Microbial Janitors Tackle Nuclear Clean-Up Problems

Researchers from the Department of Energy's Idaho National Engineering and Environmental Laboratory (INEEL) in partnership with British Nuclear Fuels, plc (BNFL) are launching a yearlong test of a microbial decontamination technology at a nuclear reactor at the Sellafield reprocessing plant in the UK. The technology will be used to remove surface contamination on a concrete wall at the reactor in a proof-of-concept test run.

The microbes' job is to eat and be merry -- it's the corrosive microbial by-product that does the researchers' work. As the microbes metabolize an elemental sulfur nutrient source, they create sulfuric acid. The sulfuric acid etches the concrete surface and loosens the contaminated layers. When enough concrete has been loosened, researchers put the kibosh on the microbial feeding frenzy by dropping the humidity, causing the microbes to die off (the microbes cannot survive in ambient humidity). Then they vacuum the degraded concrete bits off the walls, ceiling or floor, and dispose of it.

Currently, a common treatment for radioactively contaminated concrete involves chipping it away until workers reach 'clean' concrete. This means the workers, hot and sweaty in full anti-contamination suits and respirators, are being exposed to radiation as they work. And they still have a substantial pile of radioactively contaminated concrete rubble and dust to dispose of when they're done.

In some cases, radiation fields may be too high for human exposure, even for a few minutes, or the configuration of the space may not allow safe human access at all. Then, the only alternative for facility operators may be to fill the room with cement, do nothing, or demolish the entire building and treat all the rubble as radioactive waste - rather like throwing away a pear because the skin is dirty. Biodecontamination significantly reduces the cost of cleanup and risk to workers.

In this new technology, researchers whip up a batch of microbes and cellulose with a blender, and spice it with a dose of elemental sulfur. The end result is a vibrantly yellow gel with the consistency of the lemon curd you spread on a tea-time scone. The sulfur not only feeds the microbes, the bright color makes the gel easy to see, a plus when applying it to a surface for treatment. Researchers spread the gel on the concrete and then raise the humidity around the gel to 95 percent. As long as the humidity and temperature are controlled, the microbes are able to work. To kill the microbes off, researchers need only drop the temperature and lower the humidity.

The microbe of choice for this technology is Thiobacillus thioxidans. "I've been working with these little critters for eight years," said Melinda Hamilton, INEEL biologist and technical lead for the research team. She started out studying how soil microbiology affects the stability of buried cement waste forms. The research team quickly realized that the problem of microbial degradation could be put to good use actually treating contaminated concrete. This technology could significantly reduce waste treatment costs and would improve personnel safety over current methods, particularly for radioactively contaminated facilities.

This technology elegantly transforms a naturally occurring, damaging activity, into a useful technology. Thiobacilli are one of three types of bacteria known to cause microbially-influenced degradation (MID) of concrete. "Mother Nature is by far the best bio-engineer; we're just beginning to realize the wealth of naturally occurring microbes provided," said LaMar Johnson, manager of the Biotechnology Department at the INEEL. The same microbial action at work in this technology is affecting the infrastructure of our society every day, from concrete bridges to canals and waterworks, to septic systems. "You can see this phenomena all around you -- you just don't realize it," said Johnson. "We've just optimized what is happening."

At first, the research team tried using gases and liquid sprays to apply the microbe/sulfur mixture. But that meant either airborne hazards or drippy run-off, both of which created waste streams of their own, required extensive containment precautions, and multiple applications. And it was difficult to use elemental sulfur - the easiest form of sulfur for the microbes to metabolize - with either of those application methods.

The research team had a brain-storming session and created a wish list of the features they wanted the technology to have. Stickiness was a big factor -- researchers want to be able to apply the microbial colonies to any surface, including ceilings, without having it drip off. The microbes themselves produce a natural adhesive, but the population of microbes needs to be high, and the matrix suspending them has to be able to 'carry' enough food. The idea of a thick gel became immediately obvious. Researchers are now using inert cellulose as the base of the gel. An off-the-shelf, commercially available product itself, the cellulose has also been used as a thickener in cosmetics and foods.

Aside from stickiness, the other crucial factor was the ability of the matrix to sustain a high moisture level. The microbes need a moist environment to survive. The Thiobacilli can be purchased from microbial culture collections, or cultured in the laboratory.

In laboratory tests, researchers have removed as much as 10-12 mm in a 12-month period of time using this technology. The depth of contamination on the wall at the Sellafield reactor is anticipated to be around 2 to 3 mm - about the thickness of three dimes stacked on top of each other. The INEEL and BNFL have been collaborating on this technology since 1995, and ultimately hope to use it to clean up radioactively contaminated concrete at nuclear facilities worldwide.

Before the microbes create the sulfuric acid, the gel is completely safe to handle. "The microbial gel itself has a low pH - about the same as beer," said Tim Milner, research and development project manager for the Decontamination and Decommissioning group at BNFL Sellafield.

"The gel is nontoxic, and unless you're allergic to sulfur, you could touch it and there wouldn't be a problem," said Hamilton.

The research team jokes about the "geologic" pace at which the microbes do their work. "It does take some time," said Johnson. "That can be hard for us to take in this computer-oriented, fast-paced world, but the technology does offer a good solution for some of the decontamination problems we just can't solve otherwise."

According to Hamilton, time frames can be very long when you're dealing with decontaminating and decommissioning facilities. Once a contaminated building is taken out of service, it will usually sit idle for five to ten years before it moves to the top of the funding priority list for clean up. Even then, permitting and authorization to begin treatment and disposal can take years.

The Sellafield Windscale Pile 1 reactor, built in the 1940s and operated until 1957, was the first reactor built in the United Kingdom. When the reactor was in operation it was air cooled by six massive engines. The air was vented up through the reactor chimney. The portion of the wall and ceiling that will be used in the test is located in the access walkway to the reactor chimney.

Researchers will start with a microbial colony population of about one million cells per square millimeter. "We start with a much bigger population of microbes than we really need, because you have some die off when you first apply the gel," said Hamilton. The research team plans to use only one application of the gel for the duration of the Sellafield proof-of-concept test. The concrete test surface will be evenly coated with several millimeters of the gel and then enclosed in a containment system that allows the humidity and temperature to be carefully controlled.

BNFL has customized a portable modular containment system to control the humidity around the gel applied to the concrete test site. "We're actually using a humidifier that you could buy at Wal-Mart for this test," said BNFL's Milner, "although one of our engineers has souped it up a little bit." The simplicity of the technology is part of what makes it so appealing to the research team -- it is easy to apply, requires low maintenance during the activation period, is safe to humans and the environment and produces no toxic waste streams.

BNFL has conducted laboratory tests of the technology at one of its sites at Capenhurst, UK, and has spent at least eight months pursuing clearance to conduct the field scale test at Sellafield. BNFL voluntarily included the Nuclear Installation Inspectorate (NII), the UK version of the Nuclear Regulatory Commission, in the approval process to ensure that all safety concerns were adequately addressed. Long term, this will be an important step for the eventual wider proliferation of this technology.

BNFL will use data from the Sellafield reactor test to create a baseline for cost and treatment time. Based on that information, the company will develop a marketing plan for the technology. "It's always hard to predict when or how fast a technology will go to commercialization," said Milner, "but I would really expect to see a product within the next couple of years." BNFL hopes to apply the technology at nuclear facilities worldwide, particularly in European countries struggling with the legacy of aging nuclear power plants.

This research has been funded through the INEEL's discretionary research program, the Department of Energy Environmental Management Program, and by BNFL through a cooperative research and development agreement.