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Chapiter 7 (part II-2)

Isabelle Majkowski

SCK●CEN

Isabelle Majkowski, SCK●CEN and chapter 7

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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:

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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.

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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.

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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

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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

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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

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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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‘ Improved ’ Gross gamma counting CCM ESM FHT 3035

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ESM - 4 channels

Spectra of Plastic-Detectors (22x22cm2; d=10cm)

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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

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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

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7

9

10

11

12 13

14

7

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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.

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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.

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Step 3: Spectroscopy HPGe detectors Q²

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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)

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Step 3: Spectroscopy HPGe detectors Q²

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Step 3: Spectroscopy HPGe detectors Q²

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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.

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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)

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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

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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

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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 ???

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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.

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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 !!!