United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

Question 426: Are the noble gas ("submersion") DACs based

on a dose of 5 rem per year or 50 rem per year? Is the

submersion dose calculated at a depth of 1000 mg/square cm

or 7 mg/square cm?

Answer: There is no one particular dose or one particular

depth. The method for calculating submersion doses is

explained in Federal Guidance Report No. 11 on pages 10,

18, 181, and 182. When air concentration is limited by

submersion dose, the DAC for a particular radionuclide is

the maximum concentration of that radionuclide in air that,

for a 2,000-hour exposure, will result in a dose that is

equal to or less than each of the applicable limits (5 rem

effective dose equivalent, 15-rem eye dose equivalent,

50-rem dose equivalent to other organs and tissues, shallow

dose equivalent of 50 rem to the skin). That is, the DAC

for a particular radionuclide depends on which of the

applicable dose limits is the most restrictive with respect

to the concentration of that particular radionuclide. The

dosimetric model used to calculate the DACs considers

shielding of organs by overlying tissues and the

degradation of the photon spectrum through scatter and

attenuation by air. The dose from beta particles is

evaluated at a depth of 7 mg/square cm for skin, and at a

depth of 3 mm for the lens of the eye. The worker is

assumed to be immersed in pure parent radionuclide, and no

radiation from airborne progeny is considered. In most

cases, the concentration limit for submersion is based on

external irradiation of the body; it does not take into

account either absorbed gas within the body or the

inhalation of radioactive decay products. An exception to

the preceding statement is Ar-37, for which direct exposure

of the lungs by inhaled activity limits (stochastically)

the concentration in air. The skin dose is limiting for

Ar-39, Kr-85, and Xe-131m; the eye dose is limiting for

Kr-83m. Note: There are typographical errors in the

discussion of submersion doses on page 10 of Federal

Guidance Report No. 11. In the fifth sentence of the

paragraph beginning "Some airborne radionuclides...", the

word "effective" should be added before the words "dose

equivalent rate". In equation (8b), the subscript "E"

should be the subscript "T." (Reference: 10 CFR 20

Appendix B).

Page Last Reviewed/Updated Thursday, March 29, 2012