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Nuclear Rays: The atomic pressure cookers of Bohunice

Almost every tenth reactor in the world is a WWER-440. This model was developed and built in the Soviet Union since 1964.

WWER-440: The pressure cookers of Bohunice
Picture by T. Micke

The net total power of the four nuclear power plants in Bohunice is 1568 megawatt.

As a comparison: 925 megawatt was the power of the Chernobyl's reactor just before it exploded. The four comparatively small units in Western Slovakia are also Soviet craftsmanship. However, the construction isn't the same as in Chernobyl. How do those reactors really work?

The reactor type in Bohunice (Czechoslovakia) is called WWER-440. It can also be found in Bulgaria, the former GDR, Finland, Poland, Rumania, Hungary and the USSR.

Almost every tenth reactor is of this type developed in the Soviet Union in 1964. Humans operate the chain reactions in the reactor core. A Uranium atom gets fired at with a small uncharged particle called a neutron. The atom breaks. As a result two smaller atoms and two to three neutrons are formed. On top of that a lot of nuclear energy and radiation is freed.

The two to three neutrons expelled during fission look for more uranium atoms to fission. The whole thing becomes a chain reaction and more and more energy is freed. On the whole that is how an atomic bomb works: an uncontrollable chain reaction that leads to an explosion.

To avoid an explosion in a nuclear power plant the reaction has to run under controlled conditions: every fission may only lead to one new fission. Therefore out of each fissioned uranium atom only one neutron may be formed. All the other neutrons are absorbed with control rods made of boron or cadmium. The further the control rods are shoved into the reactor core the less uranium atoms will be split and the less energy will be freed. Like this the nuclear fission can be controlled.

At Chernobyl too many control rods were pulled out. If that wouldn't have been the case, this catastrophe would have never happened.

Not only the number of neutrons has to be controlled but also the speed at which they get catapulted against the uranium atoms. If they are too quick, the energy yield is reduced.

Water (at Bohunice) or sometimes graphite (at Chernobyl) is used to decelerate the neutrons.

The obtained energy of this process is used to create steam with a cooling medium that surrounds the reactor core. The steam powers a turbine that generates electricity.

Water (at Chernobyl and Bohunice), air, carbon dioxide, helium or sodium is used as coolants.

In Bohunice are four water-cooled and moderated pressurized water reactors. Water serves as coolant and is enclosed and put under high pressure, quite similar to a pressure cooker. That way the water has a higher cooking point.

This system is widely spread but it is more dangerous. Especially the old models of the Soviet type series WWER-440 are more sensitive for earthquakes and environmental impacts. This already led to incidents and leakage of radioactivity.

The experts agree on one thing no matter if they are pro or contra nuclear energy. The way the four reactor blocks are operated in Bohunice now, they are a great risk and perilous for the population.

The international atomic energy organization and the German reactor company "Siemens" are for the renovation of these ruins. Those who think Chernobyl was already too much are for the shutdown.

Renovating a nuclear power plant is not as easy as renovating an old building. In the times when most of these reactors were built more importance was attached to being able to built nuclear weapons than to safety and efficiency. Even if all concerned countries would be willing to improve the security measures they wouldn't be able to go past the current standards. Florian Faber from "Greenpeace": "Even if I would equip my old car with airbags, an anti-skid system and a catalytic converter, it wouldn't be of any use if the floor plate rusts through and the motor falls apart while I am going 130 km/h on the freeway."

< Read part 10 of the series: Atomic incidents

© A report by Tobias Micke (29-04-91) – Contact