Management of Commercially Generated Radioactive Waste - U.S. ...

D.11
0.2.3 Krypton-85
When krypton-85 is released to the atmosphere it will mix rapidly with the atmosphere
in the hemisphere in which it is released. After about 2 years it will also be fairly well
mixed throughout the world's atmosphere. For purposes of this analysis, therefore, simple
uniform worldwide mixing of 85 Kr in the world's atmosphere has been assumed. Similar
assumptions have been used by the NRC in its testimony for the AGNS fuel reprocessing facil-
ity at Barnwell (Eckerman and Congel 1974) and the EPA in its projections of population dose
commitments from the nuclear industry (EPA 1973 and 1974).
The National Council on radiation Protection and Measurements has published a discus-
sion of the behavior and significance of 85 Kr in the atmosphere (NCRP 1975). In that
report a comparison was made between the population exposure estimates made by detailed
modeling of 85 Kr dispersion and estimates assuming uniform mixing in the world's
atmosphere.
The model used in this analysis ignores the higher concentrations near the source and
during the first pass through the latitudinal band where the release occurs. As a result,
the model underestimates the local and regional dose at short times after the release. How-
ever, the net effect on the worldwide dose from long-term accumulated dose commitment expo-
sure is small--about 10 to 20%, depending on whether the nuclear facility is sited in the
Midwest or on the East Coast. The rapid mixing across the equator makes separate accounting
of the northern and southern hemisphere population doses unnecessary.
0.2.3.1 Dose Conversion Factors for Krypton-85
The world's atmosphere contains 3.96 x 1018 m 3 of air at standard temperature and
pressure (NCRP 1975). The concentration of 85 Kr at any time is simply the cumulative
amount released (corrected for radioactive decay) divided by the volume of the atmosphere.
For a continuous uniform release rate of 1 Ci/yr, the concentration (Ct) of krypton
becomes
where
Ct = (1 Ci/yr) (1012 pCi/Ci)/(3.96 x 1018 3)] [1 - exp(-At)i/A
= (2.53 x 10 - 7 ) I[ - exp(-At)]A pCi/m3 per Ci/yr released
\ = radiological decay constant for 85 Kr of 0.0648 per year
t = years since start of release.
For 30 years of continuous release at 1 Ci/yr the expression 11 - exp(-xt)1 /x becomes
13.2. This indicates that after 30 years 13.2 Ci remain in the environment out of the total
of 30 Ci released. The concentration (C 30 ) then becomes
C 30 = 2.53 x 10 -7 (13.2) = 3.34 x 10- 6 pCi/m 3 per Ci/yr.