Nuclear accidents can range in severity from being devastating to trivial with no public impact. However, it is a very emotive issue and generates a sense of horror- a hovering mushroom cloud, taking a toll on thousands of people.

The nuclear accidents at the Three Mile Island in USA in 1979 and at Chernobyl in the erstwhile Soviet Union in 1989, created enormous confusion and despair among the public, though they differed widely in severity as far as the impact on people was concerned.

In 1989, the International Atomic Energy Agency in Vienna called a meeting of experts to device a scale “for promptly communicating to the public in consistent terms to safety significance” of nuclear events. The result was the birth of International Nuclear and Radiological Events Scale (INES). The criteria used to develop the scale have been revised in 2008 to include not only nuclear reactor accidents, but also all types of radiation accidents.

Nuke reactor: How does it work?
In a nuclear reactor, the fuel is uranium. Uranium atoms undergo fission, liberating enormous amount of heat. Generally water circulates through the fuel to pick up the heat and transfer it to a steam generator, which in turn turns the turbine to produce electricity.

An important feature of a nuclear reactor from the point of view of safety is the build up of radioactivity in the fuel. Uranium is only mildly radioactive. However, the products of a fission reaction are highly radioactive. During the operation of the reactor, radioactivity in the fuel builds up to an extremely high level. In normal circumstances, this is handled safely. For any reason, if the rate of fission reaction exceeds the designed level or if there is an interruption in coolant flow, the fuel may become so hot that it may vaporise, leading to a potential accident.

To reduce the chances of such an event, nuclear reactors are built with a succession of safety layers known as “defense in depth”. The concept works at two levels. One is to secure the operational safety of the reactor with devices like control rods (which control the fission reaction rate), coolant, heat exchangers, pumps, valves, etc.
Redundancy is built in every stage so that if one system fails the other takes over. The other is to mitigate the consequences of a mishap. These are in the form of barriers to prevent the leak of radioactivity into the environment. There are five to seven containment levels to achieve this. The last of these is the familiar dome-like structure enclosing the entire reactor system.

Structure of INES
To communicate the potential impact of a nuclear event to the public, INES is designed to consist of seven levels of severity. In the increasing order of severity, these are: anomaly, incident, serious incident, accident with local consequences, accident with wider consequences, serious accident and major accident.

Because public safety is of utmost importance in the operation of a nuclear facility, the highest level in the scale corresponds to a major accident in which a large amount of radioactive material escapes to the environment. Such a release would result in the possibility of immediate health effects like radiation sickness and possibly death of exposed persons, long-term effects such as cancer and genetic damage, and environmental consequences over a wide area, possibly involving more than one country.   

Level one, the lowest in the scale is called an anomaly. It is an event beyond the authorised operating limits in which a significant part of the defense in depth remains intact, without any impact on the workers or the public.
It requires only a local correction. Events of level one to three are known as “incidents” and levels four and above are called “accidents”.
The scale is constructed on a logarithmic basis so that each level is ten times more serious than the one previous. An event at level seven is a million times more severe than an event at level one.

Though a number of events do occur in nuclear power plants, a majority of them are found to be below level three.
The only level seven event, a “major accident” is the one that occurred at the Chernobyl nuclear power plant in the erstwhile USSR. In the fire-fighting operation, 31 workers died of thermal and radiation injuries. Huge quantities of radioactive materials escaped to the environment resulting in contamination of thousands of square kilometers of land, not only in the Soviet Union but also in the neighbouring inland and Germany. The Windscale reactor accident in UK (1957) and the Three Mile Island reactor accident in USA (1979) have been graded as level five.