Causes of the accident

There was not one cause of this accident, there were several which all contributed to it. This accident happened while testing an RMBK reactor. A chain reaction occurred in the reactor and got out of control, causing explosions and a huge fireball which blew off the heavy concrete and steel lid on the reactor. These are the causes:

1. Design fault in RBMK reactor

2. A violation of procedures

3. Breakdown of communication

4. Lack of a 'Safety Culture' in the power plant

1. Design fault in RBMK reactor

An RBMK reactor is a pressurised water reactor using a graphite moderator and a water coolant, this combination is found in no other reactorsmoderator . What are the graphite and water coolant?

• The graphite moderator is a series of graphite blocks surrounding a number of pressure tubes. They slow down neutrons during fission so that the fission is contained.
• The water coolant is to deduct heat from the fission process so the reactor does not overheat.

This reactor is also unusual as it was used for both plutonium and power production. The problem with this reactor is that at low power it is instable so may undergo a rapid and uncontrollable power increase, all other reactors have built-in features to stop this instability from occurring. The design fault in the RBMK reactor was that there were excess steam pockets which led to a power increase (this is called the positive void coefficient). Because of this excess power more steam is produced and more water is needed as a coolant meaning less water can be used to absorb the neutrons, this process is very hard to stop as it supplies itself and this chain reaction continues.

2. A violation of procedures

While testing the reactor a series of compulsory safety procedures should have been followed. However several of these procedures were disregarded by the technicians. Firstly during the test only 6-8 control rods were used when there is a minimum of 20 rods required to retain control. Secondly, the emergency cooling system was disabled, which was not a real cause but if it had been available the severity of the accident may have been decreased.

3. Breakdown of communication

During the test there was little or almost no proper exchange of information between the people who were in charge of the test and the people carrying out the test on the nuclear reactor. This means that when a fault may have been detected by the tecnicians they probably did not report it immediately to their superiors because they were not aware of the severity of the fault.

4. Lack of a 'Safety Culture' in the power plant

Because of this lack of safety culture, the people at Chernobyl Nuclear Power Plant were inable to restore the design fault of the RBMK reactor even though they were aware of it before the accident. A secret USSR memorandum in the Russian archives clearly suggests this.

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