Classification of Events

The use of nuclear energy, like most human activities, carries certain risks in addition to its undeniable benefits. One of these is the risk of a non-standard situation occurring – an incident at a nuclear installation or during the transport of radioactive materials. These are different types of events, which can be distinguished according to their magnitude and the severity of their impact on the population and the environment.

Breakdown of events by impact:

Failure causing:

• a threat to nuclear safety without directly compromising the performance of safety functions
• breaking of safety barriers or other safety measures without direct consequences
• triggering the lapse of limits and conditions for safe operation and safe decommissioning
• violation of limits and conditions without direct consequences on the performance of safety functions
• activation of safety systems or their activation for real causes but without direct consequences
• violation of technical conditions or transport regulations during transport without direct consequences
• other interference with the reliability of equipment requiring corrective action to eliminate the consequences
• a release of radioactive substances or ionising radiation where the exposure limits are not exceeded

Incident that caused:

• compromising or impairing the performance of safety functions
• failure of safety systems or activation of safety systems for genuine reasons requiring action to remedy the consequences
• serious breach or failure of safety barriers
• release of radioactive substances or ionising radiation exceeding the exposure limits

Accident that caused

• a release of radioactive substances requiring the application of measures for the protection of the public

Breakdown of events by leakage pathway:

• Technological accidents – Accidents in which nuclear safety is compromised and multiple safety systems generally fail. In such accidents, for example, radioactive materials may be released outside the areas intended for that purpose. A typical example is the accident at the Bohunice A1 nuclear power plant, where the fuel partially melted and damaged the reactor to such an extent that it could no longer be operated.
• Overexposure of humans – generally does not have a long-term impact on the health and life of those exposed, although there are exceptions (about 5 such events are known worldwide in the last 10 years). Such an event occurs mainly with the use of medical and industrial emitters. It is very rare for overexposure to workers in nuclear installations to occur.
• Leakage into the hydrosphere – streams, rivers, lakes, etc. Such events are very rare. Their impact on the population depends on how much the watercourses are used for recreational and agricultural purposes and also on how drinking water is obtained in the area. The impact on people is reduced if it is a spill into a large and flowing body of water. The reference event for a spill into the hydrosphere can be the seepage of contaminated water from the Fukushima Dai-ichi plants into the waters of the Pacific Ocean. IAEA experts say the impact of this leakage on public health and marine ecosystems is very low. However, the concerns and the resulting economic damage to the surrounding population are by no means negligible.
• Leakage into the atmosphere – As with most industrial activities, nuclear power plants also have organised discharges of radioactive substances into the environment, which are subject to authorisation by the competent authorities. These discharges have very strict rules concerning their filtration, quantity and other conditions. However, in the event of accidents and incidents, unplanned releases can also occur – the largest in history was the Chernobyl nuclear power plant release, where, due to a breach in the integrity of the reactor and the subsequent ignition and burning of graphite for several days. To get a better idea of these types of accidents, consider the accidents at the US Three Mile Island and Japan’s Fukushima Dai-ichi power plants. At Three Mile Island, there was a severe breach of the reactor fuel, but only less harmful radioactive materials were released into the surrounding area – mainly noble gases (which have the advantage that they do not remain in the body when inhaled).

At Fukushima Daiichi, adverse developments following the tsunami tidal wave, power failure and fuel cooling led to fuel meltdown, the build-up of critical amounts of hydrogen, its detonation and the subsequent release of a wide range of fissile materials into the atmosphere. The scale of the release was only several times smaller than Chernobyl’s, but it was a fortunate circumstance that the wind carried some of the leaked radioactive materials out to sea. Nevertheless, it led to the evacuation and subsequent resettlement of large numbers of residents from surrounding villages and towns.

• Combination of the previous types – especially for large releases, the events are more complex and cannot be classified into one of these groups. In the Chernobyl event, in addition to the release to the atmosphere, there were extensive exposures of personnel and liquidators. In the Fukushima accident, although dangerous exposures to humans were avoided, the danger of leakage of radioactive materials into the nearby sea still needs to be addressed.

Emergency Classification of Events at Nuclear Installations

In accordance with Decree No 55/2006 on the details of emergency planning in the event of an incident or an accident, a three-tier classification of events according to their severity has been introduced for emergency classification:

• Level 1 – alert  – a condition in which the performance of safety functions is compromised or violated, safety barriers are broken or inoperable, there is a threat of radioactive leakage or radioactive substances have escaped, which may lead or leads to the unlawful exposure of persons in the building facilities, and in the event of an unfavourable development of the event, there is a threat of radioactive substances being released outside of the building facilities of the nuclear installation.
• Level 2 – on site emergency – a condition which may lead to or results in the release of radioactive substances outside the nuclear installation’s structures and onto the nuclear installation’s site
• Level 3 – general emergency / off site emergency – a condition which may lead or is likely to lead to a significant release of radioactive substances into the vicinity of a nuclear installation