Chemists find new material to remove radioactive gas from spent nuclear fuel

Jan 24, 2012
Sandia chemist Tina Nenoff heads a team of researchers focused on removal of radioactive iodine from spent nuclear fuel. They identified a metal-organic framework that captures and holds the volatile gas, a discovery that could be used for nuclear fuel reprocessing and other applications. (Photo by Randy Montoya)

(PhysOrg.com) -- Research by a team of Sandia chemists could impact worldwide efforts to produce clean, safe nuclear energy and reduce radioactive waste.

The Sandia researchers have used metal-organic frameworks (MOFs) to capture and remove volatile radioactive gas from spent . “This is one of the first attempts to use a MOF for capture,” said chemist Tina Nenoff of Sandia’s Surface and Interface Sciences Department.

The discovery could be applied to nuclear fuel reprocessing or to clean up nuclear reactor accidents. A characteristic of is that used fuel can be reprocessed to recover fissile materials and provide fresh fuel for nuclear power plants. Countries such as France, Russia and India are reprocessing spent fuel.

The process also reduces the volume of high-level wastes, a key concern of the Sandia researchers. “The goal is to find a methodology for highly selective separations that result in less waste being interred,” Nenoff said.

Part of the challenge of reprocessing is to separate and isolate radioactive components that can’t be burned as fuel. The Sandia team focused on removing iodine, whose isotopes have a half-life of 16 million years, from spent fuel.

They studied known materials, including silver-loaded zeolite, a crystalline, porous mineral with regular pore openings, high surface area and high mechanical, thermal and chemical stability. Various zeolite frameworks can trap and remove iodine from a stream of spent nuclear fuel, but need added silver to work well.

“Silver attracts iodine to form silver iodide,” Nenoff said. “The zeolite holds the silver in its pores and then reacts with iodine to trap silver iodide.”

But silver is expensive and poses environmental problems, so the team set out to engineer materials without silver that would work like zeolites but have higher capacity for the gas molecules. They explored why and how zeolite absorbs iodine, and used the critical components discovered to find the best MOF, named ZIF-8.

“We investigated the structural properties on how they work and translated that into new and improved materials,” Nenoff said.

MOFs are crystalline, porous materials in which a metal center is bound to organic molecules by mild self-assembly chemical synthesis. The choice of metal and organic result in a very specific final framework.

This illustration of a metal-organic framework, or MOF, shows the metal center bound to organic molecules. Each MOF has a specific framework determined by the choice of metal and organic. Sandia chemists identified a MOF whose pore size and high surface area can separate and trap radioactive iodine molecules from a stream of spent nuclear fuel. (Image courtesy of Sandia National Laboratories)

The trick was to find a MOF highly selective for iodine. The Sandia researchers took the best elements of the zeolite Mordenite — its pores, high surface area, stability and chemical absorption — and identified a MOF that can separate one molecule, in this case iodine, from a stream of molecules. The MOF and pore-trapped iodine gas can then be incorporated into glass waste for long-term storage.

The Sandia team also fabricated MOFs, made of commercially available products, into durable pellets. The as-made MOF is a white powder with a tendency to blow around. The pellets provide a stable form to use without loss of surface area, Nenoff said.

Sandia has applied for a patent on the pellet technology, which could have commercial applications.

The Sandia researchers are part of the Off-Gas Sigma Team, which is led by Oak Ridge National Laboratory and studies waste-form capture of volatile gasses associated with nuclear fuel reprocessing. Other team members — Pacific Northwest, Argonne and Idaho national laboratories — are studying other volatile gases such as krypton, tritium and carbon.

The project began six years ago and the Sigma Team was formalized in 2009. It is funded by the U.S. Department of Energy Office of Nuclear Energy.

Sandia’s iodine and MOFs research was featured in two recent articles in the Journal of the American Chemical Society authored by Nenoff and team members Dorina Sava, Mark Rodriguez, Jeffery Greathouse, Paul Crozier, Terry Garino, David Rademacher, Ben Cipiti, Haiqing Liu, Greg Halder, Peter Chupas, and Karena Chapman. Chupas, Halder and Chapman are from Argonne.

“The most important thing we did was introduce a new class of materials to nuclear waste remediation,” said Sava, postdoctoral appointee on the project.

Nenoff said another recent paper in Industrial & Engineering Chemistry Research shows a one-step process that incorporates MOFs with iodine in a low-temperature, glass waste form. “We have a volatile off-gas capture using a MOF and we have a durable waste form,” Nenoff said.

Nenoff and her colleagues are continuing their research into new and optimized MOFs for enhanced volatile gas separation and capture.

“We’ve shown that MOFs have the capacity to capture and, more importantly, retain many times more iodine than current materials technologies,” said Argonne’s Chapman.

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User comments : 13

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Uzza
4.7 / 5 (3) Jan 24, 2012
An alternative is to simply use a liquid fuel, which makes the gaseous fission-products bubble straight out.
Scottingham
1 / 5 (2) Jan 24, 2012
Uzza, you *may* be right, but NEVER use the word 'simply' with regards to fission.

Nothing about fission is simple. F'n awesome, yes. Simple? No.
dschlink
1 / 5 (1) Jan 24, 2012
Even with a molten salt reactor the gases do not 'bubble out'. They can be removed by distillation, but it ain't simple.
JustAnyone
2 / 5 (1) Jan 24, 2012
Molten salt reactors freak me out a little - "molten salt" sounds so much nicer than "liquid sodium", which if you've had chemistry, SHOULD freak you out.

Seems to me the way to go is the more-inherently safe gas-cooled pebble-bed reactors with melt-away floors that empty into boron-matrix cooling chambers. Making the 'pebbles' out of carbide-encased uranium pellets that are designed to reduce fission cross-section with increasing heat adds another layer of safety. Not working with water, which is corrosive at high temperature and pressure due to dissolved gasses, would be nice, too - thus the gas-fill being helium, which is inherently nonreactive. Look up on Wikipedia the 'Pebble Bed Reactor' for details.
Scottingham
4 / 5 (1) Jan 24, 2012
The germans tried to make a pebble bed reactor and saw that in practice, it wasn't that great.

My money is on the traveling wave fast reactor designs.
Tinman
not rated yet Jan 24, 2012
Okay, how is krypton volatile (it is a noble gas)? Also, elemental carbon is a solid.
Doug_Huffman
not rated yet Jan 24, 2012
The triple point of carbon is 4600°K 10600KPa.
Uzza
5 / 5 (1) Jan 24, 2012
@Scottingham
While fission is indeed not simple in itself, Molten Salt Reactors are at least very simple when compared to current Light Water Reactors.

@schlink
It was apparently simple enough that ORNL did it for the MSRE over 45 years ago.

@JustAnyone
The problem with PBR is that they still use solid fuel, and giving even more of a reprocessing headache.
If we just store the pebbles as is, then we're even worse of than today, as the volume of the pebbles is much larger than the actual waste content.
A molten salt reactor is much better than a PBR, or anything else for that matter.

On the topic of the German PBR, the problem they had was that a fuel pellet got stuck in a pipe due to friction, which is because helium gas don't really add much lubrication.
Molten salt coolant would be best. It gives good lubrication while avoiding the problem of high pressure needed by water, and is is also immune to radiation damage.
Sanescience
not rated yet Jan 24, 2012
There is a bit of "mixed metaphor" here. PBR are slow (or thermal) neutron reactors under the category of inefficient designs like PWR and that typically only extract about 1% of the energy contained in uranium and leave the 99% behind as "waste".

MSR designs and sodium metal cooled are fast neutron reactors with each their own issues, but their goal is to burn all the nasty actinides and extract most of the energy from the fuel.

With MSR designs, the versions with coolant and fuel mixed together was problematic because it exposed "everything" in the coolant cycle to a corrosive chemical cocktail of fission products and hard neutron radiation causing many materials issues and maintenance issues. With coolant and fuel separate, the thermal conduction properties of salt doesn't support the "small" core requirements for fast neutron designs. Water reactivity is less of an issue than sodium, but not "none".

Sodium has issues with water, however it has many "ideal" properties...
Sanescience
not rated yet Jan 24, 2012
Sodium has an unmatched thermal to density/weight advantage. When pipes are filled with denser metals the pumps and pipes need to be much thicker and more expensive, experience more wear, and are more prone to seismic events (earthquakes). Sodium does not react with materials used in core construction and cooling systems. It's reactivity property with water becomes an advantage considering it prevents any moister corrosion to reactor core systems. Sodium also forms only short lived neutron activation products that don't pose disposal problems and can be recycled indefinately, or reacted with clorine for cleaning or disposed as common table salt.

ShotmanMaslo
not rated yet Jan 25, 2012
Sanescience - thorium fueled MSRs such as LFTR are thermal, not fast reactors.
Uzza
5 / 5 (1) Jan 25, 2012
@Sanescience
As shotman said above, MSRs are not necessarily fast reactors. There are designs for both thermal and fast MSRs, each with it's own advantages/disadvantages.
Thorium fueled MSRs are predominantly thermal reactors, since Th-232/U-233 is an excellent breeder combination in a thermal reactor.

The issues with a mixed fuel and coolant is dependent on what element you use to create the salt, as it's the reactions with that element and the reactor materials that needs to be dealt with.
The MSRE built by ORNL used fluoride salts for the reactor because of many favorable qualities. This gives corrosion problems if not using the right materials, but they solved it by creating Hastelloy-N, which is very corrosion resistant to fluoride salts. With over 28000 hours of fuel salt operation, they saw virtually no effects of corrosion.
Sanescience
not rated yet Jan 25, 2012
My wording was a little misleading. There are indeed molten salt thermal reactors, in fact most have been.

I was functionally fixated on the comment of JustAnyone comparing molten salt and sodium cooled.
astro_optics
Feb 01, 2012
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