[ Index of Health Physics Questions and Answers | NRC Home Page ]
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)