2 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:
5 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
6 Phase 2: Development of methodologies 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
7 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 ment form Surface contamination measure - beta - 100 cm² Surface contamination measure - beta - 100 cm²
8 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".
9 Hand held monitor (dual probe) Setting of optimal HV
10 Hand held monitor (dual probe) Calibration Wide area reference source 1.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. 44 22 q1q2q3q4q5q6
11 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
12 Justification & validation Detection limit (cps) < Clearance level (cps) Detection limit - ISO 11929: k 1-a, k 1-b : function of alpha and beta error R 0 : back-ground level (cps), t 0 : duration of the BG measurement (s), t b :duration of the measurement (s). Clearance level (cps) = alarm level (cps) CL: Clearance Level (Bq/cm²), S vue : surface ’sees' by the probe (cm²), 4 h glob :global efficiency of the instrument !!!!!!!!!
14 Definition of the K factor 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
15 Assumption of the ratio… 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 dec. new path Operator BR3 Safeguards IDPBW Control Hot spot ? yes No ESM Go – no go ? no go Go Q² Result ? < CL measure ment form Surface contamination control - beta - 100 cm² Gross gamma counting - 20 kg - gamma Gamma spectrometry - 200 kg - gamma
17 Step 1: Control k 1-a, k 1-b,R 0 en t 0 are fixed t b = 1 s h glob is fixed Detectable ‘Hot spot activity’ = …. Bq
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 Test in extreme conditions (point source) Test in measurement conditions (17.5 kg) safe side: always overestimation of the activity if mass 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 Activation 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
23 Alarm in Bq/g f ct of the ratio in the integral channel Integral channel: Efficiency correction factor ratio Alarm:
24 Alarm level if function of the isotopic ratio 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 79 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 Q² calibration 1.Adjustment of the amplifiers gain Gamma 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
29 Spectroscopy HPGe detectors Q² Errors 1.Error due to systematic variation of the background. 2.Error due to the unknown material composition 3.Error caused by activity distribution 4.Error caused by the filling height of the drum. Errors are much more important for: 1.low energy gamma emitters 2.high density of matrix 3.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 position 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. ( 3 H, 14 C, 55 Fe, 59 Ni, 63 Ni and 99 Tc) smear test : efficiency ???
34 Passive & active neutron measurement Passive Neutron Drum Assay System Using large efficiency cell, instrumented by 3 He counters, measurement of Pu mass - Mass range covered 0 to 50 g of 240 Pu equivalent Detection limit: < 1 mg of 240 Pu 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 !!!