The main problem is that there is no provision to retract the fuel rods manually when there is no cooling water flowing to the reactor to cool the heat generated continuously by the fuel rods due to failure of electric power to to the cooling water pumps. Or atleast there should have been a spring-loaded mechanism to automatically retract the fuel rods, once the temperature in the reactor reaches the higher critical value or the cooling water is not flowing due to failure of even the emergency Diesel Generators which was flooded by the Tsunami waves. Clearly a case of of inadequate design for protection.
The design defect was siting the reactors right at the sea shore in a nation prone to earthquakes and tsunamis. The second design defect is storage of spent fuel rods at the roof of the containment enclosure.
ReplyDeleteSpent fuel rods and fuel rods require continuous cooling all the time. Otherwise the heat being generated in the fuel (much less than when it goes critical) leads to overheating. In Fukushima, control rods were pushed up from the bottom in position immediately on detection of earthquake and the fission reaction was stopped. Even after stoppage of the fission reaction, residual fission keeps on taking place which generates heat within the fuel rods. However stoppage of cooling water resulted in boiling away of the water and exposure of cladding (zirconium) to hot atmosphere. Oxidation of zirconium resulted in generation of hydrogen which is an explosive gas. the resulting explosion after escape of hydrogen from reactor vessel (automatic pressure release like in a pressure cooker) made cracks in the containment chamber roof which is also bottom of the spent fuel rods water pool. Leakage of water also results in overheating of these rods which on boiling away and leakage out of the holes in the pool can lead to oxidation of zirconium, generation of hydrogen which can combust and lead to burning of the spent fuel rods like a roman candle. Similarly the overheating of working fuel rods can lead to hydrogen explosion. Both these effects can spew out deadly radiation into the air and into the ground. Unfortunately the design of back up electricity both from alternate feeders and DG sets was not robust enough. The chain is as strong as the weakest link. Power supply in this case.
Indian expertise could have helped the Japanese. Power from 11KVA lines is regularly stolen in India come evening and the hooks taken off before daylight. We also have huge (4 MW) which kick in smoothly in case of failure.
Posted by Rajindhar Sandhir
For the details posted, you can as well be an engineer for designing the NPP sited at Japanese coasts..!!!
ReplyDeleteVery articulate answer Rajindhar. To that, I would like to add that Fission is an inherently un-balanced process, which always defaults to meltdown. Any time two or more of these radioactive rods are near each other, the radiation causes a chain reaction which quickly runs out of control. The fault in the design of the Fukushima Daiichi reactors, in my opinion, is relying on water alone to prevent said fission.
ReplyDeleteConsider this: in the reactor core, "control" rods automatically deployed upon detection of the quake to "stop" the reaction. However, these control rods alone are not enough to fully stop the reaction entirely, the cool water being pumped into the core is what really prevents things from spiraling out of control. Lose that, and well, we have this disaster.
Therefore, as for a solution, I offer:
1. Do not store used fuel, in a fissionable configuration, in a pool of water, in the open, where replenished cooling water is the only thing preventing fission. If their remaining heat potential is still needed, move them into an underground flooded bunker, which cannot ever run dry.
2. Build the entire reactor underground, in a reinforced concrete vault, which can be flooded with cooling water if needed, under no power, by simply opening a valve manually to let seawater flow in. Any water which is boiled away will simply be replaced by new water. Therefore, the temperature won't exceed 100°C.
I'm sure there are reasons why this plant was built the way in which it was, but to me, it seems to have some very serious design flaws, which directly contributed to this disaster. Hopefully TEPCO can get the situation under control with no more loss of life or danger to the environment.