Answer to Question #11553 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
In Canada, annual effective dose limits for nonnuclear energy workers (non-NEWs) and NEWs are 1 millisievert per year (mSv y-1) and 20 mSv y-1, respectively (from Canadian radiation protection regulations and International Commission on Radiological Protection Publication 60). Why then do we design shielding in accelerator facilities so that the maximum annual effective dose does not exceed 0.05 mSv y-1 for non-NEWs and 0.5 mSv y-1 for NEWs? I understand this is due to the principle of keeping doses as low as reasonably achievable (ALARA), but I would like to know if this is an official recommendation that may be found in any of the regulatory guides and standards.
The National Council on Radiation Protection and Measurements (NCRP) recommends:
(1) An annual limit for effective dose (E) for employees working in controlled areas of 50 mSv y-1 with the cumulative E not to exceed the product of 10 mSv and the worker’s age, exclusive of medical and natural background radiation (NCRP 1993). Further, it is recommended that for design of new facilities, E should be a fraction of 10 mSv y-1. Another consideration is that a pregnant radiation worker should not be exposed to radiation levels that exceed the monthly equivalent dose limit (HT) of 0.5 mSv to the embryo/fetus. In order to achieve the above considerations, NCRP Report No. 151 recommends one-half of that E value, or 5 mSv y-1 (NCRP 2005). Thus, an annual shielding design goal of 5 mSv y-1 is recommended for controlled areas.
(2) The shielding design goal for uncontrolled areas is 1 mSv y-1 (NCRP 2005).
Nisy Elizabeth Ipe, CHP, PhD
- National Council on Radiation Protection and Measurements. Limitation of exposure to ionizing radiation. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 116; 1993.
- National Council on Radiation Protection and Measurements. Structural shielding design and evaluation for megavoltage x- and gamma-ray radiotherapy facilities. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report No. 151; 2005.