|NEA Issue Brief: An analysis of principal nuclear issues|
|No. 10, December 1994|
Radiation protection overview: international aspects and perspective
I. What is radiation protection?
Radiation protection is a term applied to concepts, requirements, technologies and operations related to protection of people (radiation workers, members of the public, and patients undergoing radiation diagnosis and therapy) against the harmful effects of ionising radiation. It has its origins early in the twentieth century. The benefits of radiation were first recognised in the use of X-rays for medical diagnosis, very soon after the discoveries of radiation and radioactivity. The rush to exploit the medical benefits led fairly soon to the recognition of the other side of the coin, that of radiation-induced harm. In those early days, only the most obvious forms of harm resulting from high doses of radiation, such as radiation burns , were observed and protection efforts focused on their prevention, mainly for practitioners rather than patients. Although the issue was narrow, this was the origin of radiation protection as a discipline. Over the middle decades of this century, it was gradually recognised that there were other, less obvious, harmful radiation effects such as radiation-induced cancer, for which there is a certain risk even at low doses of radiation. This risk cannot be completely prevented. It can only be minimised. Therefore, the overt balancing of benefits from nuclear and radiation practices against radiation risk, and efforts to reduce the residual risk, have become a major feature of radiation protection.
Ionising radiation and radioactive substances are natural and permanent features of the environment. These sources, called natural background radiation, consist principally of cosmic rays entering the earth s atmosphere; terrestrial gamma rays due largely to uranium and thorium, including their decay products, found in various low concentrations throughout the earth s crust; potassium 40, a radioisotope which is mixed in small concentrations in nature with stable potassium; and radon decay products. Through natural background radiation, people are exposed to external radiation and internal radiation by inhalation and ingestion of radioactive substances existing in the natural environment.
Artificial radioactivity resulting from nuclear weapons programmes, principally atmospheric testing, is also spread throughout the world and exposes people to external radiation and internal radiation through inhalation and ingestion. However, the average dose to individuals in the world population from military activities is very small compared to that resulting from natural background radiation, and is declining due mainly to the atmospheric test ban treaty of 1963, but also to the general reduction of nuclear weapons programmes. Some countries are now faced with the difficult task of decontamination and stabilisation of military weapons test and production sites.
Additionally, the use of man-made radiation is widespread. These sources of ionising radiation are called practices. The use of nuclear energy and applications of its by-products (i.e., ionising radiation and radioactive substances) continue to increase around the world. In addition to nuclear power production, nuclear techniques are used in industry, agriculture, medicine and many fields of research, benefiting hundreds of millions of people and giving employment to millions of people in the related occupations. For example, medical X-rays and nuclear medicine are vital diagnostic tools, and radiotherapy is commonly part of the treatment of cancer. Large irradiators are used in many countries to sterilise medical products, preserve foodstuffs and reduce wastage, and sterilisation techniques have been used to eradicate disease-carrying insects and pests. Industrial radiography is in routine use to examine welds for defects and help prevent the failure of engineered structures. Radiotracers are used in many fields of research.
Figures 1 and 2 show the estimated percentage dose contribution of various sources, both natural and artificial, averaged to individual members of the population in the United Kingdom and the United States, respectively. The percentage contribution of the sources to any specific individual will vary from the average depending on a variety of factors (e.g., increased cosmic radiation for those living at high altitudes, individuals receiving medical radiation diagnosis or treatment). Although there are some differences in the way in which source contributors are categorised in Figures 1 and 2, it should be noted that the average dose from natural sources dominates the dose from all other sources combined, estimated to be 87% and 82% for the United Kingdom and the United States, respectively.
The process of ionisation necessarily changes atoms and molecules, at least temporarily, and thus may damages cells. If cellular damage does occur and is not adequately repaired, it may prevent the cell from surviving or reproducing, or it may result in a viable, but modified, cell. The two outcomes have profoundly different implications for the organism as a whole. Most organs and tissues of the body are unaffected by the loss of even a substantial amount of cells, but if the number lost is large enough, there will be observable harm to the individual, reflecting a loss of organ or tissue function. The likelihood of causing such harm will be zero at low doses but, above some level or threshold dose, the damage will occur almost with certainty. Above the threshold, the severity of the harm increases with increasing dose. This type of outcome, which includes acute radiation syndrome, is called deterministic, because the harm is almost bound to occur in exposed individuals if the dose exceeds the threshold dose. The adverse effects first observed in the early use of radiation were deterministic effects. Threshold doses are substantially higher than the doses to workers and members of the public expected from practices and sources in normal operation. Only an accident involving a source capable of delivering high doses is likely to cause deterministic effects.
The situation is very different if the irradiated cell is modified rather than killed. Despite the existence of highly effective defence mechanisms, the cloning of cells resulting from the reproduction of a modified, but viable, cell may result, after a prolonged and variable delay, called the latency period, in the manifestation of a malignant condition (e.g., cancer). The probability of a cancer resulting from radiation increases with increasing dose. This probability is assumed for protection purposes to be without a threshold and to be proportional to dose for doses below the thresholds for deterministic effects. Since only the probability, but not the severity, of the cancer is affected by the amount of dose, the outcome is called stochastic, meaning of a random or aleatory nature.
If the radiation damage occurs in a cell whose function is to transmit genetic information to later generations, it is presumed that some harm, which may be of many different kinds and severity, might be expressed in the progeny of the exposed person. This type of stochastic outcome is known as hereditary. The probability of hereditary harm also is taken to be proportional to the dose received. In addition, irradiation in utero can lead to effects in children, principally an increase in the stochastic risk of childhood leukaemia and a reduction in IQ (intelligence quotient) following irradiation, mainly during the eighth to fifteenth weeks of gestation.
Stochastic effects of radiation are only detectable in epidemiology studies having sufficient statistical power, and usually require large populations and years of follow-up to cover the latency period of the exposed individuals studied. The estimated risks of a radiation dose resulting in a stochastic outcome are derived from a number of epidemiology studies, the most important being the study of survivors of Hiroshima and Nagasaki atomic explosions. Radiation protection standards based on stochastic risk estimates employ assumptions about risk which are seen to be conservative in line with the degree of scientific knowledge about the risk.
Based upon the wealth of scientific knowledge to date and employing conservative assumptions, the likelihood of stochastic outcome due to normal levels of radiation exposure is estimated to be very small. For average exposure to natural background radiation, the chance is of the order of 1 in 10,000 per year. For average exposures in the population from many current practices, it is very much lower than it is for background radiation.
As previously noted, the human activities, such as those ranging from nuclear power production to radiation medicine, that add radiation exposure to that which people normally receive due to background radiation, or increase the likelihood of adding exposure are termed practices. The human activities that seek to reduce the existing radiation exposure, or the likelihood of incurring exposure which are not part of controlled practices (e.g., radon in homes) are termed interventions.
For routine conditions involving practices, most exposure of workers and members of the public is the result of normal operating conditions. However, there may sometimes be variations in operating conditions that cannot be regarded as normal. The term potential exposure is used to describe exposure that is not certain to occur, (i.e., an exposure caused by some departure from normality). Potential exposure reflects the combination of the probability of occurrence of potential events, the chance that such events will result in a radiation dose to individuals and the probability of radiation effects from the expected resulting dose.
Bearing these distinctions in mind, radiation protection for practices is founded on a conceptual framework proposed by the International Commission on Radiation Protection (ICRP) and involves three principles: justification, optimisation and limitation.
Intervention involves the application of radiation protection principles retrospectively, i.e., when it is decided to reduce existing exposure caused by an accident, contamination from past practices or high natural background which is amenable to being reduced. Two principles are involved in the case of intervention: justification and optimisation.
The conceptual framework for radiation protection, as proposed by the ICRP, provides a basis for operational criteria and guidance applicable to specific situations (e.g., nuclear power, medical radiation therapy, chronic exposure to natural radiation) developed by international intergovernmental organisations such as the International Atomic Energy Agency (IAEA), the Commission of the European Communities (CEC) and the Organisation for Economic Co-operation and Development/Nuclear Energy Agency (OECD/NEA). Essentially, all countries incorporate ICRP concepts in their radiation protection regulations and practices.
Radiation protection concepts, however, can only be implemented through an effective infrastructure which includes adequate laws and regulations, an efficient regulatory system, a well structured complex of experts and operational provisions. It is also essential to establish an attitude and behaviour shared by all those involved with protection responsibilities, from workers through management levels, which ensures that protection and safety issues receive the attention warranted as an overriding priority. This attitude and behaviour is sometimes called a safety culture.
In general, national legislation establishes a regulatory authority empowered to issue regulations, authorise a registration and/or licensing of sources, conduct inspections and take enforcement actions. While the regulatory authority is empowered and responsible to the public for discharge of these functions, registrants and licensees bear prime responsibility for the safety of the sources in their possession. They are responsible for establishing a safety culture within their organisation and are responsible for ensuring safety of their workers and members of the public with regard to their operations. Others, such as designers, manufacturers and constructors have professional and legal responsibilities that are also significant to safety.
A fundamental component of radiation protection linked to the infrastructure is the availability of adequate measurement equipment and techniques as well as modelling and assessment methods and software. These are well developed for most situations. It is also expected that the evolution of these protection technologies will continue with gradual improvements in instrumentation, modelling, assessment methods and quality control, in parallel with developments in fields such as electronics, environmental studies and the nuclear industry in general.
With respect to the quality of radiation protection infrastructures, there is a significant diversity of situations throughout the world. The OECD countries generally have well established infrastructures for radiation protection, with exhaustive and regularly updated regulations, strong and competent regulatory bodies, adequate operational protection and emergency response structures, and advanced research institutions as well as adequate measurement and assessment technologies. There are obvious variations in the level and size of these infrastructures, linked to the different levels of radiation and nuclear power applications in the various countries, but, as a whole, the standard of radiation protection across the OECD area appears good and sometimes excellent. This conclusion is supported by trends showing significant dose reduction in many practices through diligent application of the optimisation of the protection principle in several OECD countries. The situation is much more uneven in the rest of the world. Beside countries where the infrastructure and the standard of protection are fully comparable with those of the OECD countries, lie a large number of countries which do not have a sufficient or even a significant infrastructure for radiation protection. This is due to a lower degree of economic development or the presence of significant political instability and, in several cases, to a severe shortage of resources where priorities are assigned to more pressing societal needs.
The new International Standards for the Protection Against Radiation and the Safety of Radiation Sources (BSS) developed through a joint effort by the Food and Agricultural Organisation of the United Nations (FAO), the IAEA, the International Labour Organisation (ILO), the OECD/NEA, the Pan American Health Organisation (PAHO) and the World Health Organisation (WHO) provides a set of conceptual and applicative recommendations appropriate for developing protection regulations and operational requirements. The BSS will provide valuable guidance in establishing or improving national radiation protection infrastructures where they are not presently adequate.
Radiation protection is a dynamic field. The wealth of scientific knowledge upon which it is founded increases constantly. There are new advances in technology both with respect to providing protection and the use of radiation sources. Also, regardless of the general status of protection, there are a number of conceptual and practical issues which remain open.
There is a growing feeling that future advances in biology might result in breakthroughs in fundamental scientific knowledge which could change the dose-effect relationship and risk models, and provide genetic analysis techniques capable of identifying some specific radiation-induced tumours above the general background of tumour incidence. Developments such as these could affect how the principles for radiation protection are implemented. For example, experimental data on adaptive responses or stimulation of cellular repair at very low doses, if confirmed, could affect estimates of stochastic risk of low doses and lead to revised approaches to situations such as those involving intervention.
Some practices are in a constant state of evolution with new technologies and procedures replacing the old. The use of radiation in medicine is an example of such a situation. Several models of power reactors based on new safety concepts are being developed. Nuclear fusion is a truly new practice undergoing long term development which may become a reality in the decades to come. These are examples of the types of developments that can involve new radiation protection issues and strategies. Also, another area which is expected to reach an industrial dimension in the next few decades is decommissioning of commercial nuclear power plants. Here, emphasis in radiation protection should focus on optimised strategies for the protection of workers and the public. Of particular interest is the practical application of the protection principles to exempt wastes associated with huge amounts of slightly contaminated scrap materials and valuable metals.
Applying the concepts of protection against potential exposure to sources used in medicine, industry and research as well as applying the concepts to waste disposal, presents a particular challenge with much yet to be done. Mistakes or accidents involving relatively simple sources have resulted in serious injury and deaths. There is a need to improve the ability to assess and manage the risks from potential exposure, particularly with respect to accounting for the complete human-machine systems and interfaces in safety evaluations. This is particularly important for radiation therapy, because the margin of error is small when treating patients with high-radiation doses. Since devices and procedures used in radiation medicine are constantly evolving, keeping current with understanding and managing potential exposure risk is particularly difficult.
Adequate treatment of the long-time aspect (thousands of years) of waste disposal is a difficult problem with respect to potential exposure. Although there seems to be a consensus that the radiation protection objective of waste disposal is not to subject people in the future to a risk that is significantly greater than society is willing to accept now, the challenge consists in demonstrating the case against specific national performance criteria to the satisfaction of the regulatory authorities. It involves a very complex safety analysis of the disposal system. The main difficulties in providing a robust safety analysis for disposal are a lack of information about the frequency of disruptive events, lack of feedback from operating experience and design evaluation, and lack of environmental models for the future. Overcoming these difficulties requires the concerted efforts of radiation protection, waste management and other specialists at both international and national levels.
Finally, there is the social dimension of radiation protection. Radiation causes public anxiety regardless of how well present radiation practices enable persons to live in relative safety with radiation. The cause of this anxiety is only speculative and probably cannot be attributed to a single cause, but rather a combination of things, such as its association with nuclear weapons, the fact that it cannot be detected by the human senses and that it can produce cancer. This has led to a keener sensibility to the costs than to the benefits of some radiation practices.
For this and other good reasons, decision-making in several areas of radiation protection cannot be isolated from its social dimension and must involve the social parties affected. Better involvement of social parties in radiation protection decisions requires improvement in the information provided and education of interested parties about radiation, its benefits and impacts, and the protection against these impacts. Although society is showing an ever increasing interest and willingness in being involved in decisions affecting life and well-being, from the standpoint of radiation protection, a reinforced and better focused effort is needed.
The present conservative concepts and models for radiation protection provide adequate protection. The standard of radiation protection across the OECD areas appears good and sometimes excellent. A similar conclusion can be drawn for some, but not all, countries throughout the world.
Radiation protection is a dynamic field. It has undergone a significant evolutionary period during the past few decades. General improvement of protection techniques and technology is expected to continue. In addition, much is going on in fundamental research, particularly in the molecular biology area, which could alter the scientific foundation upon which today s protection is based. This could lead to more efficient use of resources allocated to protection, as well as other benefits.
There continues to be a number of challenges for protection specialists. One, for example, is satisfactorily addressing and demonstrating adequate protection for those aspects of waste disposal, which continue to be the subject of public controversy. Also, finding practical ways to apply the concept of potential exposure to a variety of practices will require continuing development effort. Another, and very important challenge, is involving the interested social parties in protection decisions and adequately taking into account the social dimensions of such decisions.
Because of the dynamic nature of the protection field, the prospect of new radiation and nuclear practices, and changing public attitudes toward risk, it is important that the wealth of expertise and resources for protection and related fields which has been accumulated so far is preserved in order to guarantee adequate and cost effective protection.