Summary
In this report, Strålsäkerhetsmyndigheten, the Swedish Radiation Safety Authority (SSM), presents an analysis of the potential radiological consequences of fallout from the nuclear explosions at distances between about 10 kilometres and about 300 kilometres from the explosion, and the effect of various protective actions. The contents of the report constitute a knowledge base and not a ready-made planning basis. However, certain conclusions can be drawn and already taken into account in emergency preparedness planning.
General background
SSM is the main regulatory body with overall responsibility regarding radiation protection and nuclear safety in Sweden. As part of the national radiation protection preparedness, SSM works proactively and preventively for radiation protection and nuclear safety and is responsible for i.a. professional expertise and expert knowledge and decision-making support in the field of radiation protection, including dispersion prognoses, radiation monitoring and radiation protection assessments. This responsibility remains unchanged during a heightened state of alert.
The Government of Sweden has emphasised the importance of a coherent total defence planning to increase the overall capability of the Swedish total defence. The current guidance1 states that planning should be based on the assumption that nuclear weapons may be used against Sweden. Increased knowledge of the possible radiological consequences of fallout from nuclear explosions can therefore constitute a valuable basis for developing the Swedish national defence.
Purpose and method
Since 2018, SSM has conducted a project to study the radiological consequences of fallout from nuclear explosions. The main purpose of the study has been to develop an understanding and further knowledge concerning radiological consequences, particularly with regard to early consequences for the general public, and the effects of various protective actions. Another aim has been to develop SSM’s capabilities in dispersion and dose calculations. This study provides a platform for further development.
A nuclear explosion produces large amounts of radioactive material, which can lead to radioactive fallout. In the explosion, the radioactive substances are mixed in a cloud with weapon residues and materials from the surroundings. As the cloud cools, radioactive particles are formed and spread by wind over large areas. The most serious radiological consequences of fallout are associated with nuclear explosions at ground level. For nuclear explosions without ground contact, fallout warranting urgent protective actions is not expected to the same extent.
On behalf of SSM, the Swedish Defence Research Agency (FOI) has constructed nuclear weapon cases that can be assumed to be representative of how an attacker would use nuclear weapons operationally to achieve military objectives. As the main scenario, SSM has chosen the nuclear weapon case that is expected to result in the most serious radiological consequences, i.e. a nuclear explosion at ground level with an explosive yield of 100 kilotons. To produce more generally applicable results, SSM’s analysis is based on statistical evaluation of results from dispersion and dose calculations for a large number of different historical weather cases.
One starting point in emergency planning for radiation protection is the level of radiation dose known as the reference level. The planning should enable radiation doses to be no higher than the current reference level. In addition, below the reference level, the optimisation of radiation protection should continue. The value chosen for the reference level depends on the circumstances of the situation under consideration. The reference level for exposure of members of the public in an emergency exposure situation that currently applies under the Swedish Radiation Protection Ordinance (20 mSv annual effective dose) is not an appropriate starting point for the optimisation of radiation protection in the event of a nuclear explosion. The risks arising from the exposure must be balanced against other risks that may exist in such a situation. The overall objectives for radiation protection in emergency exposure situations are nevertheless still applicable, i.e. that severe deterministic health effects (acute radiation injury) should be avoided or minimised, and that the probability of stochastic health effects (mainly cancer) should be reduced as far as reasonably achievable. In the report, three possible reference levels associated with fallout from nuclear explosions have been used in the evaluation: 100, 500 and 1,000 mSv annual effective dose.
Scope
The report focuses on areas at such a distance from the explosion that radiation doses from the fallout constitute the main consequences. Initial ionizing radiation and other direct effects are therefore not included. In the first instance, consequences that give rise to the need for urgent action are addressed. The effectiveness of the protective actions of sheltering indoors, evacuation, and the administration of iodine tablets is evaluated, as well as the need for management of acute radiation injuries.
A number of urgent protective actions that may be relevant in the context of nuclear explosions (personal decontamination, measures to avoid or reduce inadvertent ingestion, and early measures for food and goods) are not addressed in this report. More long-term or indirect radiological consequences that may occur as a result of the fallout, such as consequences for food production or transport, are also not addressed.
Results and conclusions
Radiation doses from fallout after a nuclear explosion can in some cases be so high that they are lethal, life-threatening or result in permanent injury to an unprotected person at distances up to tens of kilometres. It is important to plan for good protection against fallout in areas at these distances. It is also important to plan for good protection at large distances, up to hundreds of kilometres, to reduce long-term radiological consequences. Since the location of the explosion is not known in advance, this means that such planning may be needed in large parts of the country.
In the short term, radiation doses are entirely dominated by radiation from radioactive material deposited on the ground. This is an important difference compared with releases from a severe nuclear power plant accident, where the largest contribution to radiation doses during the first few days comes from inhalation of radioactive material in the air. The radiation dose that may be received from fallout decreases rapidly with time after the nuclear explosion. Sheltering in premises that offer good protection against radiation from ground deposition of radioactive material during the first days after a nuclear explosion is therefore effective and allows high radiation doses to be avoided even in areas affected by heavy fallout. Examples of such protection are protective shelters, protected spaces, command centres with fortifying protection, basements, or similar premises that protect against radiation and where it is possible to stay for several days.
After a nuclear explosion, it takes some time for the fallout to arrive (depending on distance and weather) and this time may be sufficient to seek good shelter. Evacuation in this situation increases the risk of people being unprotected if evacuation is not completed before the fallout arrives. It is also difficult to predict in time which areas will not be affected by fallout. Rapidly seeking good shelter in the event of a nuclear explosion is therefore a more appropriate protective action than evacuation.
After the need to shelter in premises offering good protection has ceased, there may be areas where it is inappropriate to remain. Such areas may need to be evacuated in order to limit longer-term radiation doses from the radioactive material deposited on the ground. The capability and planning in place for nuclear emergency preparedness, e.g. to rapidly map ground deposition over large areas, can provide a good basis for further development.
Iodine tablets have no practical function in the event of fallout from nuclear explosions. Within the distances where iodine tablets could be warranted, good protection is required to avoid high radiation doses from the radioactive material deposited on the ground. In such shelters, radiation doses to the thyroid are so low that iodine tablets are not warranted. This is a another difference compared to releases from a severe nuclear power plant accident, where taking iodine tablets can be an important measure to reduce radiation doses to the thyroid.
Need for further investigation and development
Some areas related to the results of the report appear to warrant urgent continued investigation by the responsible Government agencies.
This report shows the need to plan for shelter offering good protection for the public in connection with fallout after a nuclear explosion. The inquiry report Ett stärkt skydd för civilbefolkningen vid höjd beredskap [A Strengthened Protection of the Civilian Population during a Heightened State of Alert] (SOU 2022:57) emphasises the importance of access to protective shelters and other protected spaces for the civilian population. In its report, the inquiry proposes a number of measures with regard to protective shelters and other protected spaces.
Radiation protection legislation for the general public and workers not involved in total defence needs to be reviewed. The report shows that the regulations that apply to the public in peacetime are not suitable for all situations that may arise during a heightened state of alert. For workers who are not part of total defence, it should be investigated whether the rules that apply in peacetime are suitable for the situations that may arise during a heightened state of alert.
To enable decisions on protective actions and other response actions in the event of nuclear explosions, situation assessments based on the best possible information about the event need to be produced quickly. Which actors should collaborate in such a process needs to be investigated. The forms and content of their interaction also need to be developed. The design of reports to support decision-making is also dependent on the planning that exists for protective actions and other response actions. The development of collaboration must therefore, as with nuclear emergency preparedness, go hand in hand with the development of relevant decision support.
The system for alert, warning and communication with the public in connection with fallout from nuclear explosions needs to be developed. The development needs to consider several issues that are dependent on time and distance conditions when it comes to protection against radioactive fallout from nuclear explosions. There may be large areas that will not be affected by fallout, and where protective actions can be avoided by early analysis of prevailing weather conditions. Furthermore, there may be large areas that will eventually be affected, but where there is time to get to prepared premises that offer good protection from radiation for several days, instead of rushing to a closer but inferior and less sustainable shelter. The amount of time people need to spend sheltering from fallout on the ground depends on the amount of fallout at the site, and cannot be determined in advance. Without their own measurement capability, people in shelters depend on information from the responsible authorities about when they can leave the shelter. This information needs to be followed by recommendations on how to proceed after sheltering has ceased.
Appropriate intervention levels for food in connection with nuclear explosion fallout need to be developed. Fallout from a nuclear explosion can affect foodstuffs at great distances. The consequences for the production of food can be evaluated with the method used in this study, but this requires the availability of appropriate intervention levels, i.e. levels of ground contamination where a certain protective action may need to be taken. Fallout from nuclear weapons is so different from fallout in connection with nuclear power plant accidents that it cannot be assumed that the intervention levels for foodstuffs that have previously been developed can be used.
What capacity for radiation monitoring should be available at the local, regional and national levels, as well as what types of radiation monitoring should be carried out at various stages in connection with a nuclear explosion, needs to be further investigated. Radiation monitoring capabilities are needed at all levels of society, from national resources for qualified analyses and large-scale mapping of fallout to local capabilities for determining whether it is possible to leave a shelter. For radiation monitoring in connection with a nuclear explosion, prepared support is also needed for interpretation of measurement results, for example to provide guidance on when it may be appropriate to leave initial shelter.
Going forward, SSM intends to continuously improve its modelling capabilities for fallout from nuclear explosions. The Swedish Radiation Safety Authority will also, in collaboration with other relevant Government agencies and other stakeholders, use the current and future results to analyse and contribute to improving society’s protection against fallout from nuclear explosions.
1 Looking Forward – Action Plan to Promote and Develop Coherent Planning for the Swedish Total Defence 2021-2025