United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

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)

Page Last Reviewed/Updated Thursday, March 29, 2012