Introduction to Radionuclide Monitoring. Page 1

Transcription

1 Introduction to Radionuclide Monitoring Technology Page 1

2 Nuclear Test Environments Atmospheric Tests Underground Tests Underwater Tests Infrasound Waves Possible Seismic/Hydro Coupling Radionuclide Release (Particulate & Noble Gas) Seismic Waves Possible Infra/Hydro Coupling Possible Radionuclide Release (Noble Gas) Hydroacoustic Waves Possible Seismic/Infra Coupling Possible Radionuclide Release (Noble Gas) 2

3 Seismo-acoustic Technologies Provide real-time monitoring in underground, underwater, and atmospheric environments Pinpoint the location of the event Radionuclide Technology Provides confirmatory evidence of the nuclear nature of any suspicious event Considered as the smoking gun among the technologies 3

4 What happens during a nuclear weapons test? A nuclear (fission) explosion is a neutron-induced, uncontrolled fission of heavy nuclides. 85% Air blast, Shock, Thermal radiation, Heat 15% Radiation 5% Initial - explosive energy is released within a minute. Constitute mainly neutrons and gamma rays that are electromagnetic radiation of high energy. 10% Residual - emitted over a period of time mainly by radioactive decay of the fission products in the form of gamma rays and beta particles. 4

5 How are radionuclides transported in the atmosphere? A spherical mass of air and gaseous residues is formed after a nuclear explosion. The fireball grows in size as it sucks up particles from surrounding air and ground and becomes a radioactive cloud. As the cloud cools, vaporized radionuclides condense and attach to the aerosols in the cloud and are either transported by wind over great distances or washed-out by rain. Particulates slowly precipitate with a rate depending on the particle size. 5

6 Radioactive products of a nuclear explosion Fission radionuclides Of prime importance because released by all nuclear weapon types (fission or fusion) Neutron-activation products Products of interaction of neutrons released during nuclear fission with elements in the device material, fuel and surrounding environment Fuel products Radioactive debris from the nuclear fuel itself 6

7 What makes an atom radioactive? When its nucleus is unstable either because it is heavy or the ratio of neutrons to protons is not ideal To become stable, the nucleus disintegrates spontaneously As it disintegrates, it releases particles and the excess energy as photons (radiation). The process by which a nucleus releases energy and transforms to a lower energy state is referred to as radioactive decay. 7

8 How radioactive is it? Becquerel (Bq) is the unit of activity and equals 1 disintegration per second It tells you how many radioactive atoms are disintegrating per second Bq/m 3 is the unit of activity concentration (note that m 3 can be other units e.g., g, L) It tells you how much activity of a certain radionuclide is in a cubic meter of air 8

9 Half-life t 1/2 Half-life is how long it takes (in sec, min, days, years) for the original activity to decrease by half (process called radioactive decay) Cesium137 (half-life = 30 years) Important characteristic for detection very short-lived Cesium134 (half-life = 2 years) radionuclides will decay before analysis, while very long-lived radionuclides will have slow rate of decay Activity (arbitrary units) time (years) Activitydecrease vs. time for twocesiumradioisotopes 9

10 Energy Radiation emitted is also Typical Spectrum after 24 h of sampling, 24 h of decay and 24 h of counting characterized by its energy 7 Be kev Airflow: m 3 /h kev kev kev in kev; gamma emitters kev kev kev are identified by their kev kev kev respective energies Important characteristic for detection very low energy gammas are difficult to detect; gamma emitters with similar energies are difficult to identify Counts Pb 208 Tl 208 Tl 214 Bi 212 Bi 208 Tl 212 Bi kev 208 Tl kev 40 K 208 Tl 212 Bi kev 208 Tl kev Activity: 7 Be: 6.84 mbq/m Pb: mbq/m 3 MDC: 140 Ba: µbq/m Tl kev Energy (kev) 10

11 Main types of radiation Most fission products and neutron activation products from nuclear explosions are gamma emitters; therefore the IMS Radionuclide Monitoring technology is based on detection and analysis of gamma radiation by gamma spectrometry 11

12 γ- ray Interaction with matter Example of K-40 γ- ray spectrum (single peak) K spectrum Photopeak (1461 KeV) counts Compton edge Compton continuum Energy [kev] 12

13 Example of station sample spectra Number of γ-photons collected in the detector What energy do they have? What radionuclides and how much are in the sample? 13

14 Sources of radioactivity in the environment Naturally occurring radionuclides Terrestrial 40 K, 238 U and 232 Th decay products 212 Pb on average, 90% of spectral lines from normal IMS air filters are associated with 212 Pb and its decay products Cosmogenic 7 Be, 22 Na and 24 Na Anthropogenic radionuclides (man-made) Medical applications Industrial applications Nuclear accidents (Chernobyl) Nuclear weapons tests in the late 50s and early 60s These radionuclides constitute background radioactivity. 14

15 Radionuclides relevant to CTBT Radionuclides that are monitored are those that are most indicative of a nuclear test. Xe-135 and Xe-133 are most abundant in a 1 kiloton nuclear explosion, with activity increasing after a couple of days due to formation of beta-decay chain of precursors Most abundant radionuclides after a 1 kt explosion. 1-day decay 3-day decay 10-day decay 30-day decay Xe % Mo % Xe % Ru % I % Rh % Ba % Ce % Zr-97 6% Xe % I % Ba % Rh % Te % Mo % Zr % Pd % Ce % Te % I % Ce % I % Ru % Xe % Mo % Zr % Ce % Nd % I % I % Nd % Ce % Te % Ba % Zr % Ru % Sr % Xe % Rh % Te-129m 0.39% Ru % Pd % Sb % Te % Xe % Ru % Ce % Eu % Ce % Xe-133m 0.23% Xe-131m 0.13 Xe-133m 0.42% Xe-131m 0.05% Xe-131m % 15

16 Radionuclides relevant to CTBT These radionuclides are considered relevant as indicators of nuclear explosion because their presence or mutual ratios will discriminate from other possible sources of radioactivity. Particulates Half-life 95 Zr 64 d 95 Nb 35 d 97 Zr 17 h 99 Mo/ 99m Tc 2.75 d 103 Ru 39 d 106 Ru y 131 I 8 d 132 Te 3.3 d 133 I 20 h 134 Cs 2.1 y 136 Cs 13.2 d 137 Cs 30 y 140 Ba 12.8 d 140 La 40.2 h 141 Ce 31.5 d 143 Ce 1.4 d 144 Ce d 147 Nd d Noble Gases 135 Xe 9.1 h 133m Xe 2.19 d 133 Xe 5.24 d 131m Xe 11.9 d 16

17 Radionuclide Monitoring Station 17

18 Radionuclide station design Compressed Filter AIR IDC Compressed Filter Filter DECAY CHAMBER VSAT ANTENNA Inlet Outlet AIR SAMPLER GAMMA DETECTOR COMPUTER & ELECTRONICS 18

19 Essential elements for radionuclide particulate monitoring Sampling equipment - for collection of large volume of airborne particulates. Filter material - for trapping particulates efficiently; converted/compressed into a geometry best suited for gamma radioactivity measurements. Detection equipment - a detector crystal with good resolution, detection efficiency as high as possible, with lead shielding (low background). Multi-channel analyzer, computer system, station operation software - for production of spectral raw data for transmission to the IDC for analysis. State-of-health (SOH) sensors - status of station monitors, i.e., air flow rate, detector temperature, indoor temperature and humidity, filter position monitor, power supply status, lead shield status, that could be used to interpret the measured radionuclide data. Meteorological sensors - meteorological data monitors, i.e., precipitation, temperature, wind speed, wind direction. Very small aperture antenna (VSAT) - for transmission of data to IDC via satellite. Uninterruptible power supply & Auxiliary generator - for power stability and alternate source of electrical power. 19

20 Types of radionuclide stations Particulate (80) Manual needs a station operator to perform the daily operations Automatic Radionuclide Aerosol Sampler/Analyzer (RASA) Automatic Radionuclide Air Monitoring Equipment (ARAME) With Noble Gas Capability (40) SPALAX (Système de Prélèvement Atmosphérique en Ligne avec l Ánalyse du Xénon) SAUNA (Swedish Automatic Unit for Noble Gas Acquisition) ARIX (Analyzer of Radioactive Isotopes of Xenon) 20

21 CKP23 Manual Station Detection system Satellite antenna Equipment housing ASS500 air sampler 21

22 Manual Station Operations Filter is removed Filter is folded to reduce size Filter is compressed to 5cm diameter disc Decay for 24 h Count using HPGE for 20h SOS is used (reporting and recording) 22

23 USP70 RASA Station 23

24 CLP19 Automatic station RASA Air inlet tube Detector and shield 24

25 PTP53 ARAME Station 25

26 ISP34 Automatic Station ARAME Robot arm: suction head and cutter Detection system Measured sample New load of filter cassettes Sample on filter Filter is cut, piled and moved to counting unit Used filter cassettes Sample in decay Sample being measured 26

27 Minimum requirements Characteristics Minimum requirements Air flow 500 m 3 /h Collection time 24 h Decay time 24 h Measurement time 20 h Time before reporting 3 days Reporting frequency Daily Particulate collection efficiency filter: 80 % at = 0.2 µm, global: 60 % at = 10 µm Measurement mode HPGe High resolution gamma spectrometry HP Ge relative efficiency 40 % HP Ge resolution < 2.5 kev at 1332 kev Base line sensitivity 10 to 30 µbq/m 3 for 140 Ba Calibration range 88 to 1836 kev State of health status data transmitted to IDC Communication two-way Auxiliary data meteorological data Data availability 95 % Down time 7 consecutive days 15 days annually 27

28 Noble Gas Capability Evaluation of compliance to minimum requirements, operational performance, data analysis results are underway. SPALAX system: Tested at Tahiti, Schauinsland, Yellowknife, St.John s, Ulanbaatar, Panama, Kourou, Beijing ARIX Tested at Buenos Aires, Dubna SAUNA-II Tested at Spitsbergen, Stockholm, Takasaki, Darwin, Chatham Islands, Guangzhou 28

29 Noble Gas Monitoring Specifications Characteristics Minimum requirements Air flow 0.4 m 3 h -1 Total volume of sample 10 m 3 Collection time 24 h Measurement time 24 h Time before reporting 48 h Reporting frequency Daily Isotopes measured 131m Xe, 133 Xe, 133m Xe, 135 Xe Measurement mode Beta-gamma coincidence or High resolution gamma spectrometry Minimum Detectable Concentration 1 mbq m -3 for 133 Xe State of health Status data transmitted to IDC Communication Two-way Data availability 95 % Down time 7 consecutive days 15 days annually 29

30 Xenon measurement overview Noble gas is non-reactive and can not be sampled in the same way as aerosol particles. Requires several steps to sample the noble gas Xenon for activity measurement: Pre-processing - coarse filtration of incoming air to remove carbon dioxide, water and radon using molecular sieve or porous membrane. Xe absorption - absorption of Xe unto activated charcoal Xe desorption - thermal desorption of Xe by heating and N 2 flushing Processing and separation - by gas chromatograph Measurement β-γ coincidence counting by plastic scintillators and NaI crystals or HPGe γ- spectrometry by planar detectors SAUNA archived bottles ARIX archived bottles SPALAX archived bottles 30

31 Radionuclide Laboratories Radionuclide Laboratories perform additional sample analysis (or re-analysis) by high -resolution gamma spectrometry of a suspect sample (Level 5 IDC categorized sample) to verify the presence or absence of fission and/or activation products. Sample analysis is triggered following an event screening process at the IDC or from a request of a State Party. A Level 5 sample is split into 2 parts and sent to 2 radionuclide laboratories for remeasurement. 31

32 Support role of Radionuclide Laboratories station samples for radionuclide network quality assurance (4 samples per year) station back-up samples when a station is down samples from station visits proficiency test exercise samples Radionuclide laboratories are certified by PTS and has a quality system in line with national or international accreditation body. Total number = 16 Total certified = 10 32

33 IMS Noble gas network August 2010 NOX49 CAX16 USX74 USX79 MEX44 FRX27 CLX19 NZX46 SEX63 CAX05 CAX17 DEX 33 USX75 PAX50 MRX43 FRX31 CMX13 BRX11 ARX01 GBX68 RUX55 RUX61 RUX60 MNX45 RUX58 CNX20 JPX38 CNX22 THX65 USX77 GBX66 AUX09 FRX29 AUX04 FRX30 SPALAX SAUNA ARIX National Systems Systems under installation Page 33