The explosions lifted the pile cap, allowing the entry of air. The air reacted with the graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted.
Some 8 of the 140 tones of fuel, which contained plutonium and other highly radioactive materials (fission products), were ejected from the reactor along with a portion of the graphite moderator, which was also radioactive. These materials were scattered around the site. In addition, caesium and iodine vapors were released both by the explosion and during the subsequent fire.
Megawatts - electrical and thermal
The energy produced by nuclear reactors and thermal power stations, is in the form of heat, measured as megawatts thermal - MW(t). The heat is then used to create steam which in turn is used to produce electricity from a generator connected to a steam turbine. The electrical output is measured as MWe (megawatts electric) which is usually about one third of the thermal power rating, depending on the efficiency of the power plant involved.
Chernobyl - Main factors in the accident
- Non-routine operation of the reactor.
- Violation of operating regulations, including the removal of most of the control rods.
- Positive void coefficient characteristic of the reactor.
- Apparent lack of knowledge by the station staff of the characteristics of the reactor.
- Inadequate control rod design.
Chernobyl - Positive Void Coefficient
Positive void coefficient is a term often associated with the RBMK reactors, the type involved in the Chernobyl disaster. Reactors that have a positive void coefficient can be unstable at low power and may experience a rapid, uncontrollable power increase. While reactors other than the RBMK type have positive void coefficients, they incorporate design features to prevent instability from occuring.
In a water cooled reactor steam may accumulate to form pockets, known as voids. If excess steam is produced, creating more voids than normal, the operation of the reactor is disturbed, because
- the water is a more efficient coolant than steam
- the water acts as a moderator and neutron absorber while steam does not.
A reactor is said to have a positive void coefficent if excess steam voids lead to increased power generation, and a negative void coefficient if excess steam voids leads to a decrease in power. The coefficient is simply a measure of the speed of change of state of the reactor.
When the void coefficient is positive, the power can increase very rapidly because any power increase that occurs leads to increased steam generation, which in turn leads to a further increase in power. Such increases are, therefore, very difficult to control.
When the void coefficient is negative, excess steam generation will tend to shut down the reactor. This is, of course, not a safety problem.
Most of the world’s operating power reactors have negative void coefficients. In those reactors where same water circuit acts as both moderator and coolant, excess steam generation reduces the slowing of neutrons necessary to sustain the nuclear chain reaction. This leads to a reduction in power.
In some reactor designs however, the moderator and coolant are in separate circuits, or are of different materials. In these reactors, excess steam reduces the cooling of the reactor, but as the moderator remains intact the nuclear chain reaction continues.
In some of these reactors, most notably the RBMK, the neutron absorbing properties of the cooling water are a significant factor in the operating characteristics. In such cases, the reduction in neutron absorbtion as a result of steam production, and the consequent presence of extra free neutrons, enhances the chain reaction. This leads to excess power production.
This excess power production causes additional heating. The additional heat raises the temperature in the cooling circuit and more steam is produced. More steam means less cooling and less neutron absorbtion, and the problem gets worse.
All of this can happen very rapidly. If it is not stopped, and it is very difficult to stop because it feeds itself, there will be the sort of event that happened at Chernobyl unit 4.
In order to avoid problems with positive void coefficient there are two approaches. Either the reactor characteristics can be altered to reduce the positive void coefficient or systems can be provided that will shut the reactor down very quickly if an increase in power is detected.
Since the Chernobyl disaster, the RBMK reactor design has been altered and units have been backfitted to protect them against the effects of the positive void coefficient.