Reference levels for nuclear emergency response and post-accident recovery
Being prepared in the event of a nuclear accident is an essential part of being a responsible nuclear regulator. The Canadian Nuclear Safety Commission (CNSC) has a comprehensive emergency preparedness program, and works with nuclear operators, municipal, provincial and federal government departments and agencies, first responders and international organizations to always be ready.
Although nuclear accidents are very rare, it is still important to prepare should an accident occur. This planning is a critical component of the radiological protection system implemented in Canada and abroad, based on the recommendations of the International Commission on Radiological Protection (ICRP). Historically, these plans have been focused on the emergency response phase (see figure 1), where decisions need to be made quickly, based on very little information, to ensure the protection of the public.
Less emphasis has been placed on the development of plans for the post-accident recovery phase. This has largely been due to the longer time period available for decision making and the unique location and accident-specific factors that would need to be taken into consideration during this phase.
However, the practical experience gained from the nuclear accidents at Chernobyl and, more recently, Fukushima, has highlighted the importance of planning for the post-accident recovery phase. In Canada and abroad, nuclear regulators like the CNSC and other concerned organizations are actively working to develop and strengthen plans for this phase.
Emergency response and post-accident recovery plans are developed based on an internationally recommended framework, which includes the use of “reference levels” to help guide decision making. This factsheet will explore the concept of reference levels and how they apply to the emergency response and post-accident recovery phases of a nuclear accident.
Radiation doses and potential radiation health effects
When ionizing radiation penetrates the human body, it deposits energy. The energy absorbed from exposure to radiation is called a dose. This absorbed energy can cause damage to cells and can result in health effects.
Depending on the amount of dose received, different types of health effects are possible. At very high doses, enough cells can be damaged to impair the functioning of tissues and organs. These types of health effects are called deterministic effects. At lower doses, damage to individual cells can make the cell unable to reproduce properly and can result in an increased risk of health effects, such as cancer. These types of health effects are called stochastic effects.
In general, the goal of recommendations or limitations on dose is to keep doses low enough to prevent deterministic effects and to minimize the risk of stochastic effects. For routine nuclear facility operations, this is achieved through the public dose limit of 1 millisievert (mSv) per year prescribed in Canada’s Radiation Protection Regulations. In addition, there is a requirement that doses are kept as low as reasonably achievable, with economic and social factors taken into account.
In the unlikely event of a nuclear accident, it may not be possible or practical to keep doses below the 1-mSv-per-year dose limit. For example, evacuating members of the public may reduce or avoid an exposure to radiation, but the process can be extremely stressful and more dangerous to public health – especially if the expected dose is relatively low. These non-radiological considerations need to be taken into account during the decision-making process.
The ICRP has established reference levels to help with decision making in both the emergency response and post-accident recovery phases of a nuclear accident. The ICRP’s members include some of the world’s leading scientists and radiation protection professionals who develop recommendations on acceptable radiation dose limits, including reference levels.
A reference level is defined by the ICRP as “the level of residual dose or risk above which it is generally judged to be inappropriate to allow exposures to occur”. In other words, reference levels provide guidance to regulators and other decision makers on what doses above the normal dose limit could still prevent deterministic effects and minimize the risk of stochastic effects, while also taking into account other non-radiological considerations.
Recommended reference levels
The ICRP’s recommended reference levels are presented as “bands” or ranges of doses for different types of situations. The bands provide flexibility for the decision maker to decide on an appropriate level of exposure, while taking into account other non-radiological considerations specific to each situation. Reference levels are expressed in millisieverts (mSv – acute or per year) and in terms of residual dose – the dose received after any protective actions have been implemented.
|Band (acute or per year)||Type of situation|
Emergency situations, where events with uncertain consequences require urgent protective actions such a sheltering and evacuation to minimize the impacts of possible radiation exposures.
Existing situations, where radioactivity is already present in the environment at the time actions are taken to reduce radiation exposures. If doses are optimized below this reference level it is safe to live in the contaminated area.
How do reference levels differ from dose limits?
Dose limits are restrictions on dose that apply only to situations where the potential exposures can be predicted and radiation protection measures can be put in place in advance (i.e., planned situations). Dose limits are individual-related, meaning that all potential sources of radiation need to be taken into account when comparing to a dose limit. Although dose limits are recommended by the ICRP, they are ultimately enshrined into regulations by the national regulatory authority and represent distinct limits which cannot be exceeded through nuclear activities.
Reference levels are restrictions on dose that apply to situations where potential exposures are not known in advance (i.e., emergency situations) and existing situations. Unlike dose limits, reference levels are source-related, meaning that only the dose from a specific source of radiation is considered when comparing to a reference level. Reference levels are a tool to aid regulators and other decision makers in ensuring that the doses are appropriate to the situation. They are not typically included in regulation.
Reference levels in the emergency response phase
The 20–100 mSv reference-level band is recommended for emergency situations. Urgent protective actions such as potassium iodide pill ingestion, sheltering and evacuation – implemented according to dose-based guidelines called “intervention levels” – may be needed during this phase to manage radiation exposures. In Canada, plans for implementing these types of actions already exist at the federal level, and at the provincial and municipal levels where nuclear power plants operate. For more information on intervention levels and protective actions implemented during the emergency response phase, consult the CNSC fact sheet Managing public doses during a nuclear emergency.
Dose limits for the emergency response phase
The Radiation Protection Regulations also prescribe dose limits for the emergency response phase. They indicate an effective dose not exceeding 500 mSv and an equivalent dose to the skin not exceeding 5,000 mSv during the control of the emergency. These dose limits could apply to workers onsite at the accident or to members of the public involved in the offsite response, such as first responders.
Reference levels in the post-accident recovery phase
The 1–20 mSv reference-level band is recommended for existing situations, including the post-accident recovery phase. Any radioactive contamination of the environment resulting from the nuclear accident has already occurred when actions may need to be implemented to reduce radiation exposure. Actions during this phase could include decontamination of the environment, waste management and the return of the population following an evacuation or long-term relocation.
Currently, the CNSC is involved in a number of post-accident recovery phase initiatives, including development of a Post-Accident Management Framework for Canada and participation in the International Atomic Energy Agency Modelling and Data for Radiological Impact Assessments Programme. Working groups within this initiative are studying a variety of topics, including model testing and comparison for accidental tritium releases and the use of decision-making tools in the post-accident recovery phase.
As the post-accident recovery phase resulting from the Fukushima nuclear accident unfolds, lessons learned will continue to be incorporated into emergency preparedness activities. In the very unlikely event of a nuclear accident in Canada, municipal, provincial and federal authorities will work together throughout the emergency response and post-accident recovery phases to ensure that Canadians and the environment are safe.
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