defense-in-depth principle, Accidental situations, RDD, emergency types, operational aspects of emergency situations, generic criteria, emergency signals, IAEA International Nuclear Event Scale (INES), health concerns of members of the public during emergency.
Defence in depth concept in the design of nuclear facilities
In-spite of the best design and redundancy of safety systems, accidents are still possible with very low probability of occurrence. Defence-in-depth concept is used in the location of major nuclear facilities such as reactors (see the figure) to control radioactive releases and exposures to the public in case of any abnormal / emergency situations arising in the facility.
The site for a nuclear reactor is chosen in such a way that there is adequate distance is maintained from the reactor for any accidental releases to reach public domain (see the figure). In Exclusion zone, no members of the public are allowed to live, the Sterilised zone serves as buffer zone, and emergency preparedness plan is made available for the area covered by the Emergency Planning Zone.
The same concept is also used to safely securing the radioactive sources during normal and abnormal situations. For example, the highly radioactive fuel in nuclear reactor core is designed to be secured deep inside the various layers of physical barriers such as the fuel cladding, the coolant channels, the calandria, which is enveloped by the containment of the reactor building. Any release of radioactivity present in the fuel has to penetrate all the barriers before affecting the members of the public.
Accident is as: any unintended non-routine situation occurring by operating errors, equipment failures or other mishaps, the consequences of which are not negligible from the point of view of protection and safety. In-spite of proper built-in controls and written down precautions to avoid accidents, accidents are likely to occur, may be due to unintentional human error.
There is a potential for significant exposure of people and the environment to radiation in such situations. It is necessary to have an effective response plan in place to meet such situations to control exposures of workers, the public and the environment. The radiation hazards in a nuclear facility or radiological facility are assessed and suitable emergency preparedness plan of response is established. Based on the assessed hazards, five preparedness categories are identified in order to prepare suitable and optimized response plan in the facilities.
National regulatory bodies are the controlling authorities in nuclear or radiological emergency situations. The regulatory bodies develop their own emergency preparedness and response manuals for the management of emergency in respective facilities. The main objectives of emergency preparedness and response plan is to mitigate the consequences of the nuclear or radiological emergency if it occurs despite all efforts made to prevent the same.
In the event of an accident leading to emergency situations, the amount of radioactive materials released to environment is often not under control and hence the exposure to the public can only be reduced by introducing the protective measures or countermeasures, such as sheltering, evacuation, relocation, resettlement, prophylactic use of iodine to saturate the thyroid, and restrictions on use of food and water. These actions may also incur some additional risks and also impose some constraints on people’s activities. The levels of dose for intervention are different under accidental situations are given in IAEA documents for guidance. Unlike response to conventional emergencies, response to nuclear emergencies is likely to involve many local, regional, national/international organizations/agencies and other specialized technical experts.
In general, emergency situations are classified as:
As the name suggests, in emergency alert situation, the management is able to identify the event and declares “Emergency Alert” and take suitable operating procedures to mitigate the consequences.
In Plant Emergency situation, the event has occurred inside the plant and the consequences are not likely to cause any unexpected consequences outside the plant. The plant operations are suspended and resumed only after clearance from the controlling authority.
In this situation, the event in a radiological or nuclear facility has significant radiological consequences in the site. Elaborate response plans are put into operation to control radiation exposures and prevent spread of contamination. The regulatory authority ensures that other facilities in site are shut down until the emergency situation is called off.
The radioactivity releases wide spread and has crossed the site boundary and comprehensive mitigating measures are necessary to respond such emergency situation. All the concerned agencies and authorities are required to respond as per the emergency response plan to mitigate the consequences.
Implementation of any protective action needs a basis in terms of dose that can be projected or dose that has been already received. In an emergency situation, these doses cannot be used for quick actions during emergency since the doses cannot be measured. Operational criteria that are measurable or observable quantities on the scene of emergency can be used for decision taking on protective actions.
Significant accidents have happened worldwide involving nuclear fuel cycle facilities such as reactors, and radiation sources (sealed or unsealed form) used in medicine and industry are either lost, stolen or abandoned (orphan sources) or recycled through metallic scrap materials. Many such sources are sealed sources. Typical accidents are: in 1957 at Windscale (UK), in 1979 at Three Mile Island (USA), in 1986 at Chernobyl, in 1987 at Goiania (Brazil), at Tokaimura (Japan) in 1999, Panama (2001) and Fukushima, Japan (2011).
Present scenario has thrown in a new dimension to the accidental scenario where terrorist may use a radiation source or radioactive material along with conventional bomb (called dirty bomb) to spread radioactive contamination in public domain to create panic situation and mental trauma.
Radiological Dispersal Devices (RDD)
International community is concerned of threat of radiological weapon attack by terrorist groups using devices capable of scattering (dispersion) of radioactive materials, called Radiological Dispersal Devices, (RDD) / Radiological Exposure Device (RED) or dirty bomb in public domain. The RDD is not a nuclear bomb. However, it can cause wide spread contamination of areas, contaminate food chain, water sources, private and public property. The idea seems to exploit psychologically the fear in public mind about the harmful effects of radiation (radiophobia), and cause economic concerns, and medical care problems in the affected country.
A comprehensive plan, including good communication system, need to be in place to quickly address such events to control radiation doses to the members of the public, and to control the spread of contamination.
Personnel involved in the emergency actions, such as First Responder, should be volunteers familiar with the use of personal protective equipment (PPE) and radiation monitoring. They are made aware of the risks involved in such exposures. The exposures are recorded and dose limits for the workers generally apply unless there are over-riding reasons for not applying. Wherever, life saving actions are concerned, significantly higher doses could be justified.
Typical emergency signals
Public address systems need to be installed to communicate effectively emergency signals to the members of the public in case of emergency.
To Stay In
Siren ‘ON’ for 5 seconds ‘OFF’ for 5 seconds sounded for two minutes. When the accident is outside the main plant building and persons should remain inside the main plant. Air supply to the plant should be stopped and all windows are to be kept closed.
Siren “ON’ for 15 seconds, off for 5 seconds, sounded for 2 minutes. Persons should evacuate the plant and assemble at the designated assembly point for further instruction. This is effected when an accident takes place inside the main plant.
Continuous siren for 2 minutes indicating that the situation is normal and everybody can proceed to his work place.
Emergency power supply
In the event of failure of normal electrical power supply, automatic main failure diesel generating set takes over for power supply to important loads. In reactor accidents, heavy-duty pumps /generating sets located in strategic locations to supply cooling water to the reactor core is a necessary requirements. The typical other loads include:
- Exhaust fans, including for off-gas system.
- Control room instrumentation, and
- Radiation monitoring instruments
- Any other loads depending upon individual plant/facility.
The IAEA has developed the International Nuclear Event Scale (INES) to provide the media and the public the seriousness of the situation. Such events are rated on a scale of zero to seven (see figure). A rating of zero means that there was an abnormal situation but the safety systems worked properly and corrected it before there was any risk to the workers or the public. That means the situation has no safety significance. Ratings 1 to 3 are classified as incidents and 4 to 7 as accidents. The 1986 Chernobyl accident and Fukushima reactor disaster are major accidents and is rated as 7 in the scale.
Brief description of the different scales of INES
Type of events
|7||Major accident||Major release of radioactive material with wide spread health and environmental consequences|
|6||Serious accident||Significant release of radioactive material requiring implementation of planned counter measures|
|5||Accident||Accident with wider consequences requiring some planned counter measures.|
|4||Accident||Accident with local consequences requiring some planned counter measures|
|3||Serious Incident||Exposures exceeding the annual dose limits, non-lethal deterministic health effects, loss of highly radioactive sealed sources|
|2||Incident||Higher radiation levels in the operating areas, exposures of workers exceeding dose limits, significant exposure of the public, finding of orphan sources|
|1||Anomaly||Low activity source stolen or lost, minor problems with safety components|
|0||Below scale||Events of no safety significance|
IAEA International Nuclear Event Scale (INES)
Some examples of rated nuclear events (Sources)
|Level||Year||Examples of rated nuclear events|
|7||1986 2011||Chernobyl (Former USSR) now in Ukraine Fukushima Accident, Japan|
|6||1957||Kyshtym Reprocessing Plant (Former USSR) now in Russia, large off-site releases|
|5||1957||Wndscale (now Sellafield) Reactor pile, UK|
|1979||Three Miles Island, Reactor core damage, USA|
|4||1973 2010||Windscale Reprocessing plant, UK Mayapuri radiological incident, India|
|1980||Saint–Laurent, Nuclear Power Plant , France|
|1983||Buenos Aires, Critical Assembly, Argentina|
|3*||1989 2005||Vandellos Nuclear Power Plant core damage, Spain Thermal oxide reprocessing plant, Sellafield, UK|
Health concerns of members of the public during emergency
Members of the public are the vulnerable group of people with potential for exposure during emergency occurring in public domain or radiological or nuclear emergency situations spilling over to public domain, or due to intentional or unintentional acts from terror groups.
Moving away from the radiation source will reduce the exposure to high doses. Actions such as get inside a building, stay inside the buildings away from doors and windows for a few hours, by closing windows and doors and stay tuned to the announcements by the authorities for further guidance are recommended. While outside, cover your mouth and nose using any available respiratory protection such as breathing through mask, cloth or towel, and use of respirators will reduce intake of radioactive materials by inhalation route considerably.
Monitoring of the body and clothing, particularly outer layer, by experts is essential to rule out any surface contamination from the radioactive releases. Quick decontamination, like a shower, can lower individual exposures from harmful airborne contamination deposited on the skin. In cases of reactor accidents, prophylaxis measures such as consumption of stable iodine tablets (potassium iodate) to block intake of radio-iodine may be necessary for the protection of thyroid. These decisions are taken by the authorities in consultation with radiation safety officer, and the occupational physician.
General principles of management of exposed / contaminated persons, both externally and internally, are detailed, and discussed in the reference book, Radiological Protection and Safety – A Practitioner’s Guide, with relevant references.