The fuel in all the 3 Units is thought to have at least partially melted down despite pumping sea water and boric acid into the Units
The crisis at the three Fukushima Daiichi nuclear power stations did not come from buildings collapsing due to the March 11 earthquake of magnitude 9 but from power failure following the quake. The tsunami knocked out the generators that produced the power. Lack of power in turn caused the cooling systems of the reactors to fail.
The Fukushima nuclear reactor 1 went critical on March 1971 and is a 460 MW reactor. Unit-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 nuclear fuel.
The fuel, in the form of pellets, is kept inside a casing called cladding. The cladding is made of zirconium alloy, and it completely seals the fuel. Fuel pins in the form of bundles are kept in the reactor core. Heat is generated in the reactor core through a fission process sustained by chain reaction.
The fuel bundles are placed in such a way that the coolant can easily flow around the fuel pins. The coolant never comes in direct contact with the fuel as the fuel 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 a power plant, including nuclear, is not 100 per cent. In the case of a nuclear power plant the efficiency is 30-35 per cent. “About 3 MW of thermal energy is required to produce 1 MW of electrical energy. Hence for the 460 MW Unit-1, 1,380 MW of 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 the Fukushima units, demineralised water is used as coolant. Uranium-235 is used as fuel in Unit-1 and Unit-2, and MOX (a mixture of oxides of Uranium-Plutonium-239) is used as fuel in Unit-3.
Since a 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 absorbed the neutrons immediately after the quake, the fuel still contains fission products such as iodine-131 and caesium-137 and activation products such as plutonium-239.
“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 these radionuclides is called “decay heat.”
“Just prior to the shut down of the reactor the decay heat is 7 per cent. It reduces exponentially, to about 2 per cent in the first hour. After one day, the decay heat is about 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 radioactive decay of the fission products.
Apart from the original heat, the heat produced continuously by the fission products and activation products 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 to the Nature journal, when the temperature reached around 1,000 degree C, the zirconium alloy that encased 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 secondary confinement (outer buildings around the reactor) in all the three units (Unit-1, Unit-2 and Unit-3). The reactor core is present inside the primary containment.
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,” Dr. Parthasarathy explained. The government has said that fuel rods in Unit-3 were likely already damaged.
Effect of melted fuel
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 boric 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 all three Units. Despite these measures, the fuel rods were found exposed in Unit-2 on two occasions.
Fate of reactor core
While the use of sea-water can prevent fuel melt, it makes the reactor core completely useless due to corrosion.
The case of Unit-4 is different from the other three units. Unlike in the case of Unit-1, 2 and 3, the Unit-4 is under maintenance and the core has been taken out, and the spent fuel rods are kept in the cooling pond.
Whatever led to a decrease in water level, the storage pond caught fire on March 15 possibly due to hydrogen explosion. The radioactivity was released directly into the atmosphere.
Spent fuel fate unknown
It is not known if the integrity of the cladding has been already affected and the fuel exposed. Since the core of a Boiling Water Reactor (BWR) is removed only once a year or so, the number of spent rods in the pond will be more.
If the fuel is indeed exposed, the possibility of fuel melt is very likely. Though the fuel will be at a lower temperature than found inside a working reactor, there are chances of the fuel melting.
Since it does not have any containment unlike the fuel found inside a reactor, the consequences of a fuel melt would be really bad. Radioactivity is released directly into the atmosphere. Radioactivity of about 400 milliSv/hour was reported at the site immediately after the fire.