nuclear-news

Offline nuclear reactors still dangerous, need constant cooling

News Navigator: What danger is still posed by offline nuclear reactors? Answers by Taku Nishikawa, Science & Environment News Department The Mainichi, 8 May 12, 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……
http://mainichi.jp/english/english/perspectives/news/20120508p2a00m0na002000c.html

May 10, 2012 - Posted by Christina Macpherson | Japan, Reference, safety, technology