Researchers develop filter for nuclear waste

Apologies for the thread title, we have this essentially inevitable problem where newspaper headlines aren’t always the best description of science and technology.

http://www.theaustralian.news.com.au/story/0,25197,24370728-2702,00.html

Australian researchers say they have created a low-cost material to filter and safely store nuclear waste. The potential breakthrough for the environment was made by a team of scientists from Queensland University of Technology, led by Associate Professor Zhu Huai Yong from the School of Physical and Chemical Sciences.

(Tip to NEI Nuclear Notes for finding the story.)

Prof Zhu said the discovery was particularly important as the world increased its reliance on nuclear energy.

“You have to keep nuclear waste somewhere for hundreds of years,” Dr Zhu said.

“Water is used to cool nuclear reactors and during the mining and purification of nuclear material, so waste water is a big problem.

“For example, there is a lake in the United States filled with millions of gallons of nuclear waste water.”

But if the waste was stored conventionally in lakes or steel containers, there was a danger it could leak and pollute the land around it.

Well, I’m not so sure about what he’s talking about here. I seriously hope they’re not talking about water used at nuclear generating stations as “nuclear waste”, for example – clearly Lake Anna in Virginia, for example, does not contain millions of gallons of “nuclear waste”, or anything of any radiological significance at all, for that matter.

Alternatively, perhaps they’re talking about “tailings” waste from uranium mining and extraction? I’m not sure, but in any case, I’m not particularly concerned about waste from uranium mining and extraction, since it contains naturally occurring radionuclides such as radium which occur naturally in the ground, in exactly the same amounts that they’re naturally present in the earth, which can be put right back into the earth.

Professor Zhu said the QUT team had discovered how to create nanofibres, which are millionths of a millimetre in size and can permanently lock away radioactive ions by displacing the existing sodium ions in the fibre.

“We have created ceramic nanofibres which attract and trap radioactive cations (positively charged ions), possibly forever,” he said.

“The ceramic material can last a very long time, much longer than the radioactivity of a radioactive ion.”

Ceramic was also more chemically stable than metal, could last much longer and was much cheaper to make than steel.

The ceramic nanofibres were made from titanium dioxide, a mineral found abundantly in Australia and used to colour white paint.

The fibres were mixed with caustic soda and heated in a laboratory oven to make the ceramic material.

The nanofibres, which are up to 40 micrometres in length, look like white powder to the human eye, Prof Zhu said.

“The fibres are in very thin layers, less than one nanometre in width, and the radioactive ions are attracted into the space between the layers,” he said.

“Once the ceramic material absorbs a certain amount, the layers collapse to lock the radioactive ions inside.”

It sounds like they’ve created an artificial ceramic nanomaterial based on the well-studied chemistry of perovskite materials, in this case saturated with Na⁺ ions, forming something like Na₂Ti₃O₇ or Na₂TiO₃, perhaps.

Perovskite materials can contain all kinds of different anions – positively charged ions, usually metals – bound up inside their crystal structure. Strontium titanate and barium titanate are both well-studied materials with interesting properties. Strontium titanate is a common chemical form in which radioactive ⁹⁰Sr is used when it’s packaged in sealed radioactive sources, such as the very large ⁹⁰Sr sources that form the basis of ⁹⁰Sr radioisotope thermoelectric generators, because it’s essentially a completely inert, insoluble solid ceramic material from which the radionuclide cannot leach, dissolve or otherwise escape.

These perovskite-type materials, and their ability to “lock up” essentially any alkali metal or alkaline earth metal, as well as transition metals, actinides and just about any metal, really, inside their crystal structure is well studied technology with regards to the disposal of radioactive waste and radioactive byproduct material, both fission products and plutonium-contaminated defence wastes, and indeed, these materials are the fundamental basis of the use of Synroc type materials for radioactive waste immobilisation.

Now, it’s difficult to speculate, based on nothing more than a newspaper report, but I’m guessing that where such a perovskite material is turned into a nanomaterial, with nanoscale structure, and loaded with sodium ions, you end up with something that is kind of like an interesting inorganic ion-exchange material. (Ion-exchange materials are of course an important part of nuclear chemistry, both for recovering and recycling nuclear materials, and decontaminating waste materials.)

If you take some waste water that is contaminated somehow, say, with nuclear fission products like ⁹⁰Sr or ¹³⁷Cs, (and providing it’s not economical to recover such materials for useful purposes), I suppose you can simply run it through a bed of this material, and those ions (Sr²⁺ and Cs⁺, assuming that they’re in soluble forms) become permanently substituted into the perovskite. Then, this material can be sintered, and you have the radionuclide contamination converted straight into a chemically inert, insoluble ceramic. Unlike an ion-exchange material, however, once those cations are in the material, the idea is that they will be permanently retained there.

To paraphrase Bernard Cohen, you can take this material, if you want to dispose of it, turn it into a rock, then take the rock and put it in the rock’s natural habitat, deep inside the earth.