The great catastrophe that nuclear reactors need to guard against is runaway nuclear reaction, with generation of heat and escape of nuclear materials and the radioactive products of nuclear reactions into the atmosphere. Such accidents have happened in the past, first in the Three Mile Island case in USA in 1979 and in the Chernobyl mishap in Ukraine, in the former USSR, in 1986.
The latest in the list of nuclear radiation leak is of Japan. The great danger in the case of nuclear reactors is that their energy producing core contains a large quantity of radioactive material. A nuclear power plant is similar to a coal fired power station in that it heats water to create steam, which turns turbines, which generate electricity. The difference is that in the place of coal that heats the boiler which produces steam, the heat source in the nuclear reactor are rods of radioactive material which take part in a nuclear chain reaction. Any mishap that allows this material or the products of the reaction to escape can be disastrous.
The fuel in nuclear plants is usually an isotope of uranium, an element whose nucleus can decay, or break up, when it is struck by a neutron, breaking up into two parts and also two extra neutrons which do not find a place in the ‘daughter’ nuclei. The break-up gives off sizeable energy, which heats the fuel rods and their container, and this heat can be tapped and made use of. But the important thing is that a uranium nucleus, which needs one neutron to get it to decay, produces two more neutrons when it does, which can then push other uranium nuclei into decay. In a small piece of uranium, most of the freshly emitted neutrons do not find targets and escape the material. But if an adequate quantity of such fuel is brought together, then it becomes increasingly likely that each nuclear fission (or break up) will set off, on the average, more than one new nuclear fission. When this happens, the number of fissions rapidly multiplies and the whole fuel assembly heats up.
The heat is quickly taken away, by circulating a coolant, under pressure, and the coolant exchanges its heat with water in an exchanger, producing steam, to drive turbines. One idea of keeping the coolant, which usually is also water, under pressure is to keep from becoming steam, because steam tends to react with some of the metals used in the reactor casing, to become hydrogen. Apart from leading the heat away by the coolant, the reaction itself can be controlled by blocking the path of the neutrons between fuel rods. This is done by inserting other rods of materials that absorb neutrons and get them out of the chain reaction cycle. The whole arrangement, which is entirely remotely controlled, is encased in a specially designed concrete structure, both for concrete to provide some shielding of radioactivity which escapes from the reactor as well as to contain the intense heat and huge pressures than can develop in case the reaction goes out of control.
In the Three Mile Island case, the trouble started when the secondary loop of steam that carries heat away and to the turbines, suffered mechanical failure. This caused overheating of the reactor and a safety valve, which allows the pressure to drop, was activated, as also the automatic shutting off of the plant by lowering the control rods which block the neutron traffic.
But the valve did not close when the pressure dropped and because of the heat that fuel rods generate for some time even after the reaction has been controlled, some coolant water escaped into the atmosphere, spreading radioactive contamination. In the Chernobyl case, accidental overheating of the core led to increase in pressures and collapse of the concrete casing, which had not been designed to withstand the pressures of an accident. This led to escape of coolant and also exposure of the graphite rods, which were being used to control the reaction, to the air and they caught fire. Huge quantities of radioactive material got transported by the resulting smoke. The Chernobyl disaster brought home the need for strict enforcement of safety norms for reactor structures, to be able to withstand the pressures and temperatures that can arise during mishaps.
Failure of the cooling system
In the present emergency in Japan, it is the cooling system at the Fukushima Dai-ichi plant, which has stopped working because of power failure, resulting from damage due to the tsumami. The reactor is a ‘boiling water reactor’ or one which uses water coolant directly to run the turbines. The type of water used is ‘light water’, or water from which occasional hydrogen atoms which have nuclei with a neutron and a proton, in place of only a proton, (called heavy water), has been removed. This kind of reactor has a number of advantages and economies, but which get largely set off by high maintenance costs because of radioactive contamination arising from water circulation.
With failure of the cooling system in the Fukushima reactor, there was pressure build up, controlled by release of steam, and high radiation levels were measured both outside and within the reactor assembly. At the high temperatures, above 3400˚F, zircalloy, a special material used in the fuel rod containers, induces steam to break up into hydrogen and oxygen, and hydrogen escaped. This resulted and explosions and the fear of a ‘meltdown’, where the exposed fuel rods could continue to react. The engineers at the facility have engulfed the reactor with seawater, to cool it down and have pumped compounds of the element, boron with the seawater, to slow down the chain reaction.
Boron is one of the substances used to capture neutrons and control nuclear chain reactions. It is one of the simplest of elements, next only after hydrogen, helium, lithium and beryllium, and has five protons in the nucleus with five electrons in orbits around the nucleus. Along with the five protons, the nucleus has either five or six neutrons giving boron an atomic mass of either 10 or 11.
The case of boron 10, which has only five neutrons is useful for neutron capture, because the fifth neutron is ‘unpaired’ and ready to ‘accept’ a neutron. But Boron 11 has six neutrons, which form a compete set and Boron 11 is quite useless for neutron capture.
The engineers at Fukushima are pushing in Boron 10, along with the seawater, hoping to block the rate of chain reaction and the generation of heat within the reactor core.