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Environmental Neutron TLD Monitoring for ISFSI
Matthew J. Scannell, NextEra Energy Seabrook
James Giard - AREVA NP Inc. Environmental Laboratory
Jeffrey Raimondi, AREVA NP Inc. Environmental Laboratory
RETS/REMP Workshop June 2009
2
• Seabrook Station ISFSI Operational in 3rd Quarter, 2008– ISFSI located amongst occupied buildings (workers and
members of the public)
– Gamma dose expected to be main component
– Desire minor neutron dose to be quantified
• AREVA-NP Environmental Laboratory TLD Service– Routine gamma environmental TLD measurements
– Neutron measurement capabilities with 808/814 badge
Environmental Neutron TLD Monitoring for ISFSI
Introduction
3
• Seabrook Station ISFSI Layout– ISFSI Facility design– Surrounding Buildings
• AREVA-NP 808/14 Badge – Design features– Determination of Neutron Correction Factor– Determination of MDDneutron as a function of gamma dose– Expected Neutron Sensitivity
• Seabrook Station ISFSI Monitoring Results– 3rd Quarter, 2008, during first spent fuel load campaign– 4th Quarter, 2008, after spent fuel loaded
• Summarize 808/814 Badge Capabilities for ISFSI Neutron Monitoring
Environmental Neutron TLD Monitoring for ISFSI
Presentation Summary
8
Environmental Neutron TLD Monitoring for ISFSI
Seabrook ISFSI NUHOMS® Horizontal Storage Modules with Shield Blocks at Lower Vents
9
• 808/814 Badge Lithium Borate Elements – Three 814 elements (η+γ), natural isotopic abundances
– Three 808 elements (γ-only), depleted of 6Li and 10B
– Neutron signal subtract average 808 from average 814
Environmental Neutron TLD Monitoring for ISFSI
AREVA-NP 808/814 Badge
10
AREVA-NP 808/814 Badge (Continued)
• 808/814 Badge Environmental Holder Configuration– 808/814 badge cases
– Cases sealed in plastic pouches
– Otterbox® water-proof holder
Environmental Neutron TLD Monitoring for ISFSI
11
• 808/814 Badge Neutron Correction Factor (NCF) – Overnight Irradiate 808/814 Badges at HSM lower vents
– Neutron dose based on REM-500 tissue equivalent proportional counter (TEPC)ξ
– Gamma dose based on 451B ion chamber
– Total Dose (η+γ) is 162mrem
– Neutron dose is 9.7 mrem (6% of total dose)
– Result: NCF = 10.5 + 1.2 mR*n/mrem
ξ REMBALL neutron meter (BF3 tube in 9” Cd loaded polyethylene sphere) would have given similar NCF, within 15%
Environmental Neutron TLD Monitoring for ISFSI
Neutron Correction Factor
12
• 808/814 Badge Natural Background Measurements Prior to ISFSI Operations
– Ninety-seven Field Data Points:
Quarterly measurements at ISFSI Monitoring Locations (outside & within buildings)
normally used to establish historical background
– Five “Fade” Badge Points:
Dosed to known gamma levels (i.e. 200 mrem)
Placed in field for quarter
normally used to evaluate TL signal fade
Environmental Neutron TLD Monitoring for ISFSI
Background Measurements
13
• 808/814 Badge Neutron Sensitivity (MDDneutron)
– MDDneutron = 4.65 * σn/NCF [Eq. 1] (Currie)
σn = uncertainty in neutron signal
NCF 10.5 mR*n/mrem
• 808/814 Badge MDDneutron as Function of Gamma Dose– Propagate 814 and 808 element uncertainty curves to get σn vs
gamma dose curve. Combine with [Eq. 1] to get:
– MDDneutron = 0.494 * (LiBOgamma)0.4331 [Eq. 2]
Where LiBOgamma = average 808 element responses.
Environmental Neutron TLD Monitoring for ISFSI
Neutron Minimum Detectable Dose Calculation
14
MDD Equation Comparison
Compare MDDneutron [Eq. 1] points and [Eq. 2] Line for Field data
Environmental Neutron TLD Monitoring for ISFSI
0
1
2
3
4
5
6
7
20 30 40 50 60 70
808 LiBO Gamma Signal (mR*)
15
MDD Equation Comparison (Continued)
Compare MDDneutron [Eq. 1] points and [Eq. 2] Line on Field & Fade data
Environmental Neutron TLD Monitoring for ISFSI
0
2
4
6
8
10
12
0 100 200 300 400 500 600
808 LiBO Gamma Signal (mR*)
16
MDD Sensitivity
Expected Annual Neutron Sensitivity for Workers and Members of Public
Environmental Neutron TLD Monitoring for ISFSI
Quarterly Expected 365-Day 2000-hour 400-hour 12-hour
Gamma Quarterly Public at Workers Public at Public at
Signal MDDneutron Site Boundary On Site Fitness Center S&N Center
(mR*) (mrem) (mrem) (mrem) (mrem) (mrem)
40 2.44 9.8 2.2 0.45 0.013
17
• MDDneutron as Function of Gamma Dose
– MDDneutron = 0.494 * (LiBOgamma)0.4331 [Eq. 2]
Based on 4.65 σn/NCF
• Critical Level LCneutron as Function of Gamma Dose
– LCneutron = 0.248 * (LiBOgamma)0.4331 [Eq. 3]
Based on 2.33 σn/NCF
Environmental Neutron TLD Monitoring for ISFSI
Analysis of ISFSI 808/814 Badge Neutron Results
18
Seabrook ISFSI Gross Gamma and Neutron Results (mrem)Third Quarter, 2008 Outside Locations and Buildings
Environmental Neutron TLD Monitoring for ISFSI
10
15
20
25
30
-1
0
1
2
3
TL-67
SB
-35
SB
-38
SB
-37
SB
-31
SB
-30
TL-44
SB
-36
SB
-33
SB
-32
SB
-34
SB
-15
SB
-16
SB
-39
SB
-40
TL-68
TL-69
TL-12
TL-13
TL-11
TL-10
TL-36
19
Seabrook ISFSI Gross Gamma and Neutron Results (mrem)
Fourth Quarter, 2008 Outside Locations and Buildings
10
15
20
25
30
Environmental Neutron TLD Monitoring for ISFSI
-2
-1
0
1
2
3
20
Seabrook ISFSI Gross Gamma and Neutron Results (mrem)
Third Quarter, 2008 ISFSI PA Fence, Spent Fuel Loading Campaign
20
50
80
110
140
170
200
Environmental Neutron TLD Monitoring for ISFSI
-1
0
1
2
3
4
5
6
7
8
21
Seabrook ISFSI Gross Gamma and Neutron Results (mrem)
Fourth Quarter, 2008 ISFSI PA Fence, Shield Blocks at HSM Vents
15
20
25
30
35
Environmental Neutron TLD Monitoring for ISFSI
-3
-2
-1
0
1
2
3
4
5
22
Seabrook ISFSI Monitoring Results
• Comparison of Neutron Contribution to Dose at ISFSI PA Fence– NCF determined at HSM vent is conservative once HSM vents are shielded.– Sky-shine neutrons are lower energy spectrum than at HSM vents– Sky-shine neutron-to-gamma ratio different than at HSM vents.
Environmental Neutron TLD Monitoring for ISFSI
Measurement Location
Neutron Percentage of
Total Dose
Survey Instruments at HSM vent (From NCF Determination) 6%
Average of ISFSI Fence Locations Tl-75, 77, 78 and 79 Third Quarter 2008, During Spent Fuel Loading 5%
Average of ISFSI Fence Locations Tl-75, 77, 78 and 79 Fourth Quarter 2008, Shield Blocks at HSM Vents 22%
23
• AREVA-NP 808/14 Badge for ISFSI Neutron Monitoring
– Simple methods to determine NCF at suitable location with available survey meters
– Good neutron sensitivity for monitoring worker and public doses
– Curve established for MDDneutron as a function of gamma dose is applicable to each badge measurement.
– NCF will, if anything, yield conservative neutron doses when the neutron spectrum shifts to lower energy
When sky-shine predominates
as distance from ISFSI increases
Environmental Neutron TLD Monitoring for ISFSI
Presentation Conclusions
24
• Seabrook Station ISFSI Monitoring Results– 3rd Quarter, 2008, Spent Fuel Loading Campaign
No dose increases in surrounding buildings, or outside ISFSI
ISFSI fence locations near HSM’s had gamma & neutron dose increase. (ISFSI fence was boundry of restricted area)
– Neutron is 5% of total dose (neutron 6% at HSM vents)
– 4th Quarter, 2008, HSM lower vents shielded by Jersey barriers
No dose increases in surrounding buildings, or outside ISFSI.
ISFSI fence locations near HSM’s had gamma & neutron dose increase. Much less than 3rd quarter (due to shield blocks)
– Neutron is 22% of total dose (neutron 5% for 3rd Quarter)
– Neutrons from sky-shine may be conservatively reported due to neutron energy spectrum being lower than at HSM vents.
Environmental Neutron TLD Monitoring for ISFSI
Presentation Conclusions (Continued)
Recommended