Sandia LabNews

New technology cleans up residue from Sandia's early Cold War weapons test program

[Depleted uranium cleanup]
A CLEAN SWEEP — Anh Lai and Fran Nimick (both 6133) look over plans for cleaning up a depleted uranium site in the Tijeras Arroyo flood plain. In the background is the Thermo Nuclean rig, which uses radiation detectors, computer controls, and mechanical switches to separate "hot" soil from normal soil. (Photo by Randy Montoya)

Process removes depleted uranium left over from old weapons tests

They’d mount the weapon on a rocket-fired centrifuge and spin it like crazy, putting its systems under tremendous g-forces. They’d push, push, push the weapon, trying to discover the outer edges of its particular envelope.

The weapons used in these tests were basically the real things, except that the test engineers used depleted uranium (DU) instead of fissile material. This was back in the ’50s, and the old centrifuge used for the tests sat just above Tijeras Arroyo.

When a test was done, the engineers would discard the weapon debris — including the DU — in a gully that drained down into the main arroyo. And there the DU sat, pretty much undisturbed, for 40-some years.

In the context of weapons wastes, DU is fairly benign. The stuff buried at the Tijeras Arroyo site didn’t really pose much of an environmental threat or risk to the health or safety of anyone who might stumble on it unwittingly. But it is uranium and according to EPA and DOE guidelines, it needed to be cleaned up. In 1995, some site remediation work was done; the most obvious chunks of DU — and you can hardly miss it since it’s bright yellow — were hand-gathered. A lot of the DU at the site was inaccessible, though, covered by huge concrete slabs. More urgent clean-up tasks dictated that the buried stuff could stay buried a bit longer. So things stood until a monster thunderstorm in 1997 undermined the concrete slabs and scattered the buried weapons debris — and the DU — across the edge of the flood plain of Tijeras Arroyo.

All of a sudden, DU wasn’t concentrated in a nice neat batch under tons of concrete; it was spread out in the open. Something had to be done. The folks in Environmental Restoration for Tech Areas and Miscellaneous Sites Dept. 6133 laid out plans for cleaning up the site once and for all.

Hand-gathering not practical

Because of the way the DU is distributed, hand picking the site is no longer practical. Rather, the plan developed by project staff Anh Lai, Sue Collins, and John Copland (all 6133) called for scouring the site with a bulldozer and piling the contaminated soil in a big heap. That heap, though, measures 1,100 cubic yards, about 4,000 barrels’ worth of waste. Only a tiny fraction of that total is "hot" — to the extent that DU ever gets hot.

What the ER staff needed to do was come up with a way to dramatically reduce the volume of the contaminated soil.

That’s why Thermo Nutech is in town this month. The private company’s Thermo Nuclean radwaste technology is being used to process the DU-contaminated soil. It works pretty well. According to John, when the company is done at the site, the soil volume will be reduced somewhere between 95 and 99 percent. The technology works like this: a front-end loader dumps a bucket of dirt in a hopper. The hopper has a vibrating grid that shakes out the larger rocks and boulders. The remaining soil is moved along a conveyor belt where it is scanned by rad detectors. When the detectors get a "hot" reading — the waste criterion is a reading of 27 picocuries per gram — a computer controller operates a series of small mechanical gates that channel the "hot" soil along a separate path from that of the normal soil. The conveyor dumps the normal soil in one pile that grows fast, and dumps the "hot" soil in another pile that isn’t very big at all. Anh says the contaminated pile will be processed again to further refine it and further reduce the volume. When everything’s finished, the remaining volume of hot soil will probably fill 40 barrels, she says.

Roots at Johnson Atoll

Mike Brown, Thermo Nuclean’s on-site foreman, says the technology was developed for cleanup of weapons tests at Johnson Atoll in the Pacific about eight years ago. There’s still a Nuclean unit at the atoll; the one at Sandia is the only one in use in the continental US.

Although the Thermo Nuclean technology can be used with such contaminants as cesium, thorium, and radium, Mike says he and his four-person crew especially like working DU sites, which are more straightforward than sites with those other elements.

Anh says the cleanup of the Tijeras Arroyo site is on schedule. When Thermo Nuclean is done, she says, more soil sampling will be done to verify that the site is clean. Then John will oversee the regrading and reseeding of the site.

"When we’re done," Anh says, "it’ll look just like it did before we started."