Clearance Monitoring. Chris Goddard.

Transcription

1 Clearance Monitoring Chris Goddard

2 Outline What is Clearance? Clearance Limits around Europe Measurement techniques Plastic scintillators Long Range Alpha Detection Example systems Thermo SAM12 VF FRM-2 Babcock IonSens Measurement Issues Radionuclide vector Background Waste matrix

3 Exclusion, Exemption and Clearance Exemption a priori Small amounts of radioactive materials for practices Regulatory Control Clearance a posteriori Large amounts of radioactive materials as a result of practices Exclusion

4 Clearance Monitoring Documentation EU Council Directive Exemption levels Clearance concept (Exclusion not subject to provision) IAEA Safety Series No. 115 (BSS) Exemption levels Clearance concept Exclusion concept EU Radiation Protection Exemption levels RP65 Clearance levels RP122 General CL RP89 RP117 Metal CL RP101 RP118 RP114 Concrete CL IAEA RS-G-1.7 Clearance levels Exemption levels Exclusion concept Safety Report No. 44 Clearance levels

5 Clearance Criteria in Europe Is RP 122 Part I used for determining clearance levels? Sw eden U.K. Belgium Denmark YES NO Finland Germany Portugal Greece Austria Spain Netherlands France Luxembourg Italy Ireland

6 Clearance Levels Around Europe: Tritium

7 Clearance Levels Around Europe: Cs-137

8 Characterisation for Clearance Quantity measured: Co-60 (activation) Cs-137 (fission) Total gamma Total alpha Measurement Technique: Gamma spectrometry Plastic Scintillators Long Range Alpha Detection (LRAD) Radionuclide vector Radionuclide activity concentrations Clearance Criterion: n i= 1 c c i Li 1.0

9 Gamma Detection in Plastic Scintillators Photomultiplier Tube Plastic Scintillator Photocathode

10 Gamma Detection in Plastic Scintillators Plastic Scintillators are often chosen because: They can be fabricated to make large area detectors. Scintillator thickness can be optimised to the application They can easily be formed into a wide variety of shapes. Gammas up to 1.5 MeV are readily detected. BUT: They provide virtually no gamma energy resolution They must be wrapped in a light-tight enclosure. Most will have decreasing gamma sensitivity below 150 kev

11 Bulk Material Measurement System Measurement Chamber Detector Gamma Detector Gamma Shielding Weigh Scale Detector Scale

12 Thermo SAM12 Small Articles Monitor (SAM) MDA = 56 Bq for Co-60 for count time of 10 seconds and background 0.1 µsv/hr. Chamber size = 38 cm x 38 cm x 46 cm Detector locations = 6 Sides Maximum waste weight = 60 kg Chamber shielding = 5 cm Pb; total weight = 1.5 te, or 2.5 cm Pb; total weight Background assessment = During pauses when door closed and zero waste weight Natural Background Reduction (NBR)= minimises false alarms due to the presence of naturally occurring radioactive material (NORM).

13 VF FRM02 Large Item Monitor MDA = 300 Bq for Co-60, 700 Bq Cs-137 for count time of 180 seconds. Chamber size = 120 cm x 80 cm x 100 cm Detector locations = 6 Sides Maximum waste weight = 1200 kg (includes pallet) Chamber shielding = 5 cm Pb; total weight = 8 te Background assessment = During pauses when door closed and zero waste weight.

14 VF FRM02 Visual indication of hot spots within the waste

15 Nukem Ispra Clearance Monitor Measurement Chamber HRGS detectors

16 Range of Alpha Particles in Air 3.7cm 4.1cm 3.3cm 4.1cm 2.7cm 239 Pu 238 Pu 238 U 234 U 241 Am

17 Long Range Alpha Detection (LRAD) Principle Ion Transport Ionizing Radiation Contaminated Surface I Plate Detector

18 Babcock Ionsens MDA Bq (total alpha) for a 3 minute measurement. Longer Measurements have achieved detection limits below 5 Bq. Chamber size IonSens 208 = 100 cm x 100 cm x 80 cm Chamber size IonSens Pipe = 600 cm x 15 cm x 15 cm Sensitivity Alpha only

19 Function of the Gag Valve Monitoring Chamber Valve Closed Detector Pipe Air Flows Inside the Pipe Only

20 IonSens TM Pipe Monitoring Chamber Gag Valve in Closed Position Around Pipe

21 Measurement Issues - 1 Waste stream Radionuclide Vector Radionuclide Clearance Level (Bq/g) Pu Infer Co Measure Sr-90 1 Infer Radionuclide Vector Limiting Co-60 value (Bq/g) 90% Sr-90, 10% Co % Pu-239, 10% Co The radionuclide vector can have a large impact on the MDA required of the measureable radionuclides

22 Waste Stream Radionuclide Vectors Co-60 Ni-63 Fe-55 Ru-106 H -3 Ce- 144 Cs- 134 Am- 241 Sr-90 H-3 Pile Cap Radionuclide Vector Fe-55 Pm- 147 Pond Radionuclide Vector Pu-241 Cs-137

23 Measurement Issues - 2 Radionuclide Vector Selection The operator must be able to select the appropriate radionuclide vector from the system for each waste measurement. The waste measurement system must use this radionuclide vector data, together with the waste measurement, to produce an estimate of waste activity Waste #1 Waste #2 Waste #3 Waste #4

24 Measurement Issues - 3 Measurement System Shielding Gamma shielding produces a low background environment within the measurement chamber. A lower background results in lower MDA values. The effectiveness of chamber shielding depends on: its material composition, e.g. Fe, Pb its thickness the energy and intensity of ambient gamma backgrounds Gamma coincidence measurement can also be useful in reducing sensitivity to background for wastes containing Co-60

25 Measurement Issues - 4 Measurement System Sensitivity Minimum detectable activity (MDA) is ~ 1/sensitivity. The sensitivity (or detection efficiency) must be high to achieve low MDAs. Select optimum detector thickness e.g. thicker for 1332 kev of Co-60 than for 186 kev of U-235. Deploy detectors to achieve as close to 4π counting as possible Select counting time to maximum acceptable value (consistent with throughput needs). Remember, MDA 1/ (count time) therefore doubling the count time only reduces MDAs by ~1.4

26 Measurement Issues - 5 Natural Background of Waste Materials Measurement of added activity for clearance Need clean examples of anticipated waste materials to evaluate natural background levels (Bq/g) Systems must contain a material selection capability for these materials to allow subtraction of the natural backgrounds. e.g. K-40 in concrete. U/Th daughters in soils.

27 Measurement Issues - 6 Waste Matrix Uniformity Homogenous Co-60 activity assumed Point source At centre Point source on drum surface Waste density (g/cm3) Factor Error in calculated result

28 And Finally... Measurement systems should form one part of a quality plan that, at each stage, gives confidence that the waste in question is inactive. The measurement system should provide a final reassurance check that the waste is indeed clean before being released. These systems should not be used for segregation of active and non-active waste