Field of Science

Chemistry, fluid dynamics and an awful radioactive mess

When it comes to handling radioactive waste the Hanford site in western Washington state is the opposite of a role model. Ever since its reactors started producing the plutonium which was used in the Nagasaki bomb, Hanford has been generating waste with little foresight and responsibility. It has the dubious honor of being the most contaminated radioactive site in the country.

Scientific American has an article which gives an idea of how truly awful the problem is. It's not just that there's a lot of waste or that it's everywhere. It seems like the waste basically conforms to the devil's definition of the word "heterogeneous" and takes a form representing the average nuclear chemist's version of hell:
"Overall, the waste tanks hold every element in the periodic table, including half a ton of plutonium, various uranium isotopes and at least 44 other radionuclides—containing a total of about 176 million curies of radioactivity. This is almost twice the radioactivity released at Chernobyl, according to Plutopia: Nuclear Families, Atomic Cities, and the Great Soviet and American Plutonium Disasters, by Kate Brown, a history professor at the University of Maryland, Baltimore County. The waste is also physically hot as well as laced with numerous toxic and corrosive chemicals and heavy metals that threaten the integrity of the pipes and tanks carrying the waste, risking leakage. 
The physical form of the waste causes problems, too. It’s very difficult to get a representative sample from any given tank because the waste has settled into layers, starting with a baked-on “hard heal” at the bottom, a layer of salt cake above that, a layer of gooey sludge, then fluid, and finally gases in the headspace between the fluid and the ceiling. Most of the radioactivity is in the solids and sludge whereas most of the volume is in the liquids and the salt cake."
"Plutopia", by the way, is a very interesting book. In any case, the waste problem at Hanford looks like it will engage the services of every conceivable kind of chemist, engineer and fluid dynamics expert that I can imagine.
"All of these considerations contribute to the overall problem, which can be summed up in one word: flow. To get to the glass log stage the waste has to travel through an immense labyrinth of tanks and pipes. It has to move at a fast enough clip to avoid pipe and filter clogs as well as prevent solids from settling. This is quite a challenge given the multiphasic nature of the waste: solids, liquids, sludge and gases all move differently. The waste feed through the system will be in the form of a “non-Newtonian slurry”—a mixture of fluids and solids of many different shapes, sizes and densities. If the solids stop moving, problems ensue."
The article also talks about two serious concerns; the possibility that enough plutonium in the waste could build up to trigger a chain reaction (although one which in bomb parlance would be a "fizzle") and the possibility that the heat and radiation could split water up and lead to a buildup of hydrogen. For now these concerns are about unlikely events and are secondary in any case to the much more important problems of Sludge Management and the Battle against Viscosity. Just tells you how important it is to nip problems with reactor waste in the bud before they turn into a godforsaken headache for future generations.

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