Slightly excessive contamination was reported on four trailers that carried large, empty hoppers from the Paducah plant.
By Joe Walker firstname.lastname@example.org
This is the second time since last summer that shipping of low-level radioactive waste has stopped because of regulatory snags. Shipping resumed in December once the first problem was resolved.
"Fixed" contamination — which does not rub off when merely touched — was found on four of eight trailers delivered to the Nevada test site from April 28 to May 1, said Greg Cook, spokesman for Bechtel Jacobs, the plant's lead environmental contractor.
Although the radiation was only a fraction of that of a chest X-ray, it slightly exceeded the maximum level listed in shipping documents for as many as three trailers, he said.
The trailers had carried large, empty, funnel-shaped hoppers in square metal frames that came from a closed plant building being cleaned up. The building, known as the C-410 "feed plant," is where workers once combined uranium tetrafluoride, or green salt, with fluorine to create uranium hexafluoride. That product then was fed through the piping system during the enrichment process.
Closed since 1976, the feed plant is identified in DOE reports as perhaps the plant’s most dangerous work area, because some of the uranium was recycled from nuclear reactors and contained traces of highly radioactive plutonium and neptunium.
The contamination found on the trailer surfaces was 0.7 of a millirem per hour, a measurement of radiation exposure, he said. "That is a pretty low level of radiation. A typical chest X-ray is 10 millirem."
Cook said the contamination probably rubbed off the hoppers in shipment and was ground into the wood of the trailers.
"It appears consistent with green salt," he said. "There's no reason to think the truck beds were contaminated ahead of time. We do think they were contaminated in shipment."
Personnel at the test site, an approved Department of Energy storage facility, notified Bechtel Jacobs of the problems when the last shipment arrived. Cook said some of the waste was identified as not exceeding 0.5 of a millirem, a category that does not require warning signs.
"The next highest category of waste (0.7 millirem) does require placarding," he said. "So, we may have had three trailers out of the eight that required placarding but didn't have it."
Cook said the shipments were the only ones scheduled for the test site. However, the ban will mean delays in shipping contaminated blocks of scrap aluminum and could affect early shipments of other scrap metal, he said.
"We want to be really sure we understand how we misidentified this material," Cook said. "We think we know, but we want to be absolutely certain we have all the causes of the error and are sure how we got contamination on the trailers."
Shortly before Christmas, Bechtel Jacobs resumed shipping tons of the aluminum blocks and empty contaminated ash containers from the old feed plant. The blocks were left over from smelting to recycle metal. Nothing had been shipped to Nevada since July, when a review of past plant uses of the now-banned cleaning solvent trichlorethylene determined that soil already shipped to the site should have been classified as hazardous waste.
Although the test site disposes of low-level radioactive waste, it does not accept waste that is hazardous by regulatory definition. Shipments resumed after state regulators reviewed plant test results and agreed that the soil was not hazardous.