Principles in practice: Radiation regulation and the NRC

Abstract

The US Nuclear Regulatory Commission uses quantitative radiogenic cancer risk information in a number of official areas. In this article, the authors describe two specific areas where quantitative cancer risk information is used: (1) the system of radiation protection for workers and the public, and (2) the performance of value-impact analysis (i.e., cost–benefit analysis) in the review of imposing new regulations on the industry. The authors write that two main factors have led to a change in the recommended occupational dose limit. First, the International Commission on Radiological Protection (ICRP) moved away from comparisons with safe industries, instead basing its assessment on cancer risk resulting from a lifetime of radiation exposure. The second factor is that additional epidemiological data have accumulated since 1977 and, combined with changes in the methods used to analyze this data, have resulted in a reassessment of the risk-per-unit radiation dose. The ICRP now recommends an annual occupational dose limit of .020 sievert. There was no corresponding change in the recommended dose limit for members of the public. Currently, the NRC is evaluating these changes and considering revising its regulations accordingly. Nonetheless, the authors write, dose limits play a very small role in modern radiation protection practices, the emphasis being on optimizing situations involving radiation exposure, with the result that most licensed facilities operate at annual doses to workers and members of the public that are well below any applicable limit.

It is well-established that radiation is a carcinogen at doses above a certain level. This information comes mainly from epidemiological studies, including one that focused on the survivors of the Hiroshima and Nagasaki populations following the nuclear bombing of those two cities in 1945. That study is referred to as the Life Span Study, and in it, the health status and causes of death in these populations has been closely monitored for more than half a century. Other data come from studies of patients exposed to radiation during medical diagnostic and therapeutic procedures, miners exposed to radon and other airborne radioactive materials, radium dial painters, and other, smaller epidemiological cohorts, as well as animal and cell experiments. All of these studies together lead to the conclusion that radiation is carcinogenic at high doses and that the risk of such cancer increases in proportion to the dose. The data also allow estimation of the magnitude of the risk of cancer per unit of radiation at doses above about 0.1 sievert (Sv). ¹

Despite decades of research, however, the carcinogenic properties of radiation at low doses—the dose range within which most radiation exposure takes place—remain a matter of scientific controversy. The reason for the controversy is clear: To date, there are no generally accepted data in that dose range to indicate that radiation causes cancer.

Most data on risk, such as data from the Life Span Study, are based on high doses delivered almost instantaneously or at least over relatively short time periods. ² As the dose goes down, the number of additional cancer cases above natural incidence levels also goes down and eventually becomes too small to detect above the natural cancer background. That point is reached when the dose falls below about 0.1 Sv. Based on current estimates, the risk of cancer fatality in the region around 0.1 Sv is about 0.05 per Sv. This means that, in a population of one million people, the theoretically expected number of fatalities due to radiation exposure, if every person receives a dose of 0.1 Sv, is about 5,000.

There is considerable uncertainty in this estimate, and the actual number may be zero while still being consistent with currently available data. There are few situations in which this many people receive as much radiation as 0.1 Sv, the closest being the population exposed as a result of the Chernobyl accident. ³

Additionally, the number of spontaneous cancer deaths not related to radiation exposure in the same population of one million is about 250,000. The expected 5,000 cases are well within the natural range of variation of these spontaneous cases and therefore very difficult or impossible to detect. Technically, this situation is referred to as a very low signal-to-noise ratio. It is similar to trying to hear someone whisper while standing next to an operating jackhammer: The noise background is just too high to allow one to hear what is being said, or even if anything is being said.

This uncertainty about the carcinogenic properties (if any) of low-dose radiation has posed a challenge for both the radiation protection scientific and standard-setting communities, including organizations such as the International Commission on Radiological Protection (ICRP) and the congressionally chartered US National Council on Radiation Protection and Measurements (NCRP). The Nuclear Regulatory Commission (NRC) has the responsibility, as part of its charter, for creating and overseeing the radiation protection system for civilian workers and the public and for performing cost–benefit analyses relating to regulation of the nuclear industry. The commission has responded to that challenge by incorporating radiation regulations that are recommended by these standard-setting bodies and that use the best science available for quantifying radiogenic cancer risk. In regulations relating to low-level radiation, the commission has made assumptions grounded in the precautionary principle, so that, if the regulations were to err, they would do so on the side of minimizing human exposure to ionizing radiation.

The effects of radiation

Exposure to radiation may result in one or both of two detrimental health effects: deterministic effects (recently renamed “tissue effects”), which includes what is commonly called “radiation sickness”, and stochastic effects, which include cancer and hereditary problems. Deterministic effects are caused by exposure to relatively large doses of radiation and often occur within minutes to weeks after exposure, although some (e.g., lens opacities) may be delayed for years. Deterministic effects may include skin burns, hair loss, drop in the levels of blood components such as red and white cells and platelets, loss of the cells lining the gastrointestinal tract walls, central nervous system damage, and other health problems. Each of these has a different threshold below which they do not occur, but the thresholds are all quite high, generally above about 1 gray (Gy). ⁴ For perspective, an abdominal computerized tomography (CT) scan delivers a dose of between 0.03 and 0.1 Gy. The threshold doses for the various effects were determined from experience with accidental exposures and high-dose radiation therapy.

A characteristic of deterministic effects is that the dose must be delivered over a short period of time, usually in a matter of minutes to hours; such doses are normally encountered only in accident situations involving loss of control of a large source of radiation. For example, several emergency responders died as a result of deterministic radiation effects at the Chernobyl nuclear accident in 1986. Radiation protection efforts have as one aim the design of equipment and operating procedures that make it unlikely that accidents will result in doses approaching any threshold for deterministic effects. In other words, the goal is to prevent deterministic effects entirely.

A second type of negative health impact—stochastic effects—refers to the fact that the probability of the effect is proportional to the dose of radiation received, with the severity being independent of the dose. Stochastic effects include cancer and hereditary effects. Development of cancer following radiation exposure is often referred to as a latent effect because any cancer that results from the exposure does not manifest itself for at least a few years, and up to 20 years, after exposure.

Extrapolating toward protection

The NRC uses quantitative risk information in setting its limits for exposure to ionizing radiation. Because of the absence of data to indicate the extent, if any, to which low-dose radiation is carcinogenic, different experts in the field have formulated different hypotheses on the most appropriate method by which to extrapolate the high-dose data to the low-dose range:

There is a threshold dose below which no cancer is caused, because of the ability of tissue to repair any radiation damage that occurs.

Radiation is actually more carcinogenic at low doses than it is at high doses.

Low doses of radiation have beneficial effects (the so-called “hormesis effect”).

One can extrapolate from high doses to low doses all the way down to a zero dose, and only at zero dose will the risk drop to zero.

This last hypothesis is known as the linear no-threshold (LNT) hypothesis and is the one endorsed by advisory organizations such as the NCRP and the ICRP, as well as most regulatory authorities worldwide, including the NRC. This hypothesis falls within the domain of the precautionary principle, which states that, if data on the harmful effects of low doses of an agent are not available, it should be assumed that the agent is harmful, until data show otherwise.

Under this approach, radiation is assumed to be carcinogenic all the way down to zero dose, but it is important to realize that this is an assumption, and further research may show that it is not true. The value of the risk coefficient—the risk per unit exposure—at these low doses is also not known with certainty, and could be zero. But the value in current use is based on what is considered by experts to be reasonable assumptions and extrapolations and is consistent with current understanding of the biology of radiation carcinogenesis.

Current NRC rules and practice are based mostly on the 1977 recommendations detailed in ICRP Publication 26 (ICRP, 1977). In these recommendations, the ICRP lists the cancer mortality risk resulting from radiation exposure as 0.01 per Sv, and on that basis recommends an annual dose limit for occupational exposures of 0.05 Sv, and 10 percent of that, or 0.005 Sv, for members of the public. The ICRP reduced the public annual dose limit to .001 Sv in a subsequent publication. The limits .05 Sv for occupational dose and .001 Sv for the public are the current NRC limits specified in federal regulations (CFR-20, 2012).

The occupational dose limit was selected by the ICRP on the basis of comparisons with what were then considered to be safe industries, defined as those in which the average annual probability of death due to industrial hazards is less than 1 in 10,000. Assuming that workers in the nuclear industry should have an average equivalent safety level, the ICRP calculated an annual occupational dose limit of 0.05 Sv per year. The dose limit for members of the public was selected on a similar basis, by determining the level of risk that the public generally accepts in daily life, such as the risk involved in using public transportation. That level of risk was determined to be one in one million per year. Using a standard risk coefficient, the ICRP calculated an annual dose of about .001 Sv. ⁵

Since ICRP Publication 26, two main factors have led to a change in the recommended occupational dose limit. First, the ICRP moved away from comparisons with safe industries, instead basing its assessment on cancer risk resulting from a lifetime of radiation exposure. The second factor is that additional epidemiological data have accumulated since 1977 and, combined with changes in the methods used to analyze these data, have resulted in a reassessment of the risk-per-unit radiation dose. The ICRP currently recommends an annual occupational dose limit of .020 Sv. There was no corresponding change in the recommended dose limit for members of the public. The NRC is now evaluating these changes and considering revising its regulations accordingly. ⁶

However, dose limits play a very small role in modern radiation protection practices, the emphasis being on optimizing situations involving radiation exposure, with the result that most licensed facilities operate at annual doses to workers and members of the public that are well below any applicable limit. The use of LNT implies that any radiation exposure carries with it a risk that is proportional to the dose. Therefore, in addition to the dose limits, regulations in most countries, including the United States, require that nuclear facility licensees apply the principle of optimization, or As Low As Reasonably Achievable (ALARA), which requires that the dose received by workers or members of the public be reduced to the extent that can be justified by the impacts of any methods used to reduce that dose, including cost.

Methods used to reduce radiation exposure range widely, from installing radiation shields to training workers to do their work efficiently and correctly the first time. Application of ALARA results in average occupational doses that are a small fraction of the dose limits and public doses that are an even smaller fraction of the limit, frequently much smaller than 1 percent of the limit.

Risk, costs, benefits, and nuclear reactor regulation

The NRC performs analyses to support numerous regulatory actions that affect the civilian use of nuclear materials in the United States. Most notably these analyses and regulations involve nuclear power reactor and non-power reactor license holders and applicants. The evaluation of estimates of the costs and benefits associated with a proposed regulatory action involving NRC licensees generally involves expressing costs and benefits on a common basis; for example, constant dollars from a reference year. Because the costs and benefits need to be estimated for the entire period that members of society will be affected by the proposed regulatory action, the NRC uses a present-worth basis to allow meaningful summations and comparisons. Although this approach provides a rational basis for evaluating costs and benefits, it has a number of complexities.

In essence, NRC regulatory analyses often must place a monetary value on a unit of radiation exposure, so the value of, for instance, a reduction in exposure can be compared to the costs that a commission regulation creating such a reduction might impose on licensees. The NRC employs a value of $200,000 per person-sievert of human exposure averted for the general population as one attribute for evaluation. ⁷ The NRC is currently reevaluating this calculation due to advancements in both risk-assessment and regulatory-valuation methods, in addition to inflation from 1995 when the value was last updated.

Recent recommendations from the ICRP discourage the use of collective dose to calculate health risks; the NRC, however, finds this approach useful for comparing options for potential regulatory actions. For example, the NRC performed a cost–benefit analysis to support a regulatory change of the definition and method of calculating radiation dose from small radioactive particles that get on the skin of workers. The definition was changed from averaging the dose over 1 centimeter (cm) to 10 cm. This change allowed for the licensee to have more flexibility in monitoring for these radioactive particles and reduced the number of radiation protection technicians that monitor for the particles, thus reducing the additional dose to the technicians having to perform the monitoring activities. The analysis showed it was cost-beneficial to change the regulation, weighing the risks of these small particles to workers versus the resultant extra exposure and cancer risk to the workers performing the monitoring.

An important consideration in the regulatory change was an exchange of risk types: The change increased the potential for deterministic effects (a possibility of some skin lesions in high-dose cases), but reduced the probability of stochastic effects (cancer). The change was expected to increase the risk of skin effects by increasing the possibility of delivering higher doses to the skin before particles are detected and removed. At the same time, the change was projected to reduce whole body doses to those conducting more frequent checks for skin contamination. Reduction of cancer risk probability was deemed far more important than the corresponding slight increase in the possibility of deterministic effects; the use of the $200,000 per person-sievert value was instrumental in showing the rule change was cost-beneficial.

Looking to the future

Ongoing scientific work continues to increase our understanding of the health effects and risks associated with radiation exposure, ⁸ and the NRC monitors the latest scientific information on radiation cancer risks to ensure its regulatory programs continue to adequately protect public health and safety. Toward that end, NRC staff participate in and monitor the activities and research efforts of scientific and standard setting organizations—such as the National Academy of Sciences, the UN Scientific Committee on Exposure to Atomic Radiation (UNSCEAR), the ICRP, the NCRP, the International Atomic Energy Agency, and the US Energy Department’s Low Dose Program, including its epidemiological study of one million nuclear workers, which is now underway.

Current practices in radiation protection and risk assessment are based on sound science supplemented by expert judgment and public involvement. Expert judgment is required in areas where the science is not sufficiently developed to settle open issues such as, for example, the question of whether LNT is valid, how to quantify not just the probability of mortality but also the probability of detriments resulting from non-fatal effects, and similar issues.

The question of the validity of the LNT hypothesis remains unresolved, and it is unclear when it might be answered definitively (see Beyea, 2012, in this issue of the Bulletin). Some experts believe that epidemiological studies have reached the limit of their power in this case and therefore cannot be expected, in principle, to provide an answer. The issue may eventually be settled when a sufficiently detailed understanding of the mechanisms of radiation carcinogenesis is achieved. This would permit a determination to be made on biological rather than statistical considerations. In the meantime, use of the LNT represents what many believe is a reasonable application of the precautionary principle, at least in establishing rules of practice, such as dose limits and ALARA. The NRC can be expected to continue using this hypothesis until a more definitive resolution is achieved.