March 15 (Bloomberg) -- Don Olander, a professor at the University of California, Berkeley’s Department of Nuclear Engineering, said the damage to nuclear plants in Japan after an earthquake is different to the disaster at Chernobyl in the Ukraine in 1986.
He spoke on Bloomberg Television with Rishaad Salamat.
On comparison with Chernobyl
“It is very different from Chernobyl. This is a reactor which has two containments. One is the core itself that sits inside what’s called a reactor pressure vessel which is 6 inches thick of steel, about three or four meters in diameter and several more meters high, which contains the core during operation, at high pressure. If that is intact, if the melt has not gone through the bottom, then most of the fission products will stay inside. Chernobyl did not have that protection. Chernobyl was open and the entire core was destroyed.
On what a meltdown is:
‘‘Once you lose cooling but there is still water around, the fuel is still generating heat. Right now it is generating about half a percent of what it was when it was turned on full and that is due to the fission products decaying. That heat boils the water and makes steam. The steam rises up through the fuel rods, which are about four meters high, and they are small tubes about one centimeter in diameter. The metal tubes are a zirconium alloy, which is very reactive in steam. The steam will react with the zirconium, cladding it’s called around the fuel, and produce hydrogen and convert the metal to a ceramic, zirconium oxide. That releases more heat.
‘‘The combination of the decay heat and the heat coming from the reaction itself, plus the poor removal of heat by this upflowing steam means the temperature rises very quickly. As it rises the rate of this steam reaction increases. If the temperature gets up to around 2,000 degrees centigrade then the fuel can react with the cladding, what’s left of it, and form what’s called the melt. The thing that’s of concern is if the melt involves a large fraction of the core which has 100 tons of fuel, uranium dioxide, and about 25 tons of zirconium. If a large fraction of that is reacted and forms the melt and the melt falls to the bottom of this reactor pressure vessel then there is the possibility of a melt through and a large release of radioactivity.
On the worst case scenario:
‘‘The fuel and the metal cladding react, form a melt which falls to the bottom of the pressure vessel and melts through it, penetrates the pressure vessel and then the melt, a large knob of it, falls on the concrete floor beneath the reactor and then it starts to release fission products.
‘‘The only thing I can go by is the units 1 and 3 which went through a similar situation, a hydrogen explosion, damage to the fuel by the process that I just explained. If this unit 2 goes through the same process then it can recover. It depends if they can get cooling water in. That’s the key. If they can’t get cooling water then it is liable to go all the way through. Even a small amount of cooling water, something like 200 liters a minute for this entire core, if you can get that through then you can keep that cool.
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