Health Physics Questions and Answers - Question 63
Question 63: Must doses received in excess of the limits that were in effect before implementation of the revised Part 20 be subtracted from the 25-rem lifetime allowance for planned special exposures to obtain the total remaining dose available for planned special exposures?
Answer: Yes. See 10 CFR 20.1206 (e), which limits the dose from all planned special exposures and all doses in excess of the limits to five times the annual dose limits in §20.1201 (a) during the individual's lifetime. The following discussion applies to individuals who worked at facilities of NRC licensees. It does not necessarily apply to individuals who worked at other facilities.
The "25-rem lifetime allowance" in the question is five times the annual limit 5 rem) for the total effective dose equivalent (TEDE), which is the sum of the deep dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures). Before implementation of the revised Part 20 there were separate limits for internal and external exposure. For purposes of complying with "the 25 rem lifetime allowance," a previous intake, in units of MPC-hours, in excess of the old Part 20 limit may be converted to a committed effective dose equivalent, in units of rems, by multiplying by a factor of (1.25 rem/520 MPC-h). Previous whole-body exposures, in units of rem, in excess of the old Part 20 limit may be assumed to be equal to the deep dose equivalent component of the TEDE (in units of rem). For example, if, under the old Part 20, a worker had received a whole-body dose that was 4 rem greater than the applicable limit and had also received an intake that was 100 MPC-hours greater than the applicable limit, the TEDE available for planned special exposures of that worker under the revised Part 20 would be [25 - 4 - (100) (1.25/520)] rem, or 20.8 rem.
Although the question refers only to "the 25-rem lifetime allowance" on the TEDE, the 10 CFR 20.1206 (e) (2) lifetime limit (five times the annual limit) also applies to previous over-exposures involving the lens of the eye, the skin, and the extremities. For purposes of complying with 10 CFR 20.1206 (e) (2), previous exposures to the lens of the eye in excess of the old Part 20 limits may be assumed to be equal to the previous overexposures to the whole body (because the limit for the whole body applied to the lens of the eye) and a previous overexposure to the skin of the whole body or to an extremity may be assumed to be equal to a corresponding overexposure to the skin of the whole body or to a hand, forearm, foot or ankle, respectively, except that overexposures resulting from beta radiation from hot particles on or near the skin need not be included in the overexposures to the skin or extremities.
Note: For all future planned special exposures, the lifetime limit is applicable to each annual limit listed in 10 CFR 20.1201 (a).
(References: 10 CFR 20.1201, 10 CFR 20.1206, 10 CFR 20.2104, Technical Specifications)