WIPP Source Term Attribution Analysis

Chuck Hunter Manager, Atmospheric Technologies Group

WIPP Event Modeling Workshop August 19, 2014

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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)

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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.

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