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WIPP Source Term Attribution Analysis
Chuck Hunter Manager, Atmospheric Technologies Group
WIPP Event Modeling Workshop August 19, 2014
Working AIB Draft
Task: Perform a source attribution analysis to estimate radioactive source term released from Panel 7, Room7 Analysis based on: • Simulations of contaminant transport with the RISK indoor air model
(developed and distributed by EPA) • Available monitoring data for the WIPP underground exhaust stream
(radiological and flow)
• Results from a 1998 series of particle releases within the WIPP underground
Summary
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Effluent Monitoring Data
Station A Fixed Air Sampler
Room 7
• CAM-151 at entrance to panel 7 • Fixed air sampler (FAS) at Station A (upstream of stack HEPA filter)
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Calculate total radioactivity passing through the drift at location of CAM-151 based on: • Results from SRNL isotopic analysis for 13 filters collecting samples during release
passage • Log file of instrument diagnostics • Co-located flow data Total activity = (fd/fs) X Filter activity
CAM Data
Filter no.
Sample duration (min)
Am-241 on filter (picoCi)
Total activity through drift
(microCi)
Operating indicator
3 5.59E+03 2.4E+02 Low flow
4 1.26 3.27E+03 7.8E+01 Broken filter
5 4.22 4.82E+03 1.4E+01 Low flow
6 4.22 2.61E+06 7.8E+03 Obstructed flow
7 3.2 1.58E+05 4.7E+02 Obstructed flow
8 3.22 2.69E+06 6.5E+03 Obstructed flow
9 1.27 3.32E+06 8.0E+03 Broken filter
10 5.22 3.08E+06 7.4E+03 Low flow
11 4.22 3.14E+06 7.5E+03 Obstructed flow
12 4.22 1.57E+06 3.8E+03 Obstructed flow
13 1.80 3.93E+06 9.4E+03 Failure
Total 5.1E+04
fd - drift flow (90k – 5k cfm) fs - sample flow (2.5 cfm)
51 mCi - derived from total activity detected on CAM filters 30 mCi - total activity passing during filter change out
81 mCi – Estimated total release
80% Am-241, 20% Pu-239, 241 Sample collection issues as reported in the operational log (last column) suggest that 80 mCi passing this location is likely an underestimate.
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Calculate total radioactivity passing through the exhaust stack at location of FAS based on: Total activity = (fd/fs) X Filter activity
FAS Data
fd - exhaust flow (60k cfm) fs - sample flow (2 cfm) ~40 mCi - total activity 90% Am-241, 10% Pu-239, 241
Isotope Filter Activity (pCi)
Total Activity thru Stack (mCi)
Be-7 2.15E+02 0.006
Np-239 1.4E+02 0.004
Am-243 1.6E+02 0.005
Pu-239 4.7E+04 1.4
Am-241 1.2 E+06 36
Cm-244 5.6E+01 0.002
Total ~ 40
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‘Underground Flow Measurement and Particle Release Test’, Archer, Sanchez and Strait, 1998
1998 Particle Release Experiment
• 2 gram samples released from Room 7, Panel 1
• Particulates of size, density, etc. to represent range of Pu forms
• Fixed air monitors and portable filters along floor/walls along exhaust path
• Full range of flow conditions, including filtration mode Results indicated that 95% to 99% of a release would deposit within the underground (prior to HEPA filtration)
Working AIB Draft
RISK v1.9.7
• Available from the EPA’s website (http://www.epa.gov/nrmrl/appcd/mmd/iaq.html)
• Designed for determining indoor air quality and occupant exposures
• Simulates: – Release – Transport between rooms, following specified flow – Deposition (specified deposition velocity) – Resuspension (specified resuspension rate)
• Outputs: – Concentration and Peak Concentration for each simulated room and timestep
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Model Parameters
• Source was modeled as a near-instantaneous release – 3-minute duration at a
constant release rate (A release duration equal to or shorter than the model time-step created errors)
– Source area was set to 1 m2
Source strength (μCi/hr)
Decay Constant (0 for no decay)
WIPP
Release Start and Stop (hr)
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Panel 7-7 (137)
Panel 7; Rm 5-7 (138-139)
Panel 7 Exit (144; CAM)
V10 Flow Sensor (145,197,195)
V13 Flow Sensor (16-21; Broken into 3 segments)
V15
V23 Flow
Rm 5-6 Flow
F2 Flow
V2, V12 Flow
Layout of WIPP facility as modeled by RISK v1.9. Blue arrows indicate direction of modeled flow. Red arrows indicate flow entering modeled rooms from un-modeled rooms. Segment Numbers in parentheses represent the Branch ID number at WIPP. Each room is assumed to be well-mixed, with equal concentration throughout the room.
Initial Flow
Rm 3-4 Flow
Rm 1-2 Flow
Panel 7; Rm 3-4 (140-141)
Panel 7; Rm 3-4 (142-143)
Exhaust Stack
Exhaust CAM (“Station A”)
Exhaust Flow
Model Domain Configuration
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Modeled Flow
• Flow is specified between rooms and can be scaled (scaling affects all flows equally) – Based on information provided, initial flow
was set to: • 46 kcfm in Room 7 • 90 kcfm at the Panel 7 CAM and V10 • 160 kcfm at V13 • 285 kcfm at V15 • Unknown flows were set to match the
input/output of known flow
– Initial flow lasts for 1.5 minutes before linearly decreasing to 15% of the flow at 3 minutes and remaining at that level for the remainder of the simulation
Time (hr)
Model linearly scales from one value to the next
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Model Equations
Room i
Ci,IN Qi,IN Ci Qi,OUT
Di Si
Vi dCi/dt
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Model Equations
Deposition Resuspension
D = vd Ca A dt R = Fc Ms A dt
D Amount of deposited material (Ci) R Amount of deposited material (Ci) vd Deposition velocity (m/hr; specified) Ca Air concentration (Ci m-3) A Surface area (m2; assumed to include the floor, walls and ceiling) dt timestep (hr) Fc Resuspension fraction (hr-1) Cs Amount of material on the surface (Ci m-2)
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Model Simulations
• Deposition velocity ranging from 0.01 cm/s to 0.25 cm/s was tested • Concentration is output for each room
– The model source was scaled to match the total collection at the Exhaust Stack FAS for the 9-hour period following the release.
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Results
Vd (cm/sec)
Est. Release (Ci)
Integrated Activity thru Drift (milliCi)
Rm 7 Exit Rm 3/4 Panel 7 Exit Stn A
0.05 0.9 230 160 88 (81) 40
0.075 1.75 390 240 120 (81) 40
0.1 3.2 630 350 160 (81) 40
0.25 62 7000 2400 660 (81) 40
Vd (cm/sec)
Est. Release (Ci)
Fraction of release remaining*
Rm 3/4 P7 exit Stn A
1998 Experiment (Avg)
0.30 0.18 0.05
0.05 0.9 0.70 0.38 0.17
0.075 1.75 0.61 0.31 0.1
0.1 3.2 0.56 0.25 0.06
0.25 62 0.34 0.09 0.006
* Relative to Room 7 Exit.
Results indicate a vd needed to match deposition values for the 1998 experiment in the upper end of the Panel 7 exhaust drift (value in red for Rm 3/4) suggest a release from Room 7 on the order of 10s of curies. Results for the remainder of the exhaust (Panel 7 exit, Stn A) suggest a release of 2-3 curies. Next step is to request analysis of swipes taken in the Panel 7 drift to assess actual deposition rate for the Feb 14 event.
Integrated activity past indicated location based on 40 mCi at station A. Results for a vd of 0.05 and 0.075 compared to the Panel 7 CAM suggest a release from Room 7 of 1-2 Ci.
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Estimates of Total Inventory of Activity Involved
DOE Handbook, DOE-HDBK-3010-94, Airborne Release Fractions/Rates and Respirable Fractions for Non-Reactor Nuclear Facilities Source Term (ST) = MAR x DR x ARF x RF MAR - Material at Risk DR - Damage Ratio (often assumed as 0.5) ARF - Airborne Release Fraction (bounding values 5E-4 to 1E-2) RF - Respirable Fraction (0.5 to 1.0) Assume a 2 Ci release from Room 7 based on assessment from previous slide: Bounding Case 1: DR = 0.5, ARF = 5E-4, RF = 0.5 MAR = ST / (DR x ARF x RF) = 2 Ci / 0.5 x 5E-4 x 0.5 = 16,000 Ci Bounding Case 2: DR = 0.5, ARF = 1E-2, RF = 1 MAR = ST / (DR x ARF x RF) = 2 Ci / 0.5 x 1E-2 x 1.0 = 400 Ci This assessment is based on bounding values of release fractions obtained from HDBK-3010 for incidents involving excessive heat or fire. Path forward is to refine these fractional values using data from the ongoing investigation to obtain more realistic values of MAR in Room 7.
Working AIB Draft