HANFORD -- The Department of Energy is proposing extending a chemical barrier along the Columbia River at Hanford after a pilot project successfully trapped radioactive strontium before it entered the river.
At the same time, a system to pump contaminated water out of the ground and treat it, which had disappointing results, would be torn out.
DOE has been testing the chemical barrier technology since 2005, with the most recent results showing a 90 percent reduction in strontium contamination in ground water, according to DOE.
The test area extends 300 feet along the Columbia near Hanford's N Reactor, but DOE is proposing extending the chemical barrier to 2,500 feet to span the width of the area where strontium exceeds drinking water standards in ground water near the river.
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Rather than removing the strontium, the chemical barrier is injected into the ground and binds it in place along the riverbank while its radioactivity decays naturally.
The barrier is made by injecting chemicals to form calcium phosphate, also called apatite, about 40 feet deep near the riverbank.
The calcium is mixed with citrate to keep it from binding with the phosphate before it can spread through the ground into a barrier, an initial problem that had to be overcome. After the chemicals are injected into the soil, bacteria consume the citrate, allowing the calcium and phosphate to combine into apatite, a substance found in bone and teeth.
When strontium carried by ground water hits the barrier, it chemically binds. The combination forms a crystal with the strontium held within as its radiation naturally decays.
Half of the radioactivity of strontium decays every 28.6 years. With some of the strontium already in the ground for 30 years, scientists are figuring it will require another 270 years for the strontium to decay to meet drinking water standards.
DOE estimates the cost of building and maintaining the 2,500-foot chemical barrier would be $21 million. That includes about $680,000 to take out the pump and treat system previously used to protect the Columbia River near N Reactor.
The pump and treat system that was temporarily shut down in 2006 after a decade of use cost up to $1 million a year to operate. It was started as a stopgap method for lack of a better technology and did help reduce the flow of ground water into the river that can carry along the strontium.
But the system removed about 10 times fewer curies per year than the amount removed by natural radioactive decay.
Strontium contamination at Hanford along the Columbia River is of particular concern near N Reactor, where it is more concentrated at the river shore than at other reactors. N Reactor was the last of Hanford's nine plutonium-production reactors and used a newer design for its cooling system when it operated from 1964-86.
Its heat-exchange cooling system passed the same water through the reactor about 100 times, rather than the single pass made by cooling water in Hanford's older reactors. That was intended to reduce the discharge of water contaminated with chemicals such as hexavalent chromium to the environment.
But the multiple passes meant the cooling water had higher concentrations of some radionuclides than the water discharged from other reactors.
When N Reactor was irradiating fuel to produce plutonium for the nation's nuclear weapons program, up to 2,000 gallons per minute of contaminated water was discharged into soil near the reactor. Until strontium was detected at the shoreline in 1985, water was discharged as close as 800 feet from the river.
The discharges left an underground plume of about three-fifths of a square mile with strontium contamination that's 1,000 times the drinking water standard. Because the human body cannot distinguish strontium from calcium, it can replace calcium in the bones with cancer-causing strontium.
DOE is accepting public comments on its proposal to extend the apatite barrier to trap the strontium before it makes a decision.
The Washington State Department of Ecology, the regulator on the project, supports the proposed plan, according to DOE's proposal.
Public comment can be made until July 22 by e-mail to 100NRPP@rl.gov or by mail to Paula Call, U.S. DOE, Richland Operations Office, P.O. Box 550, A7-75, Richland, 99352.
* Annette Cary: 509-582-1533; email@example.com; More Hanford news at hanfordnews.com.