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Chapiter 7 (part II-2)

Chapiter 7 (part II-2). Isabelle Majkowski SCK●CEN. Isabelle Majkowski, SCK●CEN and chapter 7. Optimizing the development of Clearance methodologies. Difficult to reach a full set of Clearance methodology. But respecting the following steps should help:. Phase 1: Preliminary survey

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Chapiter 7 (part II-2)

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  1. Chapiter 7 (part II-2) Isabelle Majkowski SCK●CEN Isabelle Majkowski, SCK●CEN and chapter 7

  2. Optimizing the development of Clearance methodologies Difficult to reach a full set of Clearance methodology. But respecting the following steps should help: • 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)

  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 localization of radioactive sources, allowing perfect superimposition of the gamma and video images of the observed site: 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)

  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. Surface contamination measure - beta - 100 cm² measure Agent R.P. dec. new path yes go – no go ? Surface contamination measure - beta - 100 cm² No measure no go Go – no go ? Go measure ment form IDPBW methodology – flat & clean material

  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. 2 4 q1 q2 q3 q4 q5 q6 Hand held monitor (dual probe) Calibration • Wide area reference source • Class 2 reference source (ISO 8769) • C-14, Co-60, Cs-137, Cl-36, Sr-90/Y-90 and Am-241. • Instrument efficiency (ISO 7503-1) at 5 mm.

  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: • 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 !!!!!!!!!

  13. Justification and validationISO 11929

  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 beta) • BG = 10 cps, no attenuation, beta probe • Assumption of the ratio (control alpha + beta) • BG = 10 cps, no attenuation, dual probe

  16. HPGe HPGe HPGe Control Operator BR3 dec. new path yes Hot spot ? No ESM no go Go – no go ? Go Q² Safeguards < CL Result ? < CL measure ment form IDPBW methodology – scrap material Surface contamination control - beta - 100 cm² Gross gamma counting - 20 kg - gamma Gamma spectrometry - 200 kg - gamma

  17. Step 1: Control • k1-a, k1-b ,R0  en t0 are fixed • tb = 1 s hglob is fixed Detectable ‘Hot spot activity’ = …. Bq

  18. ‘ Improved ’ Gross gamma counting CCM ESM FHT 3035

  19. ESM - 4 channels Cobalt Coincidence Measurement

  20. 14 11 12 13 7 10 7 9 8 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

  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 < 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 • 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 fct of the ratioin 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. 100 79 67 36 Step 3: Spectroscopy HPGe detectors Q² • 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)

  26. Step 3: Spectroscopy HPGe detectors Q²

  27. Step 3: Spectroscopy HPGe detectors Q²

  28. ROI Spectroscopy HPGe detectors Q²calibration • Adjustment of the amplifiers gain • Gamma peaks of the 3 spectra • are in the same 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 Q²Errors • Error due to systematic variation of the background. • Error due to the unknown material composition • Error caused by activity distribution • Error caused by the filling height of the drum. • Errors are much more important for: • low energy gamma emitters • high density of matrix • and is mainly due to unknown activity distribution. • The energy of the gamma emitted by Cs-137 and Co-60 are high, and the general error will be small. • 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. position 1 position 2 10 cm 10 cm 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 °

  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

  35. Conclusions… • Still a lot of international discussion on: • Exemption / Clearance • NORM / nuclear industry • Instrumentation market offers instruments that measure at Clearance level. • Unknown (preliminary phase) -> worse case scenario: • longer measurement • less clearance • Alpha contamination !!!

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