Accounting for the Gamma Source Term

Wayne Davis URS Safety Mgmt Solutions

wayne.davis@wsms.com 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…

Chart of the Nuclides: Co-60

Chart of the Nuclides: Co-60

Chart of the Nuclides: Sr-90

Chart of the Nuclides: Sr-90

Chart of the Nuclides: Cs-137

Chart of the Nuclides: Cs-137

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