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Optimizing the development of Clearance methodologies
Phase 1: Preliminary survey
Phase 2: establishing methodologies that ensure compliance to clearance level Development of methodologies Selection of the instrument Validation of the instrument QA Material management programme (before
clearance)
Difficult to reach a full set of Clearance methodology.
But respecting the following steps should help:
3
Phase 1: Preliminary survey
Planning: Inventory and distribution of the radionuclides likely to be present:
Those data are obtained through: a good knowledge of the plant and its process streams theoretical calculations of induced activity measurement samples taken during operational and
maintenance tasks after shut down of the plant -> preliminary monitoring
survey.
4
Phase 1: Preliminary monitoring survey- Instrumentation
Gamma camera Collimated Digital image resolution: 768 x 572
pixels Standard field of view: 50° Spatial resolution: from 1° to 2.5°
depending on energy and field of view CSI(Tl) detector
Gamma scan the camera moves to scan the
surface NaI(Tl)
localization of radioactive sources, allowing perfect superimposition of the gamma and video images of the observed site:
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Phase 1: Preliminary monitoring survey- Instrumentation
Samples – smear test: taken on a representative way or at places where
the risk of contamination/activation is maximum. treatment of the sample measurement of the sample
Use to:
confirm calculation, gamma cam. or historic knowledge
Evaluate the isotopic ratio verification of the migration of radionuclide
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Phase 2: Development of methodologies
HPGe
HPGe
HPG
Instruments
Validations
Characterisation- Assumptions- conditions
Methodology:1. Certificate2. Methodology3. Validation4. QA
Grouping of material (describe in a Certificate)
• Define the scope (group)• Historic (poss. Incidents)• Decontamination process• Characterisation of the
material (solid, porous, fibrous, shape)• Radiological
characterisation• isotopic ratio• nature of radioactivity
(fixed, homogeneous distribution)
• Non-radiological risks• CL
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methodology – flat & clean material
dec.new path
Agent R.P.
IDPBW
measure
go – no go ?yes
No
measure
Go – no go ?no go
Go
measure mentform
Surface contamination measure- beta- 100 cm²
Surface contamination measure- beta- 100 cm²
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Flat surface with 2 hand held monitors
• Certificate Scope: flat clean surfaces ratio: 80% Co-60 - 20% Cs-137 (worst case
assumption !!!)
• Measurement methodology surface measured 2 times with 2 distinct
handheld monitors and by 2 distinct operators. Release measurement procedure based on:
ISO 11932: "Activity measurements of solid materials considered for recycling, re-use, or disposal as non-radioactive waste"
ISO 7503: "Evaluation of surface contamination – Part 1: Beta-emitters (maximum beta energy greater than 0.15 MeV) and alpha-emitters".
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Hand held monitor (dual probe)Setting of optimal HV
0
10
20
30
40
50
60
70
80
500 700 900 1100 1300
alpha source
beta channel
alpha channelHV
cps
0
10
20
30
40
50
60
70
80
500 600 700 800 900 1000 1100 1200 1300
beta source
beta channel
alpha channel
HV
cps
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Hand held monitor (dual probe)
Calibration Wide area reference
source1. Class 2 reference source (ISO
8769)2. C-14, Co-60, Cs-137, Cl-36, Sr-
90/Y-90 and Am-241.3. Instrument efficiency (ISO
7503-1) at 5 mm.
42
q1 q2 q3 q4 q5 q6
q2π
-nnη BGbrut
instrument Radionuclide Energy MeV Efficiency 4
C14 0,158 0% - 7 %
Co60 0,31 4% - 16%
Cs137 0,51 12% - 21%
Cl36 0,714 17% - 23%
Sr90 0,54+2,27 18% - 24%
Am241 5,48 13% - 0%
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Hand held monitor (dual probe) Measurement
Control with check sources ISO 7503: deviation < 25 % expected value SCK-CEN: deviation < 10 % beta emitters - 20 %
alpha emitters
Control with U-Source n°XQA number from to-channel ... cps ... cps-channel ... cps ... cps
Without Background!!!
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Justification & validation
b0
2β1α1
b00β1α1 t
1
t
1.kk
4
1
t
1
t
1R.kklimitDetection
Detection limit (cps) < Clearance level (cps)Detection limit - ISO 11929:
k1-a, k1-b : function of alpha and beta error
R0 : back-ground level (cps),
t0 : duration of the BG measurement (s),
tb: duration of the measurement (s).
Clearance level (cps) = alarm level (cps)
CL: Clearance Level (Bq/cm²),
Svue: surface ’sees' by the probe (cm²),4
hglob: global efficiency of the instrument !!!!!!!!!
global.vue S CL(Bq/cm²) (cps)CL
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Justification and validationISO 11929
Duration of the measurement - beta contamination
1. Clearance Level (CL) 5. Duration of the measurement (via curves)NL: 0,4 Bq/cm²
Alarm level: 6,8 cps
2. Probe:2.1 Identification:
QA n°: FC_IDP6.107
2.2 Surface probe (S)S: 100 cm²
2.3 Global efficiencyRadioisotopes: 80%Co-60, 20%Cs-137
hinstrument: 0,17 cps/Bq
Justification
K: 1 hglob: 0,17
2.4 Maximum back-groundR0: 12 cps
3. Mesurement3.1 Duration of the back-ground measurement
t0: 60 s
3.2 Facteur probability errork1-a 1,645
k1-b 1,645
4. Détermination du temps de mesure (si t0 est >>> tb)
tb: 3,56 seconds tb: 3 seconds
0
2
4
6
8
10
12
14
16
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
cps
alarm
detection limit
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Definition of the K factor
r
L
S2
rectangle
a
4L²
r²a²a2L
r²a4L²
r²2L
2rL
r²arcsin2aL
12aLtriangle2S1
hmoy
ISO 11929 : k factor
• Surface density of absorbent layer
• Distance between source and detector
SCK data bank
• maximum and minimum diameter that can be measured for a defined measurement duration
Internal
external
• attenuation with distance for our own probe
• measurement of concrete
4L²
r²a²a2L
r²a4L²
r²2L
2rL
r²arcsin-)4L²
r²(r2a)(L2aL
1hmoy
15
Assumption of the ratio…
Assumption Efficiency Duration (s) Alarm (cps)20 % Cs-137 17 % 3.1 s 6.8 cps100 % Cs-137 21 % 2.1 s 8.4 cps0 % Cs 137 16 % 3.4 s 6.4 cps
Assumption Efficiency Duration (s) Alarm (cps)20 % Cs-137 6 % 21 s 2.4 cps100 % Cs-137 12 % 6 s 4.8 cps0 % Cs 137 4 % 46 s 1.6 cps
Assumption of the ratio (control alpha + beta)
BG = 10 cps, no attenuation, dual probe
Assumption of the ratio (control beta)
BG = 10 cps, no attenuation, beta probe
16
methodology – scrap material
HPGe
HPGe
HPGe
dec.new path
OperatorBR3
Safeguards
IDPBW
Control
Hot spot ?yes
No
ESM
Go – no go ?no go
Go
Q²
Result ?< CL
< CL
measure mentform
Surface contamination control- beta- 100 cm²
Gross gamma counting- 20 kg- gamma
Gamma spectrometry- 200 kg- gamma
17
Step 1: Control
globb0
2β1α1
b00β1α1 η(Bq) hotspot'A'
t
1
t
1.kk
4
1
t
1
t
1R.kklimitDetection
k1-a, k1-b ,R0 en t0 are fixed
tb = 1 s
hglob is fixed
Detectable ‘Hot spot activity’ = …. Bq
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ESM - 4 channels
Spectra of Plastic-Detectors (22x22cm2; d=10cm)
0
5
10
15
20
25
30
35
40
45
50
0 500 1000 1500 2000 2500
Energy in keV
Rn
et(C
o),
Ba
ckg
rou
nd
in
1/s
0
20
40
60
80
100
120
140
160
180
200
Rn
et(C
s) i
n 1
/s
7*Background Co-60 Cs-137
Co-60
1
Detect. 1
2
Detect. 2
Cobalt Coincidence Measurement
20
Calibration & control
Every 6 month: Fine adjustment of the HV Calibration with Co-60 and Cs-
137 linear sources in a mass of metal tube of 17.5 kg
Before use: control with point sources on a
bloc of 7 kg criteria: deviation < 10 %
expected value
8
7
9
10
11
12 13
14
7
21
Validation of the system
Point source CCM Co-60 ROI Cs-137 IntegralCentre of detector 170 % 150 % 150 % 140 %in the blind corner 30 % 50 % 50 % 60 %
Test in extreme conditions (point source)
Source CCM Co-60 ROI Cs-137 Integralmass 13 – 23 kg 90 % to 190 % 100 % to 150 % 100 % to 140 % 100 % to 120 %wood 160 % to 230 % 130 % to 160 % 130 % to 160 % 120 % to 140 %cable 150 % to 230 % 130 % to 160 % 130 % to 160 % 120 % to 140 %plastic 90 % to 200 % 100 % to 160 % 130 % to 160 % 100 % to 140 %
Test in measurement conditions (17.5 kg)
safe side: always overestimation of the activity
if mass < 23 kg -> overestimation – less shielding
if mass > 20 kg -> alarm in Bq
alarm = detection limit -> software calculates the measurement time in function of the BG.
Algorithm to calculate Cs-137 value do not work.
22
Extention of the scope to concrete
Co-ROI
Intergal
60 %K-40
40 %K-40
10 %Ba-13390 %
Ba-133
KeV
cpsActivation product: Ba-133
80 keV (37 %)
360 keV (56 %)
300 keV (22 %)
efficiency: 16 % integral
Natural element: K-40
1.46 MeV (11 %)
efficiency: 6 % integral !!!
As = 0.05 Bq/g
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Alarm in Bq/g fct of the ratioin the integral channel
0.10
0.11
0.12
0.13
0.14
0.15
0.16
0.17
0.18
0.19
0.20
%Co
CL
In
t
) 1ε and 14.0ε ; 4.0ε CoBaCs
)A(AA
Aet r
)A(AAA
r; )A(AA
Ar
BaCsCo
CoCo
BaCsCo
BaBa
BaCsCo
CsCs
Integral channel: Efficiency correction factor
ratio
Alarm:
y
y
x
x
Co
Co
yyxxCoscreen
CL
r
CLr
CLr
εrεrr
(g) massA
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Alarm level if function of the isotopic ratio
CL rCo rCs rBa AlarmBq/g
AlarmBq
Real activityBq
RealactivityCo
Realactivity Cs
Realactivity Ba
prev. 0.8 0.2 0 0.21 4190 4762 3810 952 0prev. 0.6 0.4 0 0.22 4471 5882 3529 2353 0prev. 1 0 0 0.20 4000 4000 4000 0 0prev. 0 1 0 0.40 8000 20000 0 20000 0prev. 0.8 0.1 0.1 0.21 4141 4848 3879 485 485prev. 0.3 0.5 0.2 0.26 5151 9756 2927 4878 1951Act. 0.8 0.2 0 0.21 4293 4878 3902 976 0Act. 0.6 0.4 0 0.24 4750 6250 3750 2500 0Act. 0.8 0.1 0.1 0.21 4191 4908 3926 491 491Act. 0.3 0.5 0.2 0.29 5867 11111 3333 5556 2222
Assumption of more Co-60 than Cs-137: If in reality there is more Cs-137 alarm level
could had been higher.
Radioelement with low efficiency have high CL, there is a kind of equilibrium.
25
Step 3: Spectroscopy HPGe detectors Q²
67
36
100
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Detectors:• HPGe cooled by liquid nitrogen (2 fillings/week)• Relative detection efficiency 20 % per detector
Measurement chamber:• shielding with 15 cm low BG steel• turntable (10 rpm)• drum 220 l• load cell to measure weight from 10 to 400 kg
Total weight: 8000 kg
System already incorporated in QA approach (validation done)
28
Spectroscopy HPGe detectors Q²calibration
1. Adjustment of the amplifiers gainGamma peaks of the 3 spectra
are in the same ROI ROI
2. Calibration with 4 reference drums
• filled with material density 0.02 g/cm³ - 1.83 g/cm³
• approximation of homogeneous distribution of activity
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Spectroscopy HPGe detectors Q²Errors
1. Error due to systematic variation of the background.
2. Error due to the unknown material composition3. Error caused by activity distribution4. Error caused by the filling height of the drum.
Errors are much more important for:1. low energy gamma emitters 2. high density of matrix3. and is mainly due to unknown activity distribution.4. The energy of the gamma emitted by Cs-137 and
Co-60 are high, and the general error will be small.
5. The detection limit for Co-60 and Cs-137 is of the order of some mBq/g for a 10 minutes count of a 200 l waste drum. Which is well below the Clearance Level.
30
Other devices…In Situ Object Counting System
ISOCS: portable Ge detector, flexible portable shielding/collimator system, mathematical efficiency calculation software
that requires no radioactive sources and data analysis software.
Modelisation of the object to be measured
Simple geometry of the object
Assessment of the position of the source (homogene, linear punctual)
31
Other devices…Tunnels
2 detectors: position 1: 60° + 60° = 120° position 2: 180° + 60° = 240°
4 detectors position 1: 60° + 60° + 180° + 60 °= 360° position 2: 180° + 60° + 60°+60°= 360 °
10 cm
10 cmposition 1
position 2
32
Other devices…Air ionisation measurement
Passing a anode wire in the center of the tube -> use the tube as an ionisation chamber: detection: few Bq in 2 m in 30 secondes ~ 0.001 Bq/cm³
33
Indirect measurements: Samples
Difficult to validate their representativity: taken & treatment
used when contamination consists mainly of low energy beta or alpha emitters on surface that are difficult to access. (3H, 14C, 55Fe, 59Ni, 63Ni and 99Tc)
smear test : efficiency ???
34
Passive & active neutron measurement
Passive Neutron Drum Assay System
• Using large efficiency cell, instrumented by 3He counters,
• measurement of Pu mass - Mass range covered 0 to 50 g of 240Pu equivalent
• Detection limit: < 1 mg of 240Pu equivalent • Accuracy: better than 10% at 1g.
35
Conclusions…
1. Still a lot of international discussion on: Exemption / Clearance NORM / nuclear industry
2. Instrumentation market offers instruments
that measure at Clearance level.
3. Unknown (preliminary phase) -> worse case
scenario: longer measurement less clearance
4. Alpha contamination !!!