8 Mai 2012
May 8, 2012
As of May 5, all nuclear reactors in Japan were offline. The Mainichi answers common questions readers may have about the safety and dangers of offline nuclear plants.
Question: With the reactors offline, has the danger of nuclear accidents disappeared?
Answer: The danger is likely less than while the reactors are running, but it still exists. Nuclear plants make power by turning turbines with the heat from the chained fission of Uranium-235 in nuclear fuel. This chained fission is stopped in an offline reactor, but fuel rods continue to release "decay heat" as various unstable nuclei created during the reactors' operation until now naturally break down. This decay heat has to continually be removed.
Q: What will happen if it is not removed?
A: A situation like the Fukushima No. 1 Nuclear Power Plant disaster could happen. In the case of the Fukushima disaster, the No. 1 through 3 reactors were shut down due to the vibrations of the earthquake. The tsunami that followed cut off all the plant's power and ability to inject water into its reactors, preventing the removal of decay heat and causing the water in the reactors to boil and gradually evaporate. This led to the fuel rods being exposed, which heated up and led to a meltdown, producing hydrogen gas that exploded. The nuclear fuel of the No. 4 reactor had all been moved to the spent fuel pool on the housing unit's top floor due to a regular reactor inspection, but water could not be injected there either, so this fuel also fell into a dangerous state.
Q: How hot is decay heat?
A: Over a year has passed since the No. 1 through 3 reactors at the Fukushima No. 1 plant were stopped, and the heat has dropped to around 0.4 percent of what it was immediately after the shutdown. Still, the heat is at around 500 kilowatts, enough to boil around five cubic meters (5,000 liters) of 20 degree Celsius water in an hour. It is for this reason that currently the Tokyo Electric Power Co. is injecting six to nine cubic meters of water into each reactor per hour. It is said that used nuclear fuel normally must be kept in water for around two years before it is sufficiently cooled down. Many of the nuclear reactors offline now are in the middle of regular inspections, so much of the nuclear fuel has been moved to spent fuel pools, but if cooling there stops then the same thing that happened at the Fukushima No. 1 plant could occur.
The danger of nuclear fuel in such pools was not recognized much before the Fukushima disaster. Commissioner William Magwood of the United States' Nuclear Regulatory Commission has commented that no one thought the pools would bring about danger.
Q: What safety measures have been taken against frequent earthquakes?
A: On instruction from the government, power companies have taken emergency safety measures such as setting up more power-supplying vehicles on high ground so that if a tsunami like the one that hit after the Great East Japan Earthquake strikes and takes out a nuclear plant's power, they can still cool the reactors and spent fuel pools. (Answers by Taku Nishikawa, Science & Environment News Department)