Fukushima's Cold Shutdown

Recent News on Fukushima status:

Fukushima plant declared stable; Japanese PM says reactors are in 'cold shutdown,' setting the stage for return of some evacuees

Err... Cold Shutdown? What does that means?

I've found an interesting article (Q&A session to be more precise) to provide us with better understanding with the terms "Cold Shutdown"

Q: What does it mean when a nuclear reactor has achieved a "cold shutdown?"

Answer: It means the reactors have been stabilized and shut down. Under normal circumstances this means control rods have been inserted into the gaps between the fuel rods in the reactor, and the temperature of the water in the reactor pressure containers is below 100 degrees Celsius. In this scenario, fuel rods are contained within the pressure container, with no radiation leaking out.

Q: Is this the definition being used for the Fukushima No. 1 Nuclear Power Plant?

A: No, a slightly different definition is being used. Meltdowns have occurred at the Fukushima plant and the reactors are damaged, so when the government formulated "Step 2" of its nuclear disaster management plan, the phrase took on a slightly different meaning. The government has defined a cold shutdown for the Fukushima plant as a situation in which the temperature at the bottom of the reactor pressure containers is less than 100 degrees Celsius, and the radiation levels at the edge of the plant premises are below one millisievert per year.

Q: Why does the water in the reactors have to be cooled below 100 degrees Celsius?

A: Because otherwise the nuclear fuel could break down. In a normal nuclear plant, radioactive uranium-235 contained in the fuel absorbs neutrons and breaks down into other materials. This is called nuclear fission, and it produces a huge amount of thermal energy. Nuclear plants use this heat to generate electricity. While a nuclear plant is running, the temperature of the water in the reactor, which is under pressure, rises as high as 300 degrees Celsius. Vaporized water is condensed in equipment outside the reactor and then sent back in again.

If the water is over 100 degrees Celsius when a plant is shut down it will vaporize without being condensed again, leaving the fuel rods it normally covers exposed, and the rods will melt from their own heat.

Q: Does nuclear fuel still create heat even after a plant has been shut down?

A: Yes. The fission products that come from uranium-235 break down on their own, releasing heat and radiation. This is called "decay heat." Right after a plant is shut down, the fission products will continue to release around six percent of the energy they released compared with when the plant was running. This falls quickly to around 1.5 percent a week later, but even a month later they will release around 0.2 percent, and a year later they will still release around 0.02 percent. The meltdowns at the Fukushima plant happened because this decay heat was not dissipated.

Q: So even after the government declaration of the completion of "Step 2," we can't let down our guard, right?

A: That's right. Under normal circumstances, after shutting down a plant, heat-removing equipment would let the decay heat escape to the ocean, stabilizing the water in the reactors at 50 to 60 degrees Celsius. However, at the Fukushima plant, the loss of electricity and other problems after the March earthquake prevented the nuclear fuel cooling equipment from running and the reactors melted down, opening holes in the bottoms of the pressure containers and complicating efforts to cool the fuel. The amount of decay heat is less now than it was right after the March disasters, but the fight to keep down fuel's temperature down is expected to continue for a long time. (Answers by Toshiyasu Kawachi, Science & Environment News Department)