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doses of 40 rads (0.4 Gy). Nevertheless, for purposes of setting radiation safety standards, one postulates that there is a zero threshold for stochastic effects, and that the probability of a stochastic effect increases linearly with the size of the dose. This model for setting radiation safety standards is called the linear no threshold (LNT) model.
TABLE 3 Threshold doses for some clinically harmful deterministic effects.
Source: Ref. 6.
| Organ | Injury at 5 yr | 1–5% Dose | 25–50% Dose |
|---|---|---|---|
| Ovaries | Sterilization | 200–300 rads | 600–1 200 rads |
| Testes | Sterilization | 500–1 500 | 2 000 |
| Lens | Cataract | 500 | 1 200 |
| Kidney | Nephrosclerosis | 2 300 | 2 800 |
| Liver | Failure | 3 500 | 4 500 |
| Thyroid | Hypothyroidism | 4 500 | 15 000 |
| Breast | Atrophy | <5 000 | <10 000 |
| Bladder | Ulcer | 6 000 | 8 000 |
| Bone | Necrosis | 6 000 | 15 000 |
7 SAFETY STANDARDS
Radiation safety standards are designed to prevent deterministic effects and to limit the probability of a stochastic effect to a societally acceptable level in return for the benefits to society that result from the use of the radiation. Radiation safety standards and practices are recommended by the ICRP, a nongovernmental scientific body. These recommendations form the basis for the radiation safety standards promulgated by the various international and national regulatory agencies. On an international level, the United Nation's International Atomic Energy Agency (IAEA) is the leading standard setting organization. Its safety standards are published in the Safety Series under the title International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources 2. In the United States, the Environmental Protection Agency (EPA) sets the basic standards, and several regulatory agencies, including the Nuclear Regulatory Commission (NRC), the Occupational Safety and Health Administration (OSHA), the Mine Safety and Health Administration (MSHA), and the Food and Drug Administration (FDA), issue specific regulations for those activities over which they have legal jurisdiction. The USNRC regulations are based on ICRP Publications 26 and 30 1,3, while OSHA regulations are based on ICRP Publication 2. The Department of Energy (DOE) has a mixture of ICRP recommendations that are promulgated in Title 10, Code of Federal Regulations (CFR), part 835.
7.1 Nuclear Regulatory Commission
The NRC's task is to oversee the safe use of nuclear energy and its radioactive by‐products. The definition of by‐product materials was expanded in 2005 by amendments to the original Atomic Energy Act of 1954. To this end, it regulates the production, possession, and use of the following:
Source materials. Uranium and thorium, and their ores containing at least 0.05% U or Th.
Special nuclear material (SNM). Plutonium, 233U, and uranium enriched in 233U or 235U.
Any material, except SNM, produced or made radioactive incident to making or using SNM.
Radium sources.
Material made radioactive in a particle accelerator.
Sources of naturally occurring radioactive material.
7.1.1 Primary Radiation Safety Standards (Dose Limits)
For occupational exposure, the NRC's radiation safety standards are as follows:
To limit stochastic effects. 5000 mrem (50 mSv) total effective dose in one year.
To prevent deterministic effects. 50 000 mrem (500 mSv) in one year to any tissue except the lens of the eye, which is restricted to 15 000 mrem (150 mSv).
To protect a conceptus. 500 mrem during the entire pregnancy.
To protect individual members of the public. 100 mrem in one year.
7.1.2 As Low as Reasonably Achievable
It must be emphasized that these limits are maximum doses that may not be exceeded by design (except in special cases, such as a rescue operation following an accident). In practice, the ICRP recommends, and the regulatory agencies require, the application of the as low as reasonably achievable (ALARA) principle, which states that operations be designed so that the work is done in such a manner that the resulting dose is ALARA. NRC regulations require a licensee to demonstrate that the ALARA principle is being applied.
7.1.3 Derived Radiation Safety Standards
The primary annual dose limits include the sum of external doses and doses due to radionuclides within or on the surface of the body. To achieve the primary standard, secondary standards are prescribed, which establish an annual limit of intake (ALI) for all radionuclides in air and in potable water. The ALI is the maximum intake of radionuclide, either by inhalation or by ingestion, which results in the primary annual dose limit to the worker if radiation exposure were limited to that radionuclide taken in through that portal of entry. Thus, the ALI must be proportionally decreased if a worker is exposed to more than one radionuclide and/or external radiation.
Tertiary limits are expressed as radionuclide concentrations in air and in water. In the case of occupational exposure to airborne radioactivity, the derived air concentration (DAC) is the inhalation ALI divided by the volume of air, 2400 m3, that the worker is assumed (according to the Reference Man model) to inhale during the course of a working year. Effluent limits to the air and to the water environments are derived in a similar manner using the dose limit for the public rather than the occupational dose limit.
These standards are enforced through a system of licensing, inspection, and fines for violations. The NRC is authorized to delegate its licensing and inspection of by‐product material to Agreement States. An Agreement State is a state that has signed an agreement with the NRC to regulate the use of radioactive materials within that state to at least the same degree as the NRC. The state must show the NRC that it has the resources to meet this obligation. The agreement may be terminated if the state does not meet the NRC's criteria. At this time, 2018, there are 38 Agreement States.
The NRC regulates some, but not all, naturally occurring radioactive materials that have been concentrated (226Ra) and all radionuclides produced in accelerators based on changes in the Atomic Energy Act in 2005. The NRC does not regulate the manufacture or use of radiation‐producing machines,