Tuesday, October 27, 2009

Unaccounted For

A few months back, I had a bright idea. Every year or two, a report on plutonium accounting is released by the government and, predictably, the media have sport for a day or so with the idea that, because there is inevitably a discrepancy, that discrepant plutonium must have made its way into terrorists’ hands. Or perhaps the material is being hidden away for no-good purposes or is residing in the reservoir from which you (you!) get your drinking water.

Actually, the numbers tend to be within the bounds of accounting errors and the material that, in any processing operation, gets stuck in the pipes. The bureaucratic term for the discrepancy is material unaccounted for (MUF).

My bright idea was that we now have a way of knowing what gets stuck in the pipes. I’m using that phrase, “stuck in the pipes,” as a shorthand for all the places where materials “disappear” to. For plutonium, it would be residues in gloveboxes, too-small-to-measure residues on wipes that become trash, and, yes, stuff that remains in glovebox and reprocessing pipes, casting and machining apparatus.

The pipes at Rocky Flats, where plutonium pits for nuclear weapons were manufactured for most of the Cold War, have been disassembled and trucked away for disposal. The material stuck in those pipes must have been measured, for many reasons, before it was trucked away.

The biggest reasons to figure out what was stuck in Rocky’s pipes are accountability and safety. Accountability knowing what you’ve got and keeping it away from the terrorists. Safety is for the workers doing the disassembly and includes the possibilities of exposure and criticality. There’s a cost issue, too. It’s slower and more expensive to do this work fully suited-up and with supplied air, not to mention uncomfortable, so it’s important to choose the amount of protective gear that is needed, but not more.

And there’s one more reason: the facility accepting the building waste is subject to waste acceptance criteria describing the kinds and amounts of hazardous and radioactive wastes it can accept.

I googled up the reports on Rocky’s cleanup and e-mailed to several people who might know where to find the accounting of the plutonium that was in Rocky’s pipes. Nada in the reports, and my correspondents didn’t recall that anyone had bothered to do those measurements, although they too thought they probably should have been done.

But Rocky isn’t the only place that processed nuclear material that is being decommissioned. K-25, Oak Ridge’s gaseous diffusion uranium enrichment facility, is being taken down. Frank Munger regularly supplies photos to Manhattan Project junkies. I’ve stolen one of them for the top of this post.

And DP Site at Los Alamos, where plutonium processing was done, is about to be taken down.

Munger is asking how much U-235 was left in the pipes at K-25. And he tells me he’s not getting any answers either. He’s wondering whether a critical incident, where enough fissionable material collects in one place to give a fast-neutron burst and possibly an explosion, is likely in the parts of the plant where the U-235 was most concentrated.

It’s not clear to me how much of the piping that contained the uranium is left in the building. The asbestos would have been removed separately. There was quite a to-do for a while about what to do about the nickel from the diffusion barriers at K-25. I don’t recall what was eventually decided, but if it was sold separately, the barriers and part of the piping have been removed. It might make sense to remove and dispose of the rest of the piping separately from the rest of the building. The piping, being more highly contaminated, would need a different sort of disposal from the rest of the building materials, which might even be able to go into a normal landfill if they are uncontaminated.

My guess is that a critical incident is unlikely during K-25’s demolition. K-25 didn’t do the full enrichment; the material was finished by electromagnetic separation, so any U-235 remaining in K-25’s pipes still contains some U-238. As long as the uranium is a solid crusted on the pipes, any collapse would contain steel and other things that would physically separate the U-235 and absorb neutrons to prevent a chain reaction. Presumably someone has done a more complete criticality analysis with real numbers.

The uranium at K-25 is probably in the form of uranyl fluoride, which would be a solid crusted on the pipes. But uranyl fluoride is highly soluble in water. I can’t find a number for its solubility, but uranyl chloride is ‘way more soluble than table salt.

Munger notes that the Nuclear Regulatory Commission is seeking comment on an exemption for EnergySolutions’s Utah landfill. Whether or not it is the destination for the K-25 waste is not clear, but EnergySolutions is asking for an exemption for 100-ton shipments of piping in railcars, each containing up to 7.9 pounds of U-235 in the form of uranyl fluoride. That’s about 6 pounds of U-235 per railcar.

So let’s say that somehow the piping is exposed to water. I’m sure it will be in covered railcars and that every attempt will be made to keep water out of the burial pits. But if somehow the uranyl fluoride gets dissolved in water and that water drips to the bottom of the railcar or burial pit, a criticality accident becomes more likely. Water is a very good neutron moderator and decreases the critical mass of fissionable material. But the Nuclear Regulatory Commission did an environmental impact analysis and found no significant impact. I’m wondering if criticality incidents are a part of environmental impact analyses.

The issue of water potentially causing criticality incidents has arisen before in environmental cleanups, at least twice: for a sand filter backwash pit at Hanford, and for the underground chambers at Technical Area 49 at Los Alamos. I believe that the Hanford pit has been cleaned up, and the surface at TA-49 has been stabilized to a point where water infiltration is unlikely. (Btw, TA-49’s history is quite interesting; that link is worth clicking on – short pdf.)

The prospect of criticality from transport and burial of K-25 piping seems small in comparison to those two situations.

On DP Site, I’m a bit less sanguine. I brought up the MUF question at a meeting a few weeks back, and a fellow who claimed to have written the most recent workplan for demolition of DP Site said that there was no concern for the amounts of fissionable material in the buildings, just a great urgency to make the demolition happen. I challenged this statement, but he was adamant. Worker safety and waste acceptance criteria still exist, even for DP Site.

So I’m wondering on what basis that fellow made that statement. Bravado with little information? Setting out on an enterprise he doesn’t understand and will get whacked back by the reality of waste acceptance criteria? Or is somebody going to slip something past the public and maybe even the Department of Energy? If the last, I sure wouldn’t have bragged about it in public, but I have no way of knowing.

Back to my original point. It appears that the amounts of fissionable material stuck in the pipes of those old facilities now being demolished are not available to the public. It could be that the Department of Energy doesn’t like to admit that its past operations have been less than perfect and doesn’t want those numbers out. I would think that matching those numbers to the MUF numbers would be a public confidence-builder, unless, of course, they’re wildly off in either direction. It’s hard to see how they wouldn’t have those numbers, but I have no way of knowing this either.

Update (10/29/09): Munger has asked the DOE again, and they say that the amount of U-235 inside K-25 is Official Use Only, not to be released to the public. Another opportunity to gain credibility refused by the DOE.


Kirk Sorensen said...

Fluorides tend not to be water-soluble, yet chlorides are very water-soluble, which is the why the biochemistry of your body is based on chlorides instead of fluorides, despite the fact that fluorides are more abundant in nature.

Cheryl Rofer said...

You're correct in general. But I've worked with uranyl fluoride, and it goes into solution more easily than anything else I've seen, including sugar and salt.

MT said...

Well, this is sure going to save me trips to the dry cleaners. Thanks, Cheryl!