Can you safely evaporate radioactive water?

October 27, 2012 5:54 PM

When radioactive water is evaporated, where does the radioactivity end up?

One of the ongoing issues in the extended cleanup of the Fukushima nuclear problem is the question of how to dispose of the huge amount of radioactive water that is stored on the site. They are now sitting on more than 200,000 tons of water that has been run through the damaged reactors for cooling, and are running out of space to store it while waiting for their water treatment capability to come online.

Why can't they simply leave the storage tanks open to the sky and let the water evaporate bit by bit? Surely the molecular components (hydrogen and oxygen) that are released into the air by evaporation are not radioactive in themselves? The water left behind would presumably become gradually more and more 'concentrated', but wouldn't that solve their storage problem?

posted by woodblock100 to science & nature (12 answers total)

If the water itself is radioactive (contains tritium), then it evaporates into radioactive water vapor. But I imagine the concern is radioactive substances dissolved in the water. They would be left behind when the water evaporated, unless they are volatile enough to evaporate too.

The real issue though is that evaporation is very slow and as you said, they are running out of space. Also you'll get birds landing on the radioactive pools and becoming contaminated, etc.
posted by ryanrs at 6:02 PM on October 27, 2012

Both hydrogen and oxygen have radioactive isotopes (tritium being the longest-lived), so it's possible for the water vapor itself to be radioactive. But I think ryanrs is probably right, the contamination is other stuff dissolved in the water, and tritium evaporation isn't the main problem.
posted by hattifattener at 6:20 PM on October 27, 2012

I should perhaps have included a link to a Japan Times story about the water problem in this morning's newspaper.
posted by woodblock100 at 6:27 PM on October 27, 2012

Heavy water has a very slightly higher boiling point, and it's one of the ways of extracting/concentrating it. It's a very gradual, energy-intensive process that needs lab-grade oversight. Fukushima has far too much impure waste water to be doing this with.
posted by scruss at 6:28 PM on October 27, 2012

Doing what with, scruss? Is there some reason you think the problem is tritium-contaminated water and not other things dissolved in water? (If it were tritium, that water would probably be a valuable industrial material, actually.)

The article woodblock100 links to has very little information on the contamination except for one paragraph which mentions

new purifying equipment using Toshiba Corp. technology that is supposedly able to decontaminate the water by removing strontium and other nuclides

So presumably most of the contamination is dissolved heavier elements, not isotopes of H and O.
posted by hattifattener at 6:50 PM on October 27, 2012

Water vapor is whole molecules of water (two hydrogen and one oxygen atom). So water that has been tritiated or deuterated (where the hydrogen atom gets one or two neutrons added to the usual proton) will evaporate more or less like ordinary water. The thing is, deuterated water (aka heavy water) is not even radioactive and is almost entirely harmless, unless you ingest a lot of it. Tritiated water is mildly radioactive, but is harmless externally and excreted very quickly if ingested. The risk from radioactive water is pretty small.

It's the stuff in the water that's the issue. All kinds of nasty radioactive isotopes get formed when you expose them to neutrons, in addition to the fuel of the reactor itself, which is highly radioactive. And because they were trying to avert a worse disaster by cooling the reactor, there is a lot of water with that stuff in it. Exposing those to the air would be foolhardy.
posted by wnissen at 6:59 PM on October 27, 2012 [1 favorite]

I think the jist of this question has been slightly missed by answerers. It's not the worry that the water itself is radioactive, but rather why it can't be safely evaporated to leave radioactive "dregs" of the contaminants, whatever they might be. The harmless water would float away, and the dregs would thus take up less room, meaning that there was no longer a storage worry.

I'm not a scientist, but I think the answer may well be that you can't be sure only water would evaporate. There may be other elements or compounds in the water that would evaporate too, meaning that you would effectively create radioactive rain elsewhere. Most of the stuff likely wouldn't evaporate, but it only takes a little to create a new problem. Radon is part of the decay chain of uranium, and is always a gas. Astatine also is in the decay chain, and may be fairly likely to evaporate.
posted by Jehan at 7:38 PM on October 27, 2012 [1 favorite]

No, I don't think the Fukushima waste water is just tritium and deuterium contaminated. I was just saying that evaporation wouldn't work here.
posted by scruss at 8:12 PM on October 27, 2012

wnissen: Tritium is actually pretty bad, being an alpha emitter. Once those get past your protective layer of dead skin they can do a lot of damage. I know workers at AECL have to wear respirators to protect against inhaling tritium when working in one area.
posted by Canageek at 11:11 PM on October 27, 2012

I'm not a scientist, but I think the answer may well be that you can't be sure only water would evaporate.

This is one concern - some of the nasty stuff may be volatile, and may escape the ponds along with the water. Leaving the containers open to the environment is asking for trouble anyway: What happens if it rains and they overflow? Like ryanrs said, you will contaminate local wildlife, and also get solid detritus falling in the ponds that you now need to dispose of.

Another thing that hasn't been mentioned is that concentrating the waste in this manner doesn't necessarily make things easier: if the activity is contained in a much smaller volume, dose rates close to the waste are increased, making it more dangerous to handle. Passing a few thousand tons of low-activity water through a cleaning facility in a controlled manner may ultimately be easier, cheaper and safer than scraping high-activity muck off the bottom of 50 olympic-sized pools.
posted by Dr Dracator at 11:38 PM on October 27, 2012 [4 favorites]

I don't think you're going to get a lot of volatile contaminants in the cooling water at a nuclear power plant, so I think the issue radioactive release with simple evaporation is probably not a big deal. The issue is one of engineering.

There are filtration systems out there that can get you amazingly close to distilled water for a much lower energy investment than driving off the water with heat, and with a much lower contamination footprint than building a giant evaporation lagoon. Safeguards would have to be built in since it is potentially embarrassing (and fatal) to concentrate radioactive materials too much.

Astatine has a very short half-life and, ergo is unlikely to be much of an issue.
posted by Kid Charlemagne at 1:42 AM on October 28, 2012 [1 favorite]

Actually on further consideration I recall that they in fact do use controlled evaporation to concentrate liquid waste into solid blocks, which are safer to store. I'm sure you could look up the details in scientific journals. I imagine in this you run the evaporate through some scrubbers though.
posted by Canageek at 7:56 AM on October 28, 2012