This document discusses accounting for gamma radiation in source term calculations for nuclear waste sites. It notes that the standard DOE source term equation excludes gamma dose, but gamma radiation from certain isotopes like Cobalt-60 and Cesium-137 can contribute significantly to total effective dose, with over 40% and 26% respectively coming from ground shine. The document recommends using the Chart of Nuclides to identify isotopes of concern, running HotSpot calculations to directly model the source term including gamma, using a gamma equivalent only if too conservative, and considering site-specific meteorology and limited ground shine exposure duration.

1. Accounting for the Gamma Source Term
Wayne Davis
URS Safety Mgmt Solutions
wayne.davis@wsms.com
803.502.9789

2. Introduction
• DOE-HDBK-3010 source term:
ST = MAR x DR x ARF x RF x LPF
• RF ⇒ ST = only respirable source term
• Significant non-respirable fraction of MAR may be
deposited near receptor
• If alpha only, no concern – no ground shine dose
• If equivalent isotope is used (e.g., Pu-238eq or PEC),
then gamma dose is excluded
• If gamma also, discounted ground shine can be
significant

3. When is Gamma a Problem?
• Look at Chart of the Nuclides or similar ref.
• I selected 3 example non-actinides:
• Sr-90, Cs-137, & Co-60
(F & 1 m/s; v = 0.1, 0.3, & 1cm/s; RF=0.3 & 1)
• Sr-90 < 1% of TED from ground shine
• Cs-137 ~ 26% of TED from ground shine
• Co-60 ~ 41% of TED from ground shine
• Ground shine higher % at higher wind speed
• See following excerpts for explanation…

4. Chart of the Nuclides: Co-60

5. Chart of the Nuclides: Co-60

6. Chart of the Nuclides: Sr-90

7. Chart of the Nuclides: Sr-90

8. Chart of the Nuclides: Cs-137

9. Chart of the Nuclides: Cs-137

10. My Analytical Assumptions:
Respirable Fractions (0.3 - 1)
High-gamma MAR likely to be liquid (e.g., post actinide removal).
From DOE-HDBK-3010:

11. My Analytical Assumptions:
Deposition & Wind Speed
where:
DF(x) = Depletion Factor
x = downwind receptor distance
v = deposition velocity (I ran 0.1, 0.3, & 1 cm/s)
u = ground-level wind speed
H = effective release height

12. For a ground-level release (H=0),
this simplifies to:
DF(x) =
1
!z (x)0
x
! dx
"
#
$
%
&
'
(v
u
2
"
where:
DF(x) = Depletion Factor
x = downwind receptor distance
v = deposition velocity
u = ground-level wind speed
H = effective release height

13. Plume Depletion - Continued
Following graphs show DF for v = 1 cm/s. Plume depletion significant
for ground-level releases at stable conditions. Though depletion is
directly proportional to the deposition velocity, it is inversely
proportional to wind speed. For non-zero deposition, offsets somewhat
lower dispersion of more stable, lower wind speed releases.

14. My Suggestions for MAR w/
significant γ (> 1% inhalation dose)
• Refer to chart of the nuclides to gauge need
• Allow HotSpot to calculate ST from MAR
• If needed, use HotSpot mixture file; or,
• Use γ equivalent instead of PEC or Pueq
(probably too conservative)
• Calculate 95th percentile dose using 5-yr met
data (rather than F&1 default)
• Consider < 4 days of ground shine onsite