THE precise details of what has gone wrong at the nuclear power plants in north-eastern Japan following the magnitude 9.0 earthquake that struck the area on March 11th remain hazy. But a picture is beginning to emerge as events unfold and information is made available by the plants’ operators and the Japanese authorities.
Start with the basics. Nuclear energy is produced by atomic fission. A large atom (uranium or plutonium) breaks into two smaller ones, releasing energy and neutrons. These neutrons may then trigger the break-up of further atoms, creating a chain reaction. The faster the neutron, the fewer break-ups it provokes. This is because an incoming neutron has to be captured to provoke fission, and fast neutrons are harder to capture. As a result, the chain reaction will peter out unless the neutrons can be slowed down sufficiently.
There also need to be enough fissionable atoms about for the neutrons to bump into – in other words, a critical mass. That is why uranium fuel has to be enriched, for only one of the two naturally occurring isotopes of the metal is fissile, and it is much the rarer of the two. In water-cooled reactors like the ones at Fukushima, the right combination of slow neutrons and enriched fuel leads to a self-sustaining process which produces energy that can be used to boil water, make steam and drive a turbine to generate electricity. Besides cooling the fuel (and thus producing the steam) the water also acts as a so-called moderator, slowing down the neutrons and keeping the reaction going.
So what happens when things cease to run smoothly, as when an earthquake interferes with the plant’s systems? When designing reactors, engineers attempt to achieve what they call “defence in depth”. The idea is that if any specific defence fails, another will make good the shortfall. This is a principle that Fukushima Dai-ichi, the worst hit of the nuclear plants, has been testing to destruction. The
defences have failed badly at all three of the reactors which were running at the time the earthquake hit.
Some defences are simply barriers. The pellets of nuclear fuel are encased in hard alloys based on zirconium (which lets neutrons pass freely through), to make fuel rods. The reactor core which includes these rods, and the water it sits in, are contained within a thick steel pressure vessel. That, in turn, sits within a larger steel structure, the primary containment vessel. Around all this sits the steel and concrete of the secondary containment structure.
Other defences are actions, rather than things, some of them automatic and some of them not. The first action to be taken in the case of an earthquake is an emergency shutdown, which is achieved by thrusting control rods that sit below the reactor up into the reactor’s core. These rods, made of neutron-absorbing materials such as boron, mop up excess neutrons and quench the chain reaction.
However, there are other nuclear reactions in a core that do not depend on neutrons. Some byproducts of the nuclear fission are themselves radioactive. These decay, producing heat. Though that heat is but a fraction of a reactor’s normal power – about 3%, in the case of the Fukushima machines – it is still similar to what comes out of a commercial jet engine operating at full throttle. That can warm things up pretty quickly in the absence of a cooling system.
So, the next action needed is to get a set of pumps running to keep cool water flowing into the reactor vessel and the consequent steam coming out.