The crisis at the three Fukushima Daiichi nuclear power stations came not from the buildings collapsing after the March 11 earthquake of magnitude 9 and the tsunami that followed the quake, but from a different source — power failure.
The Fukushima nuclear reactor 1 went critical on March 1971 and is 460 MW reactor. Unite-2 and Unit-3 are 784 MW each and went critical in July 1974 and March 1976 respectively. All the three are Boiling Water Reactors (BWR) and use demineralised water for cooling the nuclear fuel.
The fuel in the form of pellets is filled inside a metal rod called cladding. The cladding is made of zirconium alloy, and it completely seals the fuel. Fuel pins in the form bundles are kept in the reactor core. Several hundred fuel pins that are assembled in the core starts the chain reaction through fission process and heat is generated.
The fuel bundles have gaps through which the coolant flows. The coolant never comes in direct contact with the fuel as it is kept sealed inside the zirconium alloy cladding. The coolant changes into steam as it cools the hot fuel. It is this steam that generates electricity by driving the turbines.
All the heat that is produced by nuclear fission is not used for producing electricity. The efficiency of power plant, including nuclear, is not 100 per cent. In the case of nuclear power plant the efficiency is 30-35 per cent. “About 3 MW thermal energy is required to produce 1 MW of electrical energy. Hence for the 460 MW Unit-1, 1,380 MW thermal energy is produced,” said Dr. K.S. Parthasarathy, Former Secretary, Atomic Energy Regulatory Board, Mumbai. “This heat has to be removed continuously.”
In the case of Fukushima units, demineralised water is used as a coolant. Uranium-235 is used as fuel in Unit-1 and Unit-2 and Plutonium-239 is used as fuel in Unit-3.
Since very high amount of heat is generated, the flow of the coolant should never be disrupted. But on March 11, pumping of the coolant failed as even the diesel generator failed after an hour’s operation.
Though the power producing fission process was stopped by using control rods that absorb the neutrons, the fuel contains radioactive elements including radionuclides like iodine, and caesium. These elements are produced during the uranium fission process. “These radionuclides decay at different timescales, and they continue to produce heat during the decay period,” Dr. Parthasarathy said.
The heat produced by radioactive decay of the fission products is called “decay heat.”
“Just prior to shut down of the reactor the decay heat is 7 per cent. It reduces exponentially, about 2 per cent in the first hour. After one day, the decay heat is 1 per cent. Then it reduces very slowly,” he said.
While the uranium fission process can be stopped and heat generation can be halted, there is no way of stopping the radioactive decay of the fission products.
Hence the original heat as well as the heat produced continuously by the fission products, including iodine, and caesium, has to be removed even after the uranium fission process has been stopped.
Inability to remove this heat led to a rise in coolant temperature. According the Nature journal, when temperature reached around 1,000 degree C, the zirconium alloy that encases the fuel (cladding) probably began to melt or split apart. “In the process it reacted with the steam and created hydrogen gas, which is highly volatile,” Nature notes.
Though the pressure created by hydrogen gas was reduced by controlled release, the massive build-up of hydrogen led to the explosion that blew the roof of the fuel hall in Unit-1 [and subsequently Unit-3].
But the real danger arises from fuel melting. This would happen following the rupture of the zirconium casing. “If the heat is not removed, the zirconium cladding along with the fuel would melt and become liquid,” he explained.
Melted fuel is called “corium.” Since melted fuel is at a very high temperature it can even “burn through the concrete containment vessel.”
According to Nature, if enough melted fuel gathers outside the fuel assembly it can “restart the power-producing reactions, and in a completely uncontrolled way.”
What may result is a “full-scale nuclear meltdown.”
Pumping of sea-water is one way to reduce the heat and avoid such catastrophic consequences. The use of boronic acid, which is an excellent neutron absorber, would reduce the chances of nuclear reactions restarting even if the fuel is found loose inside the reactor core. Both these measures have been resorted to in Unit-1 and Unit-3.
While the use of sea-water can prevent fuel melt, it makes the reactor core completely useless as it results in corrosion.