Editor's Note: This is part of the IEEE Spectrum special report: Fukushima and the Future of Nuclear Power.
This past April, when the Japanese government and Tokyo Electric Power Co. (TEPCO) jointly unveiled their plan to bring the damaged reactors of the Fukushima Dai-ichi nuclear power plant to a cold shutdown and gain control of the release of radioactive materials, they set a tentative completion date for mid-January 2012.
And "tentative" had to be the operative word, for the obstacles TEPCO faced—and to some extent still does face—are challenging in the extreme. They include:
Fuel rod meltdowns in reactors 1, 2, and 3 due to loss of cooling systems following the 11 March earthquake and tsunami;
Severe damage to the upper levels of reactor buildings 1, 3, and 4 and slight damage to building 2, stemming from hydrogen explosions;
High levels of radiation and contaminated rubble, making working conditions hazardous and difficult;
Thousands of metric tons of contaminated water accumulating on the site and leaking out of the reactors.
It appears, however, that the process is now ahead of schedule. Environment Minister Goshi Hosono, who is also in charge of the Fukushima nuclear accident recovery, told the International Atomic Energy Agency's annual general conference in Vienna on 19 September that Japan was now aiming to complete a cold shutdown of the Fukushima plant by December 2011, instead of mid-January 2012.
Progress was already evident in July, when Hosono announced that workers had completed step 1 of the two-step road map on schedule, reducing radioactive emissions and starting to bring down the core temperatures in reactors 1, 2, and 3.
Hosono attributed the success to the construction of a new cooling system, which had begun pumping water into all three damaged reactors. In addition to cooling, the system also decontaminates the water accumulating in the basements of the reactor and turbine buildings. The contamination is the result of injected water coming into contact with the molten fuel in the pressure vessels.
Critics, however, were quick to question the stability of the system and its ad hoc design. The combination of filtering and decontamination technologies—mainly from the French nuclear giant Areva and the U.S. nuclear waste management company Kurion—includes some 4 kilometers of piping.
The critics have a point. Even with the addition of a reportedly more robust system (to be used in parallel or as backup as needed) from Toshiba and IHI Corp., TEPCO admits the system underwent 39 disruptions between 10 July and 8 September. One consequence is that roughly 100 000 metric tons of water still need to be decontaminated.
Disruptions and remaining challenges notwithstanding, TEPCO has been making progress toward step 2 of the road map: a cold shutdown. According to TEPCO, that means achieving and maintaining a temperature of less than 100 °C as measured at the bottom of a reactor pressure vessel—the steel vessel containing the fuel rods—which itself is enclosed inside a protective containment vessel.
A major advance came at the beginning of September, when TEPCO was able to start up the core spray lines to cool reactors 1 and 3. The core spray lines apply water directly to the cores from above, while the system installed in July has been cooling the cores by injecting water from the bottom. TEPCO has also begun increasing the amount of water being injected into reactor 2. The core spray line could not be used until recently because TEPCO first had to survey the subsystem's piping and valves. Given the high radiation in the area, this was difficult, but workers completed the job in July and confirmed the system's operability in August.
By late September, as a result of these efforts, the temperatures in all three reactors had dropped below 100 °C for the first time since the accident. As of 29 September, the temperatures for reactors 1, 2, and 3, respectively, were 77.5 °C, 99.7 °C, and 78.7 °C.
"We are steadily bringing the postaccident situation under control," says Hosono. "To achieve step 2 this year, we'll move the schedule forward and do our best."
But Yoshinori Moriyama, deputy director-general of Japan's Nuclear and Industrial Safety Agency (NISA) is cautious. "We need to maintain this state over the midterm," he says. "Temporary lower temperatures and the nonrelease of radioactive substances do not immediately mean that this is a cold shutdown." In order for NISA to declare a cold shutdown, the temperatures must remain stable and below 100 °C into December. So NISA won't officially declare a cold shutdown until near the end of 2011.
Despite these positive developments, nuclear experts point out that achieving a cold shutdown does not make the troubled plant completely safe, given that even spent fuel continues to generate heat for years after use.
And upon achieving a cold shutdown, TEPCO must take on a new series of challenges. These include finding where the injected water is escaping, stopping those leaks, dealing with the accumulated contaminated water, removing and storing the thousands of spent fuel rods from the pools in reactors 1 to 4, and then figuring out a way to remove the melted fuel. The last is a task that could take a decade or more, according to experts.
About the Author
For more about the author, see the Back Story, "Assignment: Fukushima"