"Every Type of Reactor Poses a Threat"

Dr. Helmut Hirsch is a physicist and scientific consultant for nuclear safety for a number of organizations including the Austrian federal government and the environment ministry.

DW-WORLD.DE: Since the accident 20 years ago, it seems clear that the Chernobyl-type RBMK reactors are especially dangerous. Why is that?

Helmut Hirsch: A strengthening of any chain reaction that has begun forms a build-up in these reactors that cannot be automatically lowered as it can in most Western reactors. The chain reaction gets stronger and stronger -- this is the so-called bubble co-efficient -- and in Chernobyl's case, led to the accident. This is a weakness that has been worked on since the accident and while there have been some improvements, the problem hasn't completely been solved. The RBMK also has other disadvantages. The pressure pipes that hold the fuel elements and containment weaknesses can become brittle.

Would you say that RBMK reactors are the most dangerous reactors active today?

I think that is correct. There is also a very wide consensus that these reactors cannot be retrofitted to meet western standards and that they need to be shut down.

The VVER 440/230 reactors also do not meet Western standards. What are the problems with these?

This is a different type of reactor that is seen as practically impossible to retrofit. A major problem is that these reactors have practically no containment capability. If there were an accident where radioactive steam inside such a reactor were released, it would simply escape. There have been a few retrofittings, but this is really just tinkering and doesn't really control the problem. Another problem is that these reactors' emergency cooling systems are very weak. There have also been efforts to improve this, but the problem is so serious that retrofitting is practically impossible.

Can you see a difference with improvements to another type of reactor, the VVER 440/213?

Definitely. The 213's are the second generation of VVER reactors and there were a series of improvements made. The emergency cooling system is stronger, and they have a better containment structure, although it is still below Western standards.

The VVER 440/213 does not have so-called full-pressure containment, but a containment system dependent on pressure release. It's very complicated and prone to breaking down. There are also reports of material problems in these reactors, and there are problems with safety systems. In the case of external hazards, for example, the danger that several safety systems fail is especially large. You could say this type of reactor is clearly better than the first generation, but still has serious weaknesses.

The VVER 1000 is comparable with Western reactors from the 1970s. Does that mean they are safe?

No. This reactor has full-pressure containment and its layout is comparable with Western reactors, so you'd have to say it is an improvement on the 213 series. But the VVER 1000 also has different problems. Protection from external hazards is considerably worse than modern systems. There are also reports of material problems in the VVER 1000, especially when looking at the brittleness of the reactor's pressure container, which is a key component.

One specific problem with the VVER 1000 is a construction flaw in the reactor building, where containment should be able to take place for some time in the event of an accident. It is especially prone to melting down from below. This means that if there is an accident in this type of reactor, the chances of a relatively quick containment malfunction are higher and the release would be larger than you'd find in a German pressurized-water reactor

What role does age play in an atomic reactor's safety?

The aging process plays a large role and we have to assume that safety will decrease continually after 15 to 25 years of operation due to aging. This can be partially counteracted when operators make the necessary efforts and replace parts that have aged. But this doesn't apply in every case. The reactor pressure container, for example, cannot be replaced. This kind of aging management system is also complicated and expensive and there are some doubts as to whether the necessary measures are being taken, especially in Eastern Europe.

After how many years do you think a reactor should be closed?

A running time of about 30 years is the maximum, after that there is a clear deterioration in the level of safety. The risks also increase. I'd say between 25 and 30 years would be a general guide.

What other risks have to be considered when evaluating nuclear reactors?

Every type of reactor operating today poses a threat of a catastrophic accident with a major release of radioactivity. The likelihood is especially high in Eastern Europe's old reactors, but technical failure or external influences -- like natural earthquakes and floods or malicious acts by humans -- pose a danger to even the most modern reactors.

What should be done to increase security?

I think there are a lot of reasons for the long-term phasing out of atomic energy. On the one hand nuclear power plants are vulnerable, and they can cause serious problems should a radioactive release occur. On the other hand, nuclear technology is of military importance, which does not necessarily help achieve peaceful development on an international political level. I think for those reasons, phasing out of nuclear energy is the best course of action.

In the long-term, the only energy policy that actually meets the standards required for the survival of humanity will be based on renewable energy sources and a more rational use of energy.