They grapple with radioactive decay inside the reactor, massive volumes of contaminated water and soil, and hot spots found far from the restricted zone surrounding Fukushima Daiichi.
These are just a few of the major challenges Japanese officials face eight months after the world's second-worst nuclear accident.
A team of radiation experts dispatched by the International Atomic Energy Agency (IAEA), shown here donning protective suits as they examined reactor unit 3 on October 11, warned Japan to "avoid over-conservatism" in its cleanup choices. Meanwhile, the government and TEPCO (the Tokyo Electric Power Company) face constant pressure from communities and citizen groups who worry remediation efforts are not aggressive enough.
The focus of the workers has changed substantially since the first weeks after the March 11 tsunami inundated the coastal power plant, crippling its crucial back-up cooling system, and in turn causing explosions and radioactive emissions. The initial releases of radioactive material amounted to about 10 percent of the fallout from Chernobyl; still, emissions were high enough that Fukushima joined that 1986 disaster in Ukraine as the only incident to be classified a "major accident" on the internationally accepted event scale.
Now, outsiders agree that conditions inside the plant are relatively stable, but unknowns remain.
On November 2, for example, TEPCO workers detected radioactive elements that indicated bursts of nuclear fission in the Unit 2 reactor. The team injected boron water into the reactor to slow what workers feared was a runaway reaction. But Japanese authorities later concluded that spontaneous fission as part of radioactive decay had occurred, not the chain reaction of nuclear fission.
"It just shows there is still very limited knowledge of the state of the material within the reactors," says Edwin Lyman, senior scientist specializing in global security for the nonprofit Union of Concerned Scientists.
Water remains at the heart of the workers' efforts, but not in the same way as it was at the start of the crisis. "The real focus of activities has moved beyond the stabilization of the plant," says Neil Wilmshurst, vice president-nuclear for the electric industry's nonprofit research arm, the Electric Power Research Institute based in Palo Alto, California. "The challenge now facing TEPCO is the water cleanup."
Tens of thousands of tons of contaminated water collected in reactor and turbine buildings as a result of the tsunami, the injection of water to cool the reactor fuel, and rainfall through structures damaged by hydrogen explosions.
TEPCO dumped more than 10,000 tons of contaminated water into the sea before it was able to set up a large radioactive waste treatment facility in June. The system has had its ups and downs, and some worry about TEPCO's ability to treat water fast enough to avert additional overflow into the sea.
The safety of the treated water also is a public concern. One Japanese official recently went to the length of drinking water on live TV that had been treated after being taken from flooded basements of reactor buildings.
"They've started to partially decontaminate the water, but there is still a massive volume of water that needs to be decontaminated and stored for a long time," Lyman said.
Volunteers from Hiroshima, the first city to be destroyed by a nuclear weapon, have traveled to Fukushima to aid their countrymen whose lives were upended by a peacetime atomic accident. Here, they work on October 24 to remove radioactive substances from a fishing boat in Minamisoma, Fukushima prefecture.
The group, formed in August, includes former nuclear power plant workers and survivors of the U.S. atomic bombing in 1945. Their goal in the project pictured here is to decontaminate the fishing boat, which was swept to land by the tsunami, so it can be taken back out to sea.
Wilmshurst said no one can say how long the Fukushima plant cleanup will take, but it certainly won't be done in the next year. "This is going to be a number of years before this cleanup is complete."
He noted a video that circulated a few weeks ago showing the challenge of TEPCO workers navigating the debris in order to inspect the condensers in the reactors. IAEA noted in a report this week that TEPCO has had success in removing obstacles to the robots that are surveying the damage in the Unit 3 Reactor building, which allows for greater access and more detailed surveys of dose rates, environmental conditions and temperatures.
Despite the various difficulties, Wilmshurst says progress is being made on building enclosures around the damaged reactors. That will help contain radiation.
"As the cleanup continues and the access improves, more things will be discovered and there will be more understanding," Wilmshurst said. "It's a developing situation every day."
A man measures the radiation level before the start of the Fukushima prefectural high school baseball tournament in July in Koriyama, about 37 miles (60 kilometers) from the power plant.
Some sites just outside the restricted zone now show radioactive readings in line with the global average for natural or background radiation.
Others readings are still elevated. For example, at one station about 20 miles (32 kilometers) northwest of the plant, 300 microsieverts (0.3 millisieverts) of radiation were recorded over a 24-hour period between October 31 and November 1, according to data posted by the Japan Ministry of Education, Culture, Sports, Science and Technology (MEXT). While that's significantly less than the readings there in March, it is a higher dose than getting a chest X-ray every day and would be on pace to exceed the annual dose for which there is evidence of radiation-related cancer in adults.
Residents in areas with elevated levels of radiation have been advised to prepare for possible evacuation.
The fact that hot spots were detected last month as far away as Tokyo, 150 miles (240 km) to the south, "illustrates the very uncertain nature of the dispersal and deposition process of the isotopes," said Lyman of the Union of Concerned Scientists.
At least initially, the government didn't do enough monitoring to satisfy public concerns, and many of the hot spots have been discovered through surveys funded by citizen groups, Lyman noted.
"Because the dispersal is so highly variable, it takes a very intensive effort to survey every area," Lyman said. "I'm sure there are hot spots that haven't been discovered yet and there may be people at risk who don't realize it."
A recent Fukushima University survey of displaced households near the nuclear plant found that two-thirds don't trust government declarations that radiation levels are safe, and one-quarter don't want to return to their homes.
But the IAEA team report made a point to praise Japan's "impressive" system for tracking radiation levels. "The extensive, real-time monitoring system that is currently being set up and the transparent online availability of the resulting data are important measures to reassure the public and the international community," the IAEA report said.
An excavator removes contaminated soil at a park in Koriyama, Fukushima prefecture, on October 17.
Japan's environment ministry will budget 1.1 trillion yen ($14 billion) for decontamination projects by the end of next fiscal year, an agency officials recently said. Kyoto University researchers have estimated that decontaminating the soil alone in about a 200-square-mile (51,800-square-hectare) area will cost more than $13 billion.
The isotope Cesium 137 is the greatest worry because it takes 30 years for its radioactivity to halve. The Tokyo Institute of Technology has developed a method it hopes can be used for converting highly radioactive cesium in sewage sludge into a stable cesium oxide that could be stored in an intermediate-level facility.
Photograph by Tomohiro Ohsumi, Bloomberg/Getty Images
Surmounting the Waste Problem
Used radiation protection suits pile up last month at a radioactive waste dump at J-village, a soccer training center in Fukushima that has been converted into a command center in the cleanup effort.
Japanese officials announced that the national government will take responsibility of the disposal of waste with high levels of cesium in ash and sludge in Fukushima and several nearby prefectures.
Such high-radioactive ash already had been found in 42 waste disposal facilities in Tokyo, Fukushima and five other prefectures as of late August. An incinerator in Chiba prefecture 175 miles (282 kilometers) south of Fukushima was shut down because of high levels of radioactive cesium in incinerated ash.
Government officials said they would establish an interim storage facility in Fukushima prefecture within three years that has an estimated storage capacity of 19 million to 37 million cubic feet (15 to 28 million cubic meters). The government said contaminated soil, ash, and other radioactive materials would be stored at the temporary site for up to 30 years before they are moved to a final disposal site outside Fukushima prefecture.
"You can measure the environmental contamination; that is a known problem," said Lyman of the Union of Concerned Scientists. "The challenge is going to be how to set the appropriate levels to determine what areas to clean up and which areas to abandon. That could affect the long-term habitability of many of the areas."
At this point, he said, it appears that the public is leery of setting a high level for acceptable radiation.
But the IAEA team of experts, in their summary report after visiting Fukushima last month, encouraged Japanese authorities to "avoid over-conservatism" in establishing such standards. The team praised Japan for allocating resources necessary to develop an efficient remediation and monitoring program, but encouraged officials to involve more radiation protection experts in efforts to reduce exposure.
Restrictions were lifted as of November 7 on the distribution and consumption of leafy vegetables and turnips produced in specific areas of Fukushima prefecture. But tea leaves and beef from some nearby prefectures also remain on the restricted list.
Japanese officials told the recent IAEA team that the goal for farmland is to reduce the radioactive air dose level by 50 percent within the next two years.
Experiments have been conducted to determine the best way to remediate farmland. At one site, a layer of top soil with elevated levels of cesium was removed, and rice was planted. Deep plowing also has been tested.
The IAEA team said there is room for "removing some of the conservatism" during the next cropping season, based on research showing less transfer of radioactive cesium from soil to crops than Japanese officials are assuming. The IAEA singled out the limits that Japan had placed on cultivation of rice as an example of a standard that could be eased, once testing is complete.
The IAEA team said it's also crucial to focus on upland drainage areas, because radioactive trace elements can seep from hillsides to rice paddies and rivers through soil erosion or heavy rainfall.
Photograph by Yasuhiro Takami, Yomiuri Shimbun/AP
Room for Greater Knowledge
Teachers and parents work to help decontaminate Omika Elementary School, just outside the evacuation zone in Minamisoma, Fukushima prefecture, last month.
Omika and four other schools in the area were allowed to reopen in mid-October after decontamination was completed and an evacuation standby zone designation lifted.
Minamisoma is roughly 15 miles (24 km) north of the nuclear plant; Omika Elementary is about 13 miles (21 km) from the plant. Japanese officials ordered residents within 12.4 miles (20 km) of the plant to leave soon after the March accident, and later recommended evacuation of residents living between 12 and 18 miles (20 km to 30 km) of the plant. But by then, residents of Minamisoma already had left the area. Residents continue to be wary, and school officials, concerned about possible radiation exposure, asked parents to drive rather than walk their children to the school.
The IAEA team was told during its recent mission that 400 school playgrounds also had been remediated as of September 30. Some school sites were worked on mostly by volunteers with technical support provided by the Japan Atomic Energy Agency. The IAEA team praised Japan for giving priority in its remediation efforts to areas where children typically spend most of their time.
The effects of low-level radiation exposure may take decades to manifest, and it remains "very unclear the number of children exposed to significant levels the first few days" after the accident, said Lyman of the Union of Concerned Scientists.
"There is a possibility," he said, "that there were populations that were exposed and, as in Chernobyl, the effects won't appear for at least a decade."
But Lyman said that Japan has moved beyond viewing any criticism about the level of safety of nuclear plants as a political attack. He sees a new level of self-examination by TEPCO and Japanese authorities as a positive that has emerged from the disaster.