Waste and Materials characterization systems

Transcription

1 Waste and Materials characterization systems Machines to help you Pete Burgess September 2013

2 Waste and Materials' characterization systems We ve covered surface contamination hand held measurements earlier. Now for machines! Machines have limitations People make mistakes Clearance monitoring is often very boring Use people where initiative and imagination is needed But generally not for clearance See for lots of help 2

3 Fundamentals Assess its potential radioactive content Is it clean? Clean can only come from history We must be able to make a good case that there is no chance of contamination or activation because of where it has been or how it was protected. Very difficult for anything of any age Could it be contaminated? Could it be activated? 3

4 A hierarchy of radionuclides Bq/g (head of chain) upwards Characteristics Natural chains in equilibrium with the head of chain Alpha emitters with long half life decay products Very energetic gamma emitters Energetic beta emitters Medium energy beta + gamma emitters Less energetic beta emitters of biological significance Short half life lower energy gamma emitters Low energy, short half life beta emitters Very low energy, longer half life beta emitters Very low energy X-ray emitters Very short life nuclides Examples Ra-226+ Pu-239 and Am-241 Co-60 Sr-90 + Y-90 Cs Ba-137m C-14 Co-57 Pm-147 Ni-63, tritium Fe-55 Cs-134m 4

5 Apparent anomalies in the RP122 derived values Limiting activity values were calculated using a set of models covering volumes to be disposed of and treatment methods If the value was below 3 x 10 n it was rounded down. If above, it was rounded up Cs-137 went to 1 Bq/g Cs-134 to 0.1 Bq/g Twice the gamma energy but a much shorter half life IAEA RS-G-1.7 value is 0.1 for both The rules may have to change in the future Major implications for fission waste 5

6 The problems Contamination Soft waste Radiochemistry residues Glove boxes Ventilation ducts Pond furniture, water, sludge and concrete Filter media Tools Railway ballast from flask loading areas Activation Reactor support structures Top gas ducts Accelerator components 6

7 Rethink the fingerprint Don t think activity concentration, think fraction of exclusion level Take the fingerprint Calculate what 1 Bq/g would represent. Rework in terms of fraction of the exclusion level for each nuclide = activity/exclusion level Sum to find out what 1 Bq/g would represent 7

8 Alpha monitoring For materials with curved surfaces and for pipes and complicated shapes 8

9 Alpha monitoring IonSens TM Cut Pipe For materials with curved surfaces and for pipes and complicated shapes First Pipe Chamber (Open)

10 How does it work? It blows air through and over the object in question Alpha activity generates ion tracks These disperse quite effectively in the air stream and travel down the machine Collected and displayed as a current Maximum Missable Activity (MMA) can be as low as 10 Bq for a 2 minute measurement on a 6 m long 50 mm id pipe Surfaces have to be clean Very poor sales, mainly lack of manufacturer enthusiasm But we think it s brilliant for plants with more than a few % alpha 10

11 Side View of IonSens TM Pipe Monitor Front End Control Computer Ion Check Filter Source Actuator Monitoring Chamber Monitoring Chamber Fan Gag Detector Valve Plates HEPA Filter Electronics

12 Parallel plate detector PTFE Insulators Aluminium Plates Metallic Spacers Threaded Brass Rods

13 Potential beta contamination Thin materials are difficult to clearance monitor Coveralls are 50 mg/cm 2 approximately A 1 Bq/g limit translates to 0.05 Bq/cm 2 Using position sensitive proportional counters, multiple proportional counters or multiple scintillation detectors Some with gamma sensitivity q 13

14 Beta monitoring table Basically half an exit monitor Lay the objects on the table Start the cycle If acceptable, turn over and repeat 14

15 Gamma detection The favourite option! Scaled from non-res to hi-res Masses from a few kg to tonnes Examples: Hand held sodium iodide detectors Bucket monitors Box monitors with plastic scintillators Box monitors with sodium iodide detectors Drum and bag monitors with sodium iodide or other inorganic scintillator detectors Drum and bag monitors with hpge detectors Fundamentals Defined geometry Measurement of mass An energetic gamma as a relatively stable fingerprint fraction 15

16 Bucket monitors For the monitoring of soil, crushed concrete etc Ra-226 waste outside the nuclear industry Cs-137 (generally) within Averaging mass is a fraction of a tonne Green light is clear, red light is not clear Data logged Clever bit is making it excavator resistant

17 Calibration? MCNP model taking account of: The sample shape The matrix The contaminant The bucket wall material and thickness Confirmed using point source experiments and proper gamma spectrometric measurements on well mixed samples Two 76 mm x 76 mm sodium iodide detectors in gross counting mode Some side shielding Relatively high levels of Compton scatter encourage this Needs a reference background sample for each matrix Set to give 95 % rejection probability for a load at exactly the desired limit But the most important requirement is ROBUSTNESS 17

18 Plastic scintillator based box monitors Four or six sides made from 50 mm thick plastic scintillator Thick lead shielding to minimise background Gross counting with only rejection of very high energy (cosmic) and very low energy (electronic noise) events in the photomultipliers Good for intrinsically low background materials Calibration by MCNP or mockup 18

19 Weaknesses and strengths? Not good with high background materials Very heavy Needs careful moving as damage to the hinges or the slabs of shielding will result in an increase in background Need to try to control weights of samples to a degree BUT! Very easy to use Very sensitive for Co-60 and Cs-137 Can have 2 doors to act as posting route Not too expensive

20 Big brother Multiple detectors give positional information for any active areas Built-in rails or conveyor Often built in weigh scales Maximum Missable Activity for a 600 kg load and 60 seconds counting is a few 100 Bq for Cs-137 and Co-60 Tiny fraction of the exclusion level Automated 20

21 Move up in energy resolution Use the same detectors and mechanics as the bucket monitor This time use energy counting windows or (increasingly) full spectrometry Allows better resistance to legitimate natural activity Particularly K-40 in clay pipes, bricks and tiles 1.41 MeV 11 % gamma emitter found at Bq/g levels in many materials Gives approximately the same gross gamma count rate as materials at the Out of Scope level Identify K-40 concentration from its photopeak Predict the Compton scattered component under the Cs-137 peak Subtract to give net Cs

22 Bag monitors using hpge detectors 22

23 hpge detectors Large germanium diode large area gives high fraction of emissions intercepted, large depth gives good photodetection probability A very large intrinsic volume, i.e. a volume that behaves as a solid state ion chamber Radiation interactions generate holes and free electrons About 3 ev per pair means a large number of ion pairs generated Gives a good intrinsic statistical variability compared to scintillation crystals Three advantages A very good estimate of the photopeak energies A very good capacity to differentiate between similar energies BUT, for clearance, the huge advantage that the area in the continuum under the peak is about 1/60 th of that from a scintillator giving a limit of detection which is perhaps 8 times better from this cause alone. 23

24 hpge detector components 24

25 Size etc Dimensions of cm Kept at low temperature to ensure crystal quality and minimise electronic noise Liquid nitrogen or refrigerated 25

26 Bag monitor use Known volume (ish) Weighed to determine mass and rotated to give an average measurement Known matrix soil, concrete, sludge etc Detector fitted with collimator to reduce background Proprietary software used to calculate response, such as Canberra s ISOCS 26

27 Limitations Dealing with gross non-uniformity such as point sources Often used in open areas such as waste stores Hence influenced by local background and other waste bags Typically a 5 minute counting time will give an acceptable false negative/false positive balance at exclusion levels for a gamma dominated fingerprint 27

28 Shielded drum monitors Same technology can have the advantage of heavy local shielding Reduced mass but much lower background Much better maximum missable activity for any counting time Can have an installed pop-up source for transmission measurements through the waste Allows very good selfattenuation correction 28

29 Example Four nuclides Co-60 at 10 % and 0.1 Bq/g Ni-63 at 25 %, Fe-55 at 30 % and tritium at 35 %, all at 100 Bq/g 1 Bq/g = 0.1/ /100 = units of exclusion Limit then equals 0.99 Bq/g Co-60 limit is then just less than 0.1 Bq/g Easy measurement for many materials 29

30 Higher levels of complication Tomographic machines which produce a waste density profile and use this to produce point correction Multiple detector machines each with a collimator which defines the volume monitored Machines which count neutrons for the assessment of potentially fissile waste Direct neutron counters which potentially will detect any neutron Limitations are the (alpha,n) reaction where any alpha emission may generate a neutron and the presence of any neutron absorbers Better are coincidence counters, where coincidence detection is used to identify fission neutrons (generally in pairs) But sensitivity is always low Machines which use neutron irradiation to stimulate fission 30

31 Gate monitors - the final line of defence Everything leaving the site has been characterised either by direct monitoring or by history - out of a non-active area therefore not active Check! Why? - fly tipping, breakdown of procedures, sheer bad luck.

32 Fundamental points 5 The vehicle monitor is not for measurement, it s there to detect

33 Gate monitors Originally designed mainly to look for objects in scrap steel - plenty of things end up in scrap steel Mexican chair legs and Taiwanese steel beams (Co-60) Irish Steel/RTZ zinc+lead concentrate and Spanish discharges to air (Cs-137) Avesta contaminated slag (Pu-238) Now adapted for security monitoring

34 Gate monitors Background runs at about 2000cps. Smallest likely Cs-137 source to be detected will be 10s of kbq. Detection depends critically on where it is in the load The more the excess count rate is on one detector, the better the detection. They will also detect bulk activity such as low level steel activation giving Co-60 and generalised low level surface gamma contamination. They will also detect tiles, granite road metal, fertiliser, toilets, the more active bricks and drivers after I-131 therapy.

35 Land in-situ Large sodium iodide detectors on a slow moving vehicle Alarm is on gross count rate or window ratio Not enough time to collect a proper spectrum 1 metre measurement pitch Alarm, back up slightly, accumulate spectrum, recover object 35

36 Summary Identify target nuclides Determine activity concentration limits Identify legitimate background nuclides Review possible measurement methods, including time and cost for each Predict performance Test Review fingerprint regularly PUBLISH SO WE CAN ALL LEARN 36