United States Nuclear Regulatory Commission - Protecting People and the Environment

Health Physics Questions and Answers - Question 428

Question 428: 10 CFR 20.2102 (a) requires the use of the units curie, rad, rem, including multiples and subdivisions, on records required by Part 20. May a licensee continue to use roentgen-based units (e.g., R, mR, R/h, mR/h) in exposure control, radiation survey, and instrument and dosimeter calibration records without conversion to rad or rem, provided that assessed doses for individuals are recorded in units of rad or rem?

Background: The purpose in asking this question is to establish whether or not the units of measurement specified in 10 CFR 20.2101 (a) -- curie, rad, rem, and multiples and subdivisions -- must appear in all records required by Part 20 or only in those records that specifically deal with activity, absorbed dose, or dose equivalent. The intent is to be scientifically correct in recording exposure rate measurements made with radiation survey instruments and estimates of exposure obtained with direct-reading dosimeters and to avoid unnecessary changes to existing recordkeeping practices. Nuclear fuel cycle, radiography, medical, well-logging, and low-level waste licensees perform hundreds of thousands of radiation surveys each year with instruments that are calibrated for exposure rate and that read out in units of §R/h, mR/h, or R/h. Thousands of workers at nuclear power plants and licensed radiographers wear direct-reading dosimeters that are calibrated for exposure and that display mR or R. These radiation surveys and dosimeters are used to estimate exposure rates and exposures for the purpose of controlling individual doses, but they are not normally used to assess dose equivalent.

Therefore, it is not normally necessary to convert roentgen-based units to rad or rem in records of surveys and dosimeter readings. Rather than change the hundreds of forms, survey maps, logs and calibration sheets that are used at a facility to record exposure control data, radiation surveys, and calibrations, each licensee would prefer to continue recording radiation levels and exposures in roentgen-based units and to explain the relationship of these units to rem in a single program document, such as the facility's radiation protection plan. An example of such an explanation for a nuclear power plant is "exposures and exposure rates measured and recorded in roentgen-based units are numerically equal to or greater than deep-dose equivalent rates in rem-based units for the x-ray and gamma radiation energies normally present in locations other than inside or near open reactor plant components." The use of a single program statement would permit a licensee to record what was actually measured in the true units of measurement. This approach to recording exposures and exposure rates appears to be consistent with 10 CFR 20.2101 (a), which implicitly prohibits the use of the SI units becquerel, gray, and sievert, but which does not prohibit the use of roentgen and other appropriate units when measuring and recording quantities other than activity, absorbed dose, and dose equivalent. It is also consistent with the use of roentgen-based units in 10 CFR Part 34 (§§34.21, 34.24, 34.33) and in 10 CFR Part 39 (§§39.33).

Answer: Yes, except that the "assessed doses for individuals" must be recorded and reported in terms of dose equivalent quantities in units of rem for demonstrating compliance with the limits of Part 20.

As indicated in the background to the question, 10 CFR 20.2101 (a) prescribes the units to be used for the quantities activity, absorbed dose, and dose equivalent on records required by Part 20. 10 CFR 20.2101 (a) also requires that each licensee clearly indicate the units of all quantities on records required by Part 20. The roentgen is a unit for the quantity exposure; it is not a unit for the quantities absorbed dose or dose equivalent.

Thus the use of this quantity and unit are not inconsistent with the requirements of 10 CFR 20.2101 (a). However, the quantity exposure and its unit roentgen are commonly used as surrogates for the quantity absorbed dose and the unit rad or the quantity dose equivalent and the unit rem. When this is the case for use of the quantity exposure and its unit roentgen on records required by Part 20, the quantitative relationship between exposure (roentgen) and absorbed dose rad) or dose equivalent (rem) must be clearly documented and understood by individuals using these quantities and units in meeting the requirements of Part 20. The documentation of this relationship may be in the licensee's "radiation protection plan" or other radiation protection program document (s), including survey procedures; it is not necessary that this relationship (e.g., conversion factor) appear on each form, map, or log used in surveys and calibrations. It may be assumed that one roentgen equals one rem, or a more accurate conversion factor may be used. The relationship between exposure roentgen} and absorbed dose (rad) or dose equivalent (rem) should also be included in the instruction (training) of individuals who make the measurements of exposure (in roentgen units), and records of those measurements, that are required by Part 20.

Note: The answer to Question 96 (a) has been revised to be consistent with the answer above. Questions and answers 116 and 117 and answers also discuss dose quantities and units to be used in records.

(References: 10 CFR 20.2101, 10 CFR 20.1003; 10 CFR 34.21, 34.24, 34.33; 10 CFR 39.33)

Page Last Reviewed/Updated Friday, October 09, 2015