DMNR - Detector Mesh for Nuclear Repository

Transcription

1 DMNR (Detector Mesh for Nuclear Repositories) On-line monitoring of short/medium term radioactive waste storage, A.Pappalardo, V.Bellini, V.Febbraro, S.Scirè and P.Finocchiaro INFN Laboratori Nazionali del Sud, Catania, Italy Dipartimento di Fisica, Università di Catania, Italy Ansaldo Nucleare, Genova, Italy

2 The problem radioactive waste is produced worldwide and generally packed into special drums the storage site must be monitored for leaks/breaks to prevent possible contamination of the environment and/or people

3 Current situation worldwide In Nirex Report N/077, Vol.1, par Monitoring the Nuclear Decommissioning Authority (NDA) recommends that when designing waste repositories, care should be taken in order to allow for future implementation of more effective monitoring systems ANDRA ( ANDRA (France) ESDRED (EU project in FP6) PAKS Nuclear Power Plant, Hungary ( IAEA ( ONDRAF/Belgoprocess, Belgium ( ONKALO, Finland ( NEA ( NIREX ( OHIO State University ( UKAEA ( USNRC ( No repository with online monitoring ONDRAF (Belgium)

4 The idea to develope an integrated solution for on-line monitoring (borrowed from HE physics experiments & DAQ) A mesh of detectors with radiation hardness robustness low efficiency, high sensitivity reliability position sensitivity ease of handling low cost In order to achieve 3D radioactivity map Alarm and leak detection

5 Our proposed solution A mesh of scintillating fibres readout at both ends by means of Silicon PhotoMultipliers (SiPM) A mesh of detectors with radiation hardness years close to a drum with mGy/h robustness yes, plastic scintillators; SiPM not damaged by ambient light exposure low efficiency, high sensitivity yes reliability yes position sensitivity yes ease of handling yes low cost yes

6 Basic sensor unit Photon Radiation SciFi SiPM Coincidence output SiPM Silicon PhotoMultiplier array of photodiodes biased beyond breakdown (Geiger mode) the avalanche is quenched by means of integrated resistors sensitive to single photons 1-cell n-cells charge = k charge = nk

7 SiPM charge response to light STMicroelectronics peak n=31 is still resolved STM 10x10 sum of several spectra taken at the same bias with different light intensity Hamamatsu peak n=17 is resolved cross-talk Hamamatsu MPPC 10x10 S U

8 Noise: dark rate vs threshold Hamamatsu STMicroelectronics N(1.5)/N(0.5) 0.5% N(1.5)/N(0.5) 27% Correlated noise (i.e. crosstalk, afterpulses) of the order of 0.5% (STM) and 27% (Hamamatsu)

9 Preliminary test results gamma source One-SiPM charge spectrum SiPM threshold Sci Fibre Test set-up with gamma sources The presence of correlated noise prevents the use of a very low threshold, thus biasing the detection efficiency

10 Counts [a.u.] Counts [a.u.] Counts [a.u.] Preliminary test results (position sensitivity) Test done with a 60Co source in three positions close to a 240cm long plastic scintillating fiber. The estimated efficiency is of the order of 0.5% (threshold at 6 photons)

11 Constant gamma detection efficiency (50 kev 10 MeV) Measured with a 60Co source (1 MeV) and a AmBe source (50keV) with a couple of SiPM Hamamatsu, threshold at 6ph ε 0.5% mass energy-absorption coefficient

12 Possible extensions sources+ moderator a 6LiF thermal neutron converter captures thermal neutrons SiPM 6 Li 6 Sci Fibre Li + n t + α MeV the triton penetrates the fibre and produces scintillation light that can be detected at both ends cross section 103 b tests under way at ISIS (UK) Test set-up: AmBe + PuC sources

13 a possible overall structure local data storage DAQ & commun. boards remote data storage waste drums unit

14 Front-end and DAQ scheme Discr Coinc Discr counters Discr Coinc Discr waste drums unit FPGA ZigBee & wire transm.

15 Conclusions efficient real-time monitoring of radioactive waste fast identification of leak position low cost for detectors and data transmission modular interconnectivity for facilitate the waste management to reduce potential radiation doses to workers

16 Thanks for your attention

17 Possible extensions a special semi-autonomous robot for inspections and general operations is currently under study (remote handling)

18 6 Li + n t + α MeV T total 4.78 MeV T tritium 2.73 MeV T alpha 2.05 MeV cross section of thermal (0.025 ev) neutron capture with 6Li, about 1000b

19 Charge response to light Poisson multi-gaussian fit dp 1 F( x ) = A = A P o is so n( µ,n ) e dx σto t ( n ) 2 π n =1 STMicroelectronics [ x c ( n )] 2 2 σ to2 t ( n ) σtot2(n) =σe2 +nσ12 Hamamatsu Single photons are well separated in a wide range. STM shows a better poissonian behaviour (less correlated noise)

20 number n of photon peaks Gain Q/ n = charge per photon gain = ( Q / n) / e = electrons/photon1 ) photon here means detected photon (=firing cell) 1 known charge Q from QDC Breakdown V 29.5 Breakdown V 68.5 STMicroelectronics Hamamatsu

21 STM Noise 2: afterpulse probability P(tAP>t0) STMicroelectronics < after 1000 ns operated at 1µs holdoff is clean Hamamatsu < after 1000 ns operated at 1µs holdoff has still 1% afterpulses Hamamatsu

22 Photon Detection Efficiency single photon counting method How to get rid of the photo-current noise? the threshold is set on the 1-photon plateau ( ph) SiPM and reference NIST photodiode watch into an integrating sphere the monochromatic light intensity is kept low (no 2-photon events on the SiPM) the pulse to the counter is wide enough to suppress correlated noise pulses are counted with and without light, signal counts are obtained by subtraction deadtime correction is applied the better fill-factor of Hamamatsu shows up clearly (78% vs 36%)

23 efficiency (gamma of 1 MeV) 60 Co source, activity of Bq placed at 1mm from the fiber, 1mm x 240mm. covered solid angle about 15% 4π counting rate 4 Hz 4 ε 0.5 % efficiency (gamma of 50keV) AmBe source da 10 Bq placed at 10cm from the fibre 1mm x 240mm 4 covered solid angle about 0.2% 4π counting rate 0.1 Hz 0.1 ε 0.5 % two SiPM Hamamatsu, threshold 6ph

24 DATI RADIOLOGICI DEL SINGOLO FUSTO La matrice inglobante il rifiuto avrà le seguenti caratteristiche: - densità = Kg/l; volume = 360 l; altezza = 894 mm ( dal fondo del fusto). La composizione del materiale che costituisce la matrice è assunta essere quella del calcestruzzo standard. E prevista una operazione di sigillatura che consiste nell inserire nel fusto un ulteriore strato di malta sulla matrice cementizia inglobante il rifiuto. I principali dati caratteristici della malta di sigillatura sono: - densità = 2 Kg/l; volume = 60 l; altezza = 147 mm. Per quel che riguarda il contenuto di radioattività presente in un singolo fusto valgono i dati riportati nella seguente Tabella 1 Tabella 1 Isotopo Attività specifica Prodotto Finito (Bq/l) Attività totale nel fusto (20 litri) (Bq) Co-60 Sr-90 Tc-99 Cs-137 Eu-152 Eu-154 Pu-241 Pu-238 Pu-239 Pu-240 U-232 U-233 U-234 U-235 U-236 U-238 Th232 Th-228 Tl-208 Am E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+08

25 Energia (Mev) Intensità di sorgente (γ/s) E E E E E E E E E E E E E E+09