Biomedical Engineering Reference
In-Depth Information
deterministic effects under NCRP Report No. 116. Specific equivalent-dose limits are
needed only for the lens of the eye, the skin, hands, and feet, as shown in Table 14.4.
(The effective-dose limit protects the skin sufficiently against stochastic effects.)
The newer ICRP/NCRP recommendations were not fully implemented into U.S.
regulations by the federal and state reviews, which were completed in the early 1990s.
For example, the present rules of the Nuclear Regulatory Commission (NRC) and the
Department of Energy (DOE) have continued based on dose equivalent.
5)
The basic
limits are those shown in Table 14.5, except that there is no lifetime cumulative limit
in the federal rules. Also, the 20-mSv annual committed limit was not adopted fed-
erally, and so the 500 mSv deterministic limit for “all other tissues and organs” in
Table 14.5 is still in effect. Like all parts of the current regulations in the U.S., these
decisions followed extensive studies by various organizations, public comments, and
reviews of past operating experiences. It was judged that additional changes at the
time would result in very little reduction in annual doses, which remain well below
the limits, averaging only a few mSv or less. Application of the ALARA principle,
which was further emphasized in the revised federal regulations, has played an effec-
tive role in keeping occupational exposures low within current ICRP/NCRP recom-
mendations.
Example
In the next-to-last example, a worker received external whole-body doses of 8.4 mGy
from gamma rays and 1.2 mGy from 80-keV neutrons. (a) Calculate his total effective
dose equivalent based on NCRP Report No. 91. The values of the gamma and neutron
quality factors are, respectively,
Q
= 1and
Q
= 13. (b) In addition to these exposures
during a year, how much additional internal beta dose (
Q
1) to the thyroid alone
could be received without technically exceeding the limits in Table 14.5? The thyroid
weighting factor is 0.03.
=
Solution
(a) The total effective dose equivalent is, in place of the effective dose, Eq. (14.9),
TEDE
=
1
×
8.4 + 13
×
1.2
=
24 mSv.
(14.14)
26 mSv would reach the stochastic limit in Table 14.5.
Given the tissue weighting factor, the thyroid dose equivalent for this contribution
would be (26 mSv)/(0.03)
(b) The addition of 50 - 24
=
867 mSv. This amount exceeds the deterministic limit
of 500 mSv in Table 14.5. Since the thyroid already has 24 mSv from the external
radiation in Part (a), an additional dose equivalent of 500 - 24 = 476 mSv from the
beta irradiation would reach the deterministic limit for the organ. In order to comply
with Table 14.5, the maximum additional internal thyroid dose, therefore, could not
exceed 476 mGy. (The TEDE for the year is thereby limited to 24+0.03
=
×
476
=
38 mSv
by the deterministic dose limit to the thyroid.)
5
See 10 CFR Part 20 and 10 CFR Part 835
listed in Section 14.11. These laws became
effective, respectively, in 1991 for NRC and
1993 for DOE. At this writing, DOE is
considering adoption of ICRP-60
terminology
for occupational radiation exposures. See, for
example, Federal Register/Vol. 71, No.
154/Thursday, August 10, 2006/Proposed
Rules.
