What happened in Chernobyl?

On 26 April 1986, reactor 4 of the Chernobyl nuclear power plant was shut down for maintenance. The accident started during a safety test on a new emergency system. Since the power of the reactor dropped too quickly, the operators tried to drive the power back up by extracting many more control rods (which assist in controlling the reactivity of the reactor core) from the reactor core than permitted. They had also switched off an automatic safeguard.

Against all expectations, the power of the reactor rose more than intended. The operators tried to shut down the reactor manually by inserting all of the control rods into the reactor core. However, a number of rods did not get fully inserted into the core.

Then, observers outside the power station heard two explosions, one shortly after the other. Burning material was shot into the air, and set fire to the turbine building. The reactor cover was lifted up from the reactor. The reactor core, made of graphite blocks containing the fission elements, was on fire.

What was the cause of the nuclear accident in Chernobyl?

The nuclear accident in Chernobyl was the result of a combination of different factors:

• The unsafe design of the power station, e.g. with an inherently unstable reactor, combustible graphite as a moderator, flawed control rods and only a single containment.
• Human errors, because the staff had an insufficient understanding of the important role of certain safety systems and was not sufficiently trained to respond correctly to anomalies.
• A low safety awareness, because the staff knowingly ignored safety regulations and shut down safety systems for extended periods.
• An environment in which authoritarian leaders imposed their will from above, without taking into account the doubts or critical comments made by their staff or experts.

A test carried out in unsafe circumstances and a combination of errors and wrong decisions led to an uncontrolled nuclear chain reaction and power increase in the reactor, which caused it to overheat and explode.

Can this also happen in the Belgian nuclear power stations?

No, because

• The design of the Belgian nuclear power plants is inherently much safer and does not have the deficiencies which led to the nuclear accident at Chernobyl;
• The staff of the Belgian nuclear power plants are much better trained than the Chernobyl operators and know how to use procedures correctly, to work together efficiently, to stop in the event of doubt, and to prevent human errors by applying special techniques;
• In the Belgian nuclear power plants, there is an open culture in which everyone can freely express their doubts or concerns, raise problems and report errors transparently.
• The safety of the Belgian nuclear power plants is strictly monitored by a number of independent bodies. No other sector is regulated so strictly. The nuclear power plants have on average about 50 independent audits every year, which means almost one audit per week. The reports are often public. Both national and international nuclear experts confirm that the Belgian nuclear power plants are safe.

Through this combination of a strong design, correct working methods and safety-conscious behavior, we ensure the safe operation of our nuclear power plants.

In what ways is the design of the Belgian nuclear power stations different to Chernobyl?

The Chernobyl nuclear power plant was of the RBMK type: a reactor with water as cooling agent and graphite as a moderator (= material which slows down the neutrons which are released by the nuclear reaction). RBMK reactors were only built in the Soviet Union. This type of reactor was inherently unstable and did not meet the Western safety standards at the time of construction.

The Belgian nuclear power plants are of the PWR type (pressurized water reactor). In this type of reactors, water is used both as cooling agent and as a moderator. Pressurized water reactors are inherently safe and stable. Most of the operational nuclear power plants worldwide today belong to the PWR type.

Generally, a PWR reactor has some important advantages compared to an RBMK reactor, for example:

• A PWR has a ‘negative void coefficient’ and therefore is therefore intrinsically stable: when the temperature of the primary cooling water rises, the nuclear reaction slows down and the power output drops. In the case of an RBMK reactor, the exact opposite happens: when the temperature of the primary cooling water increases, the nuclear reaction speeds up, thus increasing the temperature of the cooling water, etc. This can lead to an uncontrolled nuclear reaction and power increase, which was also the case in Chernobyl.
• An RBMK reactor uses graphite as a moderator, i.e. to slow down the fast neutrons in the core. Graphite, however, is rather combustible. A PWR uses water as the moderator.

The Belgian nuclear power plants have other important advantages:

• With the aid of control rods, the nuclear reaction can be shut down fully within 2 seconds. The control rods slide down into the reactor core purely by means of gravity and do not need to be driven down, as was the case in Chernobyl.
• The Belgian nuclear power plants are equipped with extensive safety systems with threefold redunancy. These safety systems are housed in bunker-type buildings which are resistant to all possible events.
• The Belgian nuclear power plants have double-walled reactor buildings in reinforced concrete, which are designed to keep the radioactivity within the building in the event of an accident. The Soviet RBMK reactors were not protected by such a strong construction.

How do we prevent human errors in the Belgian nuclear power stations?

We reduce the risk of human errors through our working practices and behavior.

Our staff strictly adhere to detailed procedures for every action. Independent checks ensure the quality of the work carried out.

Furthermore, we train our people in safe behavior and apply effective techniques to prevent human errors:

• We constantly stress the importance of nuclear safety.
• The operators in the control room are trained and drilled to work efficiently under stress and in emergency situations.
• We teach them to always stop in the case of doubt and to ask for assistance or additional checks if necessary.
• Through effective communication, we prevent misunderstandings: for example, if someone asks for confirmation, staff reply by repeating the information instead of saying ‘yes’ or ‘OK’.
• For important or difficult interventions, staff are drilled in advance on scale models or in a simulator.
• We encourage our people to report problems or errors transparently and to identify the root cause through research and analysis.
• We create an open culture in which doubts or concerns are discussed freely.

In addition, the safety systems of our nuclear power plants are designed to detect and compensate for human errors.

What lessons has the nuclear industry learned from the Chernobyl accident?

After the nuclear accident in Chernobyl, the World Association of Nuclear Operators (WANO) was established. This organization unites more than 120 operators of over 430 nuclear power plants across the world and has the mission of ensuring that an accident such as Chernobyl can never happen again.

Through WANO, all players in the nuclear power sector work together to operate their nuclear power plants as safely as possible and to strive for continuous improvement. They do this by defining international directives and standards, by auditing each other through peer reviews, by exchanging best practices and sharing important operational information and experiences. WANO always pays a lot of attention to the human factor, which played an important role in the Chernobyl accident.

What is the current situation in Chernobyl?

Since the disaster in 1986, radiation levels have dropped by several hundreds. Therefore, most of the contaminated areas are now safe for habitation and economic activity. However, the zone in the immediate vicinity of the power station (the Exclusion Zone, a radius of 30 kilometers around the power station) will have to be decontaminated intensively and is not yet suitable for permanent occupation. In the meantime, brief visits to the zone are possible, even without protective clothing.

In August 2010, the construction of a sarcophagus over the reactor started. That way, the decontamination work inside the reactor building can continue.

This sarcophagus is built to withstand extreme temperature fluctuations between -40°C and +40°C, extreme weather conditions and radioactivity. The structure is composed of two layers. Pressure in te space in between these two layers is slightly lower than atmospheric pressure, which prevents radioactivity from escaping to the outside world.