Appendix 12
Radionuclide Production in
Soil at SDL
Introduction
The purpose of this appendix is to estimate the potential for soil activation resulting from SDL operation. It should be noted that the soil activation region associated with SDL lies fully beneath the footprint of the building – therefore there is no potential for rainwater leaching tritium or sodium – 22 from the soil into the aquifer. Nonetheless, it is worthwhile to know if any significant soil activation is created in the soil below the facility as the result of SDL operation.
Tritium Production
Appendix 9 gives the HEN production rate from a middle z thick target to be 1 x 103 n/cm2 - s at 1 meter.
Neutrons created in the stop must penetrate through 8" of concrete before entering the soil. The attenuation length in concrete for neutrons in this energy range is 65 g/cm2. In addition, the distance from the stop to the soil is 1.6 meter. Therefore,
F = 1 x 103 (1/1.6)2 x e-8" x 2.54 x 2.35 / 65 = 187 n/cm2-s
The tritium production rate A can be calculated from:
A= NsF(1 - e-lt)
where
s is the cross section for tritium production from spallation in oxygen and silicon and is taken as 5 millibarns (mb) (attachment 3 from reference 1)
N is the number of target atoms per gram in soil (SiO2)with density 1.6 g/cc:
N = 6.02 x 1023 molecules per gram-mole x 3 atoms/molecule ¸ 60 g/ g-mole
N = 3 x 1022 atoms / g
t is the irradiation time (taken as 2000 hours per year) and the tritium decay constant l = 0.056 yr-1.
Therefore the tritium production rate "A" in disintegrations per second per gram of soil from 2000 hours of operation in one year is
A(1 yr) = 3 x 1022 x 5 x 10-27 x 187 x (1 - e-0.056 x 2000/24x365) =
A(1 yr) = 3.56 x 10-4 d/s/g = 1.2 x 10-2 pCi/g H3 in soil
A(1 yr) = 1.2 x 10-2 pCi/g H3 x 1.6 g/cc = 1.92 x 10-2 pCi/cc H3 in soil
The Accelerator Subject Area provides a methodology for determining the acceptability of induced activity in soil. For tritium, the model assumes that tritium in soil will result in soil water leachate that is 1.1 times the soil water concentration
Therefore C (H3 soil water leachate) is
C (H3 soil water leachate) = 1.92 x 10-2 pCi/cc x 1.1 = 2.1 x 10-2 pCi/cc = 21 pCi/l
The Subject Area establishes that calculated H3 leachate values in excess of 1000 pCi/l would require further safeguards and monitoring. Therefore, no corrective actions are required for this level of tritium production.
The subject area also requires that Na22 production be calculated. Cross-sections for Na22 production are not available, therefore the measurements and calculations from SLAC RP-2000-07 (Ref. 2) are used to estimate Na22 productions rates. From that report , it can be estimated that
ASat.(Na22 )= 1/2 ASat.( H3 )
From the calculation for A(1 yr) above, ASat.( H3 ) in the soil beneath the SDL can be calculated to be
ASat.( H3 ) = 2.2 x 10-1 pCi/g
Therefore , ASat.(Na22 ) = 1.1 x 10-1 pCi/g
We can then calculate the production in one year to be:
A1 yr.(Na22 ) = ASat.(Na22 ) x (1 - e-0.266 x 1)
A1 yr.(Na22 ) = 1.1 x 10-1 pCi/g x (1 – 0.766)
A1 yr.(Na22 ) = 2.57 x 10-2 pCi/g = 4.1 x 10-2 pCi/cc
The subject area prescribes that we assume that 7.5% of the sodium induced in soil becomes leachable and that a concentration factor of 1.1 be applied to the leachable fraction. Using these values, the concentration in soil water leachate for Na22 is
C (Na22 soil water leachate)= 4.1 x 10-2 pCi/cc x .075 x 1.1= 3.4 x 10-3 pCi/cc = 3.4 pCi/l
The Subject Area establishes that calculated Na22 leachate values in excess of 20 pCi/l would require further safeguards and monitoring. Therefore, no corrective actions are required for this level of Na22 production.
Conclusion
Tritium and sodium-22 production in soil from operation of SDL beam dumps do not require any additional engineering controls or monitoring.
References
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