Recycling nuclear waste

Fri, 05/29/2015 - 11:12am

Kate McAlpine, Univ. of Michigan

The simulation of the reactor core confirms that the dead zones allow the reactor to operate safely. This image shows where atoms split, or fission. The fuel rods run vertically, with the red, high-fission fuel regions and blue, low-fission dead zones. Image: Seker et al, Univ. of Michigan

The simulation of the reactor core confirms that the dead zones allow the reactor to operate safely. This image shows where atoms split, or fission. The fuel rods run vertically, with the red, high-fission fuel regions and blue, low-fission dead zones. Image: Seker et al, Univ. of Michigan

An advanced nuclear reactor under development by Hitachi could help solve the nuclear waste problem, and Univ. of Michigan researchers were involved in verifying its safe performance through computer simulations.

The U-M team worked with colleagues at the Massachusetts Institute of Technology and the Univ. of California, Berkeley. After more safety analysis, Hitachi plans to move forward with a prototype of the "resource-renewable boiling water reactor" in the next few years.

One of the major technological hurdles for nuclear energy is developing systems to dispose of the waste produced by typical reactors. It must be sealed away for hundreds of millennia while the radioactivity naturally decreases.

Hitachi's new design would burn off the longest-lived radioactive materials, called transuranics, shortening that isolation period to a few centuries. This would recycle the nuclear waste to produce yet more energy and reduce the amount that must be stowed away.

"Because of transuranics, we're talking about lifetimes for storing fuel that we can't even fathom," said Thomas Downar, U-M professor of nuclear engineering and radiological sciences. "You get this down to a hundred years, then you're talking about the ability to engineer a container that you have confidence will last that long."

In the conventional boiling water reactors that currently produce about 30 percent of all the nuclear-generated electricity in the U.S., the neutrons that split uranium atoms have been slowed by the boiling water. In contrast, the Hitachi design uses fast neutrons since they are more likely to split, or fission, transuranic atoms.

Prototype fast reactors have been running since the 1970s, but they use a sodium coolant. Sodium burns when it comes into contact with air and reacts violently with water. This is one of the reasons why U.S. utilities that operate reactors have been hesitant to consider sodium-cooled designs.

A water-cooled fast reactor, though, could offer safer and more familiar operation. The challenge was designing a water-cooled core that would stop itself if it started overheating and the water turned to steam. In conventional reactors, the water's slowing action acts as a failsafe because steam is less effective at decelerating neutrons. Since fewer neutrons are at the right speed to cause fissions, the reaction rate slows down too.

For a boiling water reactor that's burning transuranics, this scenario is trickier. The faster neutrons could mean a faster fission rate, creating more heat, steam and fast neutrons.

"If something goes wrong and the power increases, you want to have the fission rate decrease," Downar said.

To create this safety feature in their reactor, Hitachi engineers plan large dead zones in the fuel rods, made of materials with a much lower probability of fissioning with fast neutrons.

Hitachi calculated that as the presence of steam reduced the density of the water, fast neutrons were likely to travel further. By keeping the active regions of the fuel assembly small, more neutrons would be lost to these "blanket" regions in an overheating scenario, slowing the fission rate.

Before beginning the expensive process of prototyping, Hitachi wanted to confirm with outside experts that the design would perform as expected.

With funding from the Department of Energy, members of Downar's group spent the last three years developing codes that could simulate the more complex layout and physics of Hitachi's reactor core design. For example, uranium fission reactions are reasonably steady and easy to predict, but transuranic reactions are irregular and difficult to calculate accurately.

The U-M team developed a method to generate data that simulates the way transuranics burn. They then applied this data to established codes currently used for boiling water reactor analysis. By looking at what happened when the steam bubbles appeared, the team found that the fast neutrons tended to leave the reactive part of the fuel assembly, slowing the reaction rate as planned.

Now, the university teams are about to begin a careful comparison of their methods with the predictions from the Hitachi computer codes to discover any differences in the simulation of the advanced reactor's performance. Hitachi will fund the teams at U-M, MIT and Berkeley for the next phases of the project.

Source: Universty of Michigan

Nuclear waste: Bury nuclear waste down a very deep hole, say scientists

 

Scientists at the University of Sheffield calculate that all of the UK's high level nuclear waste from spent fuel reprocessing could be disposed of in just six boreholes 5km deep, fitting within a site no larger than a football pitch. The concept -- called deep borehole disposal -- has been developed primarily in the UK but is likely to see its first field trials in the USA next year. If the trials are successful, the USA hopes to dispose of its 'hottest' and most radioactive waste -- left over from plutonium production and currently stored at Hanford in Washington State -- in a deep borehole. University of Sheffield researchers are presenting the latest findings relating to these trials and new concepts for sealing the waste into the boreholes at the American Nuclear Society (ANS) conference in Charleston this week (April 13-16). Professor Fergus Gibb, of the University of Sheffield's Faculty of Engineering, explains: "Deep borehole disposal is particularly suitable for high level nuclear waste, such as spent fuel, where high levels of radioactivity and heat make other alternatives very difficult. Much of the drilling expertise and equipment to create the boreholes already exists in the oil and gas and geothermal industries. A demonstration borehole -- such as is planned in the US -- is what is now needed to move this technology forward." At the ANS conference next week, Professor Gibb, with co-researcher Dr Karl Travis, will be presenting modelling work carried out by the University of Sheffield team on the Hanford waste, which confirms that around 40 per cent of the waste, in terms of radioactivity, currently stored at the US site could be disposed of in a single borehole. Fundamental to the success of deep borehole disposal is the ability to seal the hole completely to prevent radionuclides getting back up to the surface. Professor Gibb has designed a method to do this which he will be presenting at the conference next week: to melt a layer of granite over the waste, which will re-solidify to have the same properties as natural rock. Professor Gibb's colleague at the University of Sheffield, Dr Nick Collier, will propose a method of fixing and surrounding the waste within the borehole using specialist cements able to handle the temperatures and pressures at that depth. Deep borehole disposal (DBD) has a number of advantages over the current solution envisaged for all UK nuclear waste, which is in a mined repository at 500m depth: DBD is effectively 'pay-as-you-go' disposal. A mined repository can cost from hundreds of millions to tens of billions of dollars to construct before any waste can be disposed of; DBD costs a few tens of millions of dollars per borehole. There are more geological sites suitable for DBD as the granite layer that is required can be found at appropriate depths under most of the continental crust. A borehole could be drilled, filled and sealed in less than five years, compared to the current timescale for a UK mined repository, which is to open in 2040 and take its first waste by 2075 (although a site has not yet been agreed). As DBD disposes of nuclear waste at greater depths and with greater safety and because there are more potential sites available, it should be easier to obtain public and political acceptance of the technology. DBD has limited environmental impact and does not require a huge site: the holes are a maximum 0.6m in diameter and can be positioned just a few tens of metres apart. Once a borehole is complete, all physical infrastructure on the surface can be removed. While seismic activity might damage the containers within the borehole, fracture the surrounding rock and disrupt some of the nearest barriers in the borehole, it would still not destroy the isolation of the waste or make it possible for radioactivity to reach the surface or any ground water. The demonstration borehole in the USA will be drilled just under half a metre in diameter and trials will be conducted to ensure waste packages can be inserted into the borehole and recovered if required. Initial results are expected in 2016. If these results are positive, disposal of the Hanford waste capsules would then take place in another borehole, just 0.22m in diameter.

Radioactive kitty litter may have ruined our best hope to store nuclear waste

 

Some of the most dangerous nuclear waste in the US is currently scattered between 77 locations all over the country, awaiting permanent storage. Until February, many experts suggested that the best place to put it was a facility about 40 miles east of Carlsbad, New Mexico, called the Waste Isolation Pilot Plant (WIPP). For 15 years, WIPP has operated as the first and only permanent, deep geologic nuclear waste storage facility in the country, holding "low level" radioactive materials — mostly clothing and tools exposed to radiation from nuclear weapons production — in steel barrels more than 2,150 feet below the Earth’s surface.

But earlier this year two emergencies brought that suggestion — and WIPP’s future — into question. And now it seems kitty litter may be to blame.

 

What happened?

WIPP is in a salt desert, and much of the work there involves burrowing through the salt and using huge elevators to deposit the stuff at surface level. The resulting underground caverns are then filled with radioactive waste and eventually closed shut, sealed forever.

Investigators search for a radiation leak at the Waste Isolation Pilot Plant in Carlsbad, NM. (Department of Energy)

In February, according to a press release from the US Department of Energy — which oversees operations at WIPP — two things happened to stall that relatively simple process.

First, on February 5th, a salt-hauling truck caught fire. That would be an inconvenience on the side of a highway, but in an enclosed salt cavern surrounded by nuclear waste, it’s a potential catastrophe. Workers evacuated the site. Fortunately no one was hurt. But the fire was significant enough to shut down underground operations until investigators could figure out what happened and how to stop it from happening again. Surface-level operations continued.

But not for long. Nine days later, late at night on Valentine’s Day, an alarm sounded, indicating that radioactivity was present in the air underground. No one was below ground at the time, but employees on the surface activated massive fans designed to ventilate the underground air. The next day, another monitor went off — this one on the surface — indicating airborne radiation. Employees who worked outside on the surface were told to take shelter inside buildings completely separated from storage operations. Valves allowing air to flow underground were sealed with high-density expanding foam. Everything came to a standstill, indefinitely.

 

Fallout

There are no indications that anyone has been injured from the radiation leak. (All employees went through examinations for radiation exposure; a DOE press release says most workers were not affected, and those who were "received less exposure than a person receives from a chest X-ray.") But for months, nothing has changed. The standstill remains. WIPP’s 850 or so employees are mainly sitting around, waiting (or "performing surface facilities maintenance or assisting with procedure reviews and revisions") while investigators from the US Department of Energy (DOE), the New Mexico Environment Department and elsewhere attempt to figure out what happened.

Initially, there were two hypotheses. The first was that something had gone wrong with the supports inside the cavern where waste was being stored. If that were the case, it meant a piece of salt rock or a steel support had fallen into one of the sealed barrels, puncturing it and releasing radiation into the air.

"That was an unlikely possibility," says Norbert T. Rempe, PhD, a retired geologist who spent decades as a principal engineer at WIPP. The cavern where the radiation monitor went off had been dug only recently, so the chances that supports had eroded or collapsed were probably slim.

Organic kitty litter likely caused a steel barrel's seal to puncture at the Waste Isolation Pilot Plant in Carlsbad, NM. The damage and the resulting radiation leak could close the facility, experts say. (Department of Energy)

More likely, he said, was the second hypothesis: that something had gone awry inside one of the radioactive containers — that the radioactive material had become hot for some reason, expanding and puncturing a steel barrel from the inside.

Last month, DOE investigators went into the cavern. Pictures showed that the latter hypothesis was true; a waste container’s lid was unsealed, and dust around the lid had turned yellow from the unusual heat emanating from inside. Each barrel is labeled to track where it came from. The punctured barrel originated from Los Alamos National Labs.

Jim Conca, PhD, a geologist who worked for years at WIPP who now blogs at Forbes about energy issues, believes he knows what blew the lid off at least one of WIPP’s radioactive barrels. The culprit, he wrote, was kitty litter.

"When someone decides to deviate, that is a bad, bad thing."

As Conca explains it, inorganic cat litter has properties that make it ideal for stabilizing nitrates in radioactive material — for ensuring that it doesn’t dry out and become dangerously hot. So kitty litter is often mixed in barrels with the low-level waste that’s eventually sent to WIPP. What happened at WIPP, he believes, is that one of the radioactive shipments was mixed with organic instead of inorganic material. "‘Green’ cat litter," he writes, is "made with materials like wheat or corn. These organic litters do not have the silicate properties needed to chemically stabilize nitrate the correct way." The result: "solutions can ignite when they dry out."

In other words, the whole problem at WIPP — the radiation leak, the months-long stall in operations, the worries over safety — results from one person’s mess up. (Yesterday, a DOE press release confirmed that kitty litter "may have caused a chemical reaction" that lead to the leak.)

"Everything nuclear is proceduralized," he tells The Verge. "It’s well laid out and everything everyone does is supposed to go up and down the chain of command. When you decide on a procedure for doing something like treating this waste, you don’t deviate from it. Ever. And when someone decides to deviate, that is a bad, bad thing."

He continues: "In this case, it could shut down the most successful nuclear repository in history."

 

Moving forward

Apart from the absurdity in a multibillion-dollar project halted by the decision to use off-brand kitty litter, the real lesson here may lie in the fragility of even the best nuclear storage facility. Corrective action at WIPP could be a massive undertaking. How many other barrels contain the dangerous organic cat litter? Are all of those barrels underground at WIPP? Are they awaiting shipment at surface level in places like Los Alamos? Or are they located at the temporary storage site in Andrews, Texas, where some containers were transferred when problems arose at WIPP?

"Expert assessment will be needed," writes Per Peterson, a professor and researcher at UC Berkeley’s Department of Nuclear Engineering, in an email shared with The Verge, to merely "determine whether the safety benefits of stabilizing or repackaging the material in these drums are justified by the risk to personnel who would attempt to do this work."

And if the DOE decides stabilizing or repackaging the material is unjustified, that would close WIPP for good.

"If that happened, it would be a shame and a disaster — particularly for taxpayers," says the retired WIPP geologist Rempe. The DOE estimates the total cost of the WIPP plant to be $7.2 billion.

Even if WIPP does begin operating again, "we have no idea how long this will take until WIPP is back to normal operations, or what the new normal operations will be," Rempe says. "No one knows right now. And it could be a long time before anyone knows."