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The NRC's regulations are published in Title 10, Code of Federal Regulations. To assist applicants for NRC licenses, or to assist licensees in complying with the regulations, the NRC publishes Regulatory Guides and NUREG documents. These guides describe procedures and methods that are acceptable to the NRC for the purpose of demonstrating compliance with the regulations.
7.2 Occupational Safety and Health Administration
OSHA is a federal regulatory agency whose function is to assure safe workplaces. It does this by establishing occupational safety and health standards and by inspecting workplaces to determine whether they are in compliance with the regulations. Radiation exposure from all nonmining radiation sources that are not regulated by the NRC, such as X‐ray machines, are subject to OSHA regulation. Because of OSHA's limited resources, however, safety regulation of these radiation sources in Agreement States is left to the state. OSHA regulations are based on ICRP 2 recommendations. It should be noted that although OSHA is authorized to regulate and inspect to determine compliance with the regulations, it is not authorized to license any facility. OSHA's radiation regulations are found in 29 CFR 1910.1096.
7.3 Mine Safety and Health Administration
Radiation exposure in mines is regulated by MSHA, a federal regulatory agency located within the Department of Labor. MSHA's regulatory authority extends to underground and to surface mines, as well as to the mining infrastructure, such as ore‐processing facilities and tailings. In the case of uranium, MSHA shares mining health and safety responsibilities with the NRC. Contributions of uranium tailings to the off‐site environment are regulated by the EPA. MSHA's health and safety regulations are published in 30 CFR 57.
7.4 Department of Energy
Rather than being regulated by the NRC, federal law requires that the DOE establish its own radiation safety standards for its workers. These standards, which are consistent with EPA guidance to Federal agencies, are generally similar to the recommendations contained in ICRP 60 and 66, although the primary dose limits are the same as those given by the NRC. DOE radiation safety standards are published in 10 CFR 835.
8 PRINCIPLES OF RADIATION SAFETY
The objective of health physics practice is to keep the radiation dose to the worker ALARA below the statutory limits. The practice of radiation protection is a special application of the control of the working environment by engineering means. In principle, if the potentially hazardous source cannot be eliminated, one must first try to isolate the source. If that is not sufficient, an attempt is made to control the environment to minimize the worker's exposure, and if that still is not good enough, the worker is isolated. The exact manner of applying these general principles to radiation safety depends on the individual situation. In health physics practice, the problem is broken down into protection from external radiation and protection from personal contamination resulting from inhaled, ingested, or tactilely transmitted radioactivity.
8.1 External Radiation
External radiation originates in machines designed to produce radiation, such as X‐ray machines and accelerators; sealed sources such as those used as gauges; in industrial radiography; in medical radiotherapy, and in other devices specifically designed to produce radiation; and unsealed radionuclides, such as those used in research laboratories and in medicine. Of special importance to the industrial hygienist are those devices in which X‐ray production (bremsstrahlung) is a side effect in an instrument or device in which electrons are accelerated across a high voltage. If it is not feasible to eliminate the radiation source, then exposure must be controlled by application of one or more of the following three techniques:
Minimizing exposure time
Maximizing distance from the source
Shielding the radiation source
8.1.1 Time
For the purpose of control of occupational radiation dose, the reciprocity relationship
is valid. Thus, if work must be performed in a relatively high radiation field, restriction of exposure time, so that the product of the dose rate and exposure time does not exceed the maximum allowable total dose, allows the work to be done in compliance with the radiation safety criteria. If the job cannot be finished within the restricted time period, then the operation must be redesigned (by shielding the source, for example) in order to decrease the intensity of the radiation field, the worker must be better trained so that he can do the job more efficiently, or another worker must be substituted when the first worker reaches the dose limit.
8.1.2 Distance
It is intuitively evident that radiation dose decreases with increasing distance from the source. In the case of a “point source” (For most practical purposes, a source appears like a point at a distance equal to about 10 times the longest physical dimension of the source.), the dose rate decreases rapidly according to the inverse square law:
(9)
where I1 and I2 are the dose rates at distances d1 and d2, respectively. Thus, a dose rate of 100 mrad h−1 at a distance of 0.5 m from a point source decreases to 25 mrad h−1 at 1 m, and to 6.25 mrad h−1 at 2 m.
For the case of a line source, such as a brine‐carrying pipe that has a buildup of scale along the pipe wall containing radium, the dose rate decreases more slowly with increasing distance than from a point source. The same is true for an area source, such as a spill of a radioactive liquid. However, at distances approaching 10 times the maximum linear dimension of the spilled area, the dose rate begins to fall off as though the spilled area were a point.
8.1.3 Shielding, Gamma
Shielding a radiation source decreases the gamma ray dose rate to a level given by
(10)
where I0 is the dose rate incident on the shield, I is the emergent dose rate, μ is the attenuation coefficient for the shielding material and for the gamma ray energy (μ is a function of the shielding material and of the gamma ray energy), and t is the shield thickness. (Values for μ may be found in appropriate handbooks and textbooks 4,5.) For example, if one wishes to use a lead shield to reduce the dose rate from 100 to 2.5 mrem h−1 at a certain distance from a 137Cs source (0.661 MeV gamma), calculate the shield thickness after finding that μ (Pb, 0.662 MeV) = 0.058 cm−1:
8.1.4 Shielding, Beta
Two factors must be considered when designing a shield for betas: the beta energy and the generation of bremsstrahlung. To minimize the bremsstrahlung, choose a beta shield of low atomic number. To stop the betas, choose a shield thickness, using the range–energy curve, Figure 2, which is equal to the range of the beta particle. For example, calculate the thickness of a Lucite shield, whose density is 1.12 g cm−3, to stop all the betas from 32P.
Phosphorus‐32 emits a 1.71‐MeV beta. From Figure 2, one finds the range of a 1.71‐MeV beta