely study the composition of every alloy being provided.

The one with the very best content material of corrosion-resistant parts, comparable to nickel, would be the best option. Inside a nuclear reactor, that alloy ought to reply to a bombardment of radiation not by corroding extra quickly however by forming a protecting layer that helps block the corrosion course of.

“That will look like a trivial outcome, however the precise threshold the place radiation decelerates corrosion relies on the salt chemistry, the density of neutrons within the reactor, their energies, and some different components,” says Brief. “Subsequently, the whole pointers are a bit extra sophisticated. However they’re offered in a simple method that customers can perceive and make the most of to make a good selection for the molten salt–based mostly reactor they’re designing.”

This story is republished courtesy of MIT Information (web.mit.edu/newsoffice/), a well-liked website that covers information about MIT analysis, innovation and instructing.