Twenty-five years after the tragic runaway fission and fire at Chernobyl, tons of concrete shield workers and visitors from the dangerously radioactive puddle of melted fuel that lurks in the basement of the building housing reactor No. 4. Similarly, more than 30 years after the partial meltdown at Three Mile Island in Pennsylvania, concrete shaved 2.5 centimeters deep guards a hollow reactor vessel, its partially melted down fuel rods having been taken out over the course of a decade and shipped to Idaho National Laboratory (INL) for study. And now, nearly two months after the serial partial meltdowns in the reactors and spent-fuel pools at Fukushima Daiichi, Tokyo Electric Power Co. (TEPCO) has announced at least nine months of work to control the nuclear accident—the initial part of a cleanup that could last for decades.

"It's like mining," says nuclear physicist Douglas Akers of INL of the Three Mile Island cleanup effort. "Go in and remove the previously molten fuel, pack it up in shipping containers, and remove it to Idaho."

What happens when nuclear reactors reach the end of their useful lives—either accidentally as in the cases above or as a planned shutdown for a series of power plants throughout the U.S. and the world, more and more in coming decades? The answer to that question ranges from a green field suitable for farming to sacrificial zones that, in effect, become nuclear parks, such as the 25-square-kilometer Rocky Flats National Wildlife Refuge in Colorado—a former bomb-making site—or the 30-square-kilometer "exclusion zone" surrounding Chernobyl, respectively.

"Today we know that about 77,000 square miles of territory in Europe and the former Soviet Union was contaminated with radioactive fallout, leaving long-term challenges for flora, fauna, water, the environment and human health," wrote Mikhail Gorbachev, the Soviet premier at the time of the Chernobyl explosion, in Bulletin of the Atomic Scientists's March/April 2011 issue. "Tens of billions of dollars have been spent in trying to contain and remediate the disaster," including a massive sarcophagus currently being constructed to re-entomb the melted radioactive fuel.

The cleanup at Fukushima Daiichi will face similar challenges, including ascertaining how much of the nuclear fuel melted down and how bad is the radioactive contamination on the power plant grounds—as well as in surrounding areas. That challenge is exacerbated by the fact that three reactors and two spent fuel pools have been affected by the crisis but are also surrounded by five further spent fuel pools and two more unaffected reactors—and the fact that the fuel rods remain uncooled even today. Step one will therefore be cooling the nuclear fuel, a process that could take at least three months per TEPCO's current plan, which involves entirely filling the stricken reactors with seawater—although leaks may foil this plan.

View a slideshow of the plight of reactors at Fukushima Daiichi

"You have four reactors and you could easily have two or three approaches to decommissioning," says Kurt Kehler, vice president of decommissioning and demolition at engineering company CH2M HILL, whose company might bid for the job. "Some where the fuel melted, you entomb. [There are] some where you can extract it and go into safe store, and then some you could recover and keep operating."

Like many of the personnel operating the U.S. nuclear fleet, the name for the end-of-life process for a nuclear power plant got its start in the U.S. Navy—to decommission a reactor is to tear it down and restore its site to one of several conditions within 60 years.

"Ideally for most utilities the intent is to remove everything from the site and restore it to other uses than power generation," says John Hickman, project manager of the reactor decommissioning branch at the U.S. Nuclear Regulatory Commission (NRC). Removal targets radioactive contamination, be that the radioactive cobalt and cesium that typically seeps into the concrete as a result of normal operations or plumes of water contaminated with tritium—the radioactive form of hydrogen—that often leaks from such power plants over the course of decades. Even so, tearing apart a nuclear reactor does not mean undue exposure to radioactivity; many forms of shielding—from concrete to cooling water itself—remain to protect disassembly workers as well as specially designed tools, such as particle-containment boxes for sawing through radioactive metals.

Right now, at least five such nuclear site decommissions are underway in the U.S., ranging from Zion nuclear power plant in Illinois to the cleanup of a sprawling nuclear bomb–making site at Hanford in Washington State. In the case of Zion nuclear waste disposal company Energy Solutions has actually taken ownership of the former power plant, through a subsidiary, and will tear it down in its entirety and ship all of the resulting waste to Utah for disposal. Specially designated landfills, such as Energy Solution's site in western Utah or Barnwell nuclear dump in South Carolina, hold the radioactive remnants from such deconstructions. "The current estimate for the Zion facility—and the amount in the [decommissioning] trust fund—is $900 million for this two-unit site," Hickman says. "The volume of material is enormous compared to past decommissions."

Energy Solutions takeover is a response to prior decommissionings, such as Maine Yankee in the 1990s, which rapidly grew in expense as workers attempted to sort radioactive material from its nonradioactive counterparts. Like Zion, the Maine Yankee decommission aimed to restore the site to a pristine condition, one in which a farmer could live on the former nuclear power plant grounds, grow crops and eat them without an "unacceptable dose" from any remaining radioactive contamination, says NRC spokesman Scott Burnell.

There are other options, of course, such as Sacramento's former Rancho Seco nuclear power plant shut down in 1989, which has now been turned into a solar farm and natural gas–fired generator. "Our regulations require cleanup of the facility to the point that when they are done you can only be exposed to 25 millirems per year from [nuclear power] plant–generated materials," Hickman says of that option. (A rem is a dosage unit of x-ray and gamma-ray radiation exposure.) And, in the case of Three Mile Island (TMI), reactor No. 2 will sit dormant until reactor No. 1 finishes its useful life, in 2034, when both will be demolished. "The advantage of doing it at that time is that any residual radioactivity associated with TMI reactor No. 2 will have decayed quite a bit," says Wayne Johnson, division manager of the Earth Systems Science Division at Pacific Northwest National Laboratory.

And there is often something left behind, at least in the U.S. where there is no long-term repository for spent nuclear fuel. As a result, on each and every decommissioned nuclear power plant site sit some number of concrete and steel casks encasing the used uranium fuel rods. For example, the Humboldt Bay nuclear power plant being torn down in California has a concrete pad with such cylinders sunk into it so they cannot tip over in an earthquake. In addition, the pad is situated on a hill high enough to avoid a tsunami. "The safety briefing is if the tsunami warning goes off, then run to the top of where the spent fuel is because that's the highest ground around," Hickman says.

In the case of Fukushima, it will be years before the uranium fuel rods—if not too melted and deformed—are cool enough to be shifted to such long-term storage. If the fuel rods are more melted, however, the cleanup will be even more challenging, particularly if they have formed a "puddle". "You can't very easily cool it off because you can't get water to the center," notes nuclear physicist Douglas Akers of INL.

That makes it nearly impossible to remove, hence the puddle that sits in the basement of Chernobyl. "Once you get fuel that is deformed or spread like in Chernobyl, the risk to the worker and engineering controls that need to be put in place to protect the worker become very expensive," CH2M HILL's Kehler explains. "If there is a meltdown that is that extreme, then you are looking at an entombment state for a number of years." It remains unclear exactly how melted the fuel in the three damaged reactors and two crippled spent-fuel pools at Fukushima Daiichi are, although U.S. Secretary of Energy Steven Chu estimated that as much as 70 percent of the fuel rods may have melted.

At the same time, the puddle is relatively impervious. "It's ceramic armor surrounding fission by-products," Akers says.

And then there are the sites that are really contaminated, like Hanford in Washington State—some 1,500 square kilometers laced with the residue of U.S. bomb-making operations stretching back to World War II. "You hope that whoever left it for you cleaned it out," says Kehler, whose company is tasked with cleaning the site. "Sometimes they did and sometimes they didn't."

Nuclear accident
Of course, even unregulated military sites for the production of nuclear weapons often made at least some attempt to contain radioactive material, including some of the most polluted sites in the world, which Soviet technicians contaminated in pursuit of plutonium. That is not the case for unplanned meltdowns, like the ones that ended the use of TMI reactor No. 2, Chernobyl reactor No. 4, and now Fukushima Daiichi reactor Nos. 1, 2 and 3.

TMI was relatively easy to clean, thanks to safety systems that contained most of the radioactive material. "Only 3 to 4 percent of the reactor inventory [of radioactive noble gases*] was released," Akers says, and most of what did escape remained in the reactor buildings. "Effectively, everything producing any off-site damage was all retained in the facility."

But Fukushima and, even more so, Chernobyl saw the failure of such safety systems to contain radioactive material and—in the case of Chernobyl because of a fire—dispersal occurred over a wide area. Cleaning that is impossible: "From a soil standpoint, there really is no [in-place] treatment for radioactive contamination," Kehler explains. "It's either removal or fix-in-place and control the footprint." As a result, the amoebalike contours of the exclusion zone in Ukraine and Belarus stretch out around the remnants of the defunct nuclear power plant in Ukraine.

Something similar may happen in Fukushima given the dispersal of radioactivity, including plutonium. Already, the Japanese government has declared the region within 20-kilometers of the stricken nuclear power plant a no-go zone, enforced by fines up to $1,200 and detention. And although such plutonium is not in itself soluble in water, it is clear from studies of heavily contaminated sites in Russia, such as former plutonium-making facility known as Mayak, that plutonium and other insoluble radioactive material tends to hitch a ride on tiny particles in the soil, known as colloids.

Other radioactive material, such as cesium 137 with a half-life of roughly 30 years, dissolves like salt in water, traveling into groundwater supplies and even plants. Uptake by plants is a real concern for any nuclear remediation, given the potential for human or animal consumption, although some fungi seem to thrive on radioactive material. "The trouble with particles with any living organism is breathing it, eating it, ingesting it, getting it into the skin," Kehler says. "A little bit of contamination can give a big dose because it's going to be there [inside] for a long time."

But there are other biological responses: Cooling water from TMI became a microbial farm—as is also likely to happen with the saltwater used in the emergency at Fukushima. "They started growing algae and bacteria in the reactor core," Akers recalls. "It's like a swimming pool that nobody's cleaning up."

What to do about the water?
Of course, dealing with contaminated water is a primary concern at any defunct reactor site and the primary response is filtration, such as with ion exchange columns that employ special resins to attract the radioactive elements and pull them out of the water, often employed in series. "The residual radioactive material itself [that is caught in those filters] is then solidified in some way suitable for low-level waste disposal," PNNL's Johnson explains. French nuclear giant Areva will employ such filtration techniques to help decontaminate radioactive seawater at Fukushima Daiichi.

In the case of TMI, after the water was filtered for heavier radioactive isotopes, it was left to cool so that light radioactive isotopes like tritium—an isotope of the hydrogen in water molecules—could break down. After 14 years, 8.7 million liters of it was simply allowed to evaporate. "They had the option of either dumping it in the [Susquehanna] River or evaporating it," Akers says. "They chose to evaporate it."

Tritium cannot be separated from water and has a half-life of more than 12 years, although it emits relatively little harmful radiation when it decays. TEPCO, in the case of Fukushima, will let some of the radioactive seawater sit on its site in massive storage containers.

At the same time, TEPCO has already dumped 11,500 metric tons of contaminated seawater into the ocean. That again will make radioactive material available to sea life, potentially ending up in fish or marine mammals that feed on them, based on previous such releases in the past. That may alter such animals' genetics as well as disrupt reproduction and development, although the exact effects remain unclear for lack of study.

In the end such uptake by living organisms is the fate of all such radioactive contamination in the wild. That's why the greatest risk facing a site like Chernobyl is something as natural as a wildfire, similar to the ones that swept Russia and Ukraine this past summer. "One of the worst accidents Chernobyl could have at this point is a forest fire," Kehler notes. "All the radioactivity in the plants would then become airborne."

*Correction (4/29/11): This sentence was edited after posting to note that only noble gases escaped the reactor at Three Mile Island.