Limits for Radiation Exposure

The main contributions to the average radiation exposure in the United States are summarized in Table 12.2. It should be noted that the dose from the naturally occurring radioisotopes varies considerably from one location to another, depending on the local geology in certain regions of India, for example, the soil is so rich in thorium that the annual dose equivalent from this source may be as high as 20 mSv/yr. 

The setting of limits for long-term exposures to very low levels of radiation is difficult because most of the experience of radiation effects in humans has been obtained from the study of individuals who have been exposed to large doses over a short period, such as the survivors of the atomic bomb attacks on Hiroshima and Nagasaki, or to chronic exposures at levels much above the background level, such as watch dial painters who ingested considerable amounts of radium. Studies of long-term exposure of animals to 

In setting radiation limits, it is customary to assume that, for exposure of the whole body to radiation, the effect produced is proportional to the absorbed dose. Since this assumption ignores the possibility of damaged tissue repairing itself during long-term exposure to low radiation doses, it is believed to represent a conservative approach to the estimation of possible damage. Somatic effects where the probability of injury (e.g., the initiation of cancer) is proportional to the dose are known as stochastic effects. In contrast to these, an effect such as cataract of the eye is nonstochastic in this case, a threshold dose exists such that no cataract will be induced if the absorbed dose is below this amount. 

In order to protect both radiation workers and members of the public from significant radiation injury, the ICRP has made recommendations for the maximum dose equivalent per annum for each of these groups. The most recent recommendations (1977) are shown in Table 12.3. In cases where the whole-body dose has been nonuniformly distributed (e.g., by ingestion of radioactive iodine, which accumulates in the thyroid gland), the dose equivalent to the particular organ is multiplied by a weighting factor (specified by ICRP) to obtain the corresponding whole-body dose equivalent. 

Example 12.1. At a certain point in the year, a radiation worker has been exposed to the following whole-body absorbed doses lOmGy of y, 5 mGy of slow neutrons, and 2 mGy of fast neutrons. What further dose equivalent is he permitted before reaching the prescribed annual limit  

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Radioactive materials released to the environment are sources of exposure and potentially harmful. Such releases may be from different activities in the nuclear fuel cycle, mining operations or industrial users. Strict control measures must be employed to keep the resulting doses as low as reasonably achievable . This implies the implementation of protective and control measures and includes the setting of limits for radiation exposure. 

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