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Accounting for the Gamma Source Term
Wayne Davis URS Safety Mgmt Solutions
[email protected] 803.502.9789
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
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…
My Analytical Assumptions: Respirable Fractions (0.3 - 1)
High-gamma MAR likely to be liquid (e.g., post actinide removal). From DOE-HDBK-3010:
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
For a ground-level release (H=0), this simplifies to:
DF(x) = 1! z (x)0
x
! dx"
#$
%
&'
(v u2"
where: DF(x) = Depletion Factor x = downwind receptor distance v = deposition velocity u = ground-level wind speed H = effective release height
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.
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