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A Chernobyl lesson for aerial monitoring: integration of passive measurements with active sampling in the emergency early phase. D. Castelluccio1, S. Chiavarini2, E. Cisbani1, U. Delprato3, G. Fragasso4, R. Fratoni1, S. Frullani1, M. Gaddini5, F. Giuliani1, C. Marchiori6, A. Mostarda1, G. Paoloni6, E. Pianese5, L. Pierangeli1, A. Sbuelz7, G. Siciliano4, P. Veneroni1 1Technologies and Health Department, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161-Rome, Italy; 2Centro Ricerche della Casaccia, ENEA, 00100-S. Maria di Galeria-Rome, Italy; 3IES Solutions srl, Via del Babuino 99, 00187-Rome, Italy; 4Galileo Avionica S.p.A., Via Albert Einstein 35, 5001- Campi di Bisenzio (FI), Italy 5Central Direction for Emergency and Technical Rescue, Ministry of Interior, Piazza Scilla 2, 00178-Rome, Italy; 6Dipartimento di Meccanica e Aeronautica, Facoltà di Ingegneria, Università di Roma “La Sapienza”, Via Eudossiana 18, 00184 Rome, Italy; 7Iniziative Industriali Italiane S.p.A., Viale Gorizia 6, 00198-Rome, Italy;

1 Aerial monitoring of Chernobyl releases: Italian experience

Twenty years ago a large effort was developed in Europe to quantify land contamination levels [1] resulting from Chernobyl accident releases (started on April 26, 1986 and ended ten days later) as a consequence of some deposition of radionuclides as the plume passed over different countries. Due to the extension of the territory interested by the contamination many countries took advantage of monitoring systems mounted in aircrafts in order to scan, in a relatively short time, large fractions of the affected areas. In Italy, as in many other countries, such systems have been developed in the previous years to face emergencies connected with uncontrolled re-entry in the atmosphere of nuclear powered satellites [2] or with the search of orphan/lost sources (see [3] for a modern version of a device for such purpose). Large

Thematic Session: Incidents and Accidents

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volume γ radiation detectors, or array of detectors, are mounted on fixed wing aircrafts or helicopters and relatively big sources or diffused contamination at ground level can be detected when flying at low altitude. The same systems are used to check and measure surface contamination of nuclear fuel cycle plant sites, or other facilities connected with the handling of nuclear materials.

Figure 1 Historical photos of Chernobyl time. a) NaI(Tl) detector with its anti-vibrating frame under the helicopter fuselage; b)

control and acquisition system with dedicated PC and operator. At beginning of ’80 [4], in the framework of a collaborative research program between the Physics Laboratory of Istituto Superiore di Sanità (Italian Institute of Health) and the Atomic Defence Laboratory of Corpo Nazionale dei Vigili del Fuoco (Italian Fire and Technical Rescue Brigade), a detection system, consisting of a large (16”x4”x4”) NaI(Tl) module mounted in an anti-vibrating frame under the fuselage (fig 1a) of an Agusta-Bell 412 helicopter with the control and acquisition system installed on board (fig 1b) hosted in the space made available with the removal of a row of seats, was commissioned and made available. The pattern and wind transportation of the releases resulted in plumes crossing Europe and passing over countries for weeks [5].

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Figure 2 Gamma spectra measured in flight at dates indicated With the aerial monitoring system available a series of missions have been made, between the beginning of May and mid June, covering large portions of Centre-South of Italy [6]. While the presence of contamination was clearly seen since the first mission and the identification of the main radioisotope groups present, as well as the change in their composition with time (fig 2), was possible, a quantitative assessment on the contamination parameters had not been possible until the mission of May 21. In fig. 3 the integral counts under the last two peaks of the spectra are reported as measured during different missions between May 3 and May 21 in the different areas flown in the time sequence. There is a clear evidence of a systematic increase with time of the detected radioactivity in the first two missions due to the accumulation of contamination of the helicopter fuselage while flying in the plume. Also in the mission of May 9 some radioactivity was still present in air because measurements done at different heights did not scale according to the function expected when the source of contamination is dispersed at the ground level.

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Figure 3. Integral counts under the two higher energy peaks during

the flight missions in the date indicated.

Only with the mission of May 21, having the measurements taken at different heights successfully passed all the tests of consistency, the quantitative determination of the level of ground contamination was possible (fig. 4).

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Figure 4 Ground contamination derived from measurements of the May 21 mission.

2 Learned lessons

In our experience, the first quantitative measurement of a contamination parameter has been only possible after many days from the beginning of the accident. Indeed with aerial monitoring systems in use, the quantification of the source activity, or the ground contamination, through the analysis of the γ ray spectra measured, is only possible with the assumption of a source pattern (localized for a point-like source, diffused for ground surface contamination). In case of a more complex situation, there is not a suitable knowledge to model the radiation source; therefore the measurements can only supply qualitative information. This is the case, both in near and far field, when the radioactive plume released by an accident is passing over the country. The lack of quantitative measurements and the derived uncertainty in forecasting the propagation of the radioactive contamination, does not help the emergency management in the most critical phase, i.e. when countermeasures have to be decided in a preventive way and some risk of negative effects is inevitably linked to their enforcement. A different tool for the emergency management should be provided. An aerial platform instrumented for in-plume measurements, aiming to characterize the extension, composition and concentration of the radioactive mixture in the plume, as well as to measure in situ meteorological parameters could be of invaluable help in the emergency early phase. During last years research and manufacturing activities have been developed to reach these goals [7,8]. 3 Active sampling unit and radiation detectors

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To measure air concentration contamination, the aircraft has to be equipped with a sampling unit that allows representative sampling in presence of aerosols. In the adopted unit (fig. 5), called SNIFFER, aerosol is collected on a filter, positioned along the sampling line, through a valve that controls the opening and shutting of the line, while the carrier gas is driven behind the filter and reaches the exhaust line after a Venturi throat.

Figure 5. SNIFFER sampling unit. The inlet flow scales with the aircraft speed and can be regulated within an error of ± 1% in the aircraft speed operative range, typical values of flow are between approximately 30 and 70 l/minute. Pressure and temperature measurements allow to convert the sampled volume to normal conditions. The processing of the data collected from sensors allows the calculation of the aircraft speed and of the actual sampling flow. Active isokinetic sampling of aerosol is reached by computerised adjusting of a needle valve, aiming at assuring a flow speed at the inlet equal to the aeroplane relative speed. A rotating filter-case disk is placed downstream the line and has four 47 mm diameter filter holders, one for each of the possible independent aerosol collections during the same flight mission.

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Optical marks assure the filter correct positioning and its identification. A motor driven system allows the opening of the sampling line for the filter rotation while guaranteeing the continuity of the line during the sample collection. Behind the filter and aligned with the suction line, a Geiger counter (external diameter 1 cm) and a small (1 cm3) BGO (Bismuth Germanium Oxyde) (Fig. 6) crystal allow for real-time measurements of gross-beta and total gamma activities as well as of low resolution γ spectra.

Figure 6 Geiger and BGO counters along the sampling line

A high resolution spectrum is measured, instead, with a HPGe (High Purity Germanium) detector, located off the sampling line in front of the position that the filter, previously used for sampling, reaches after a 900 rotation of the filter disk. In this way, in abnormal situation, it is possible to identify each γ-emitting radionuclide in the aerosol sample previously collected. In presence of diffused contamination, maintaining one filter not exposed along the sampling line, allows the subtraction of the background, and then a quantitative measurement of the radioisotope concentration even if the fuselage carries a certain degree of contamination, due to the radioactive plume the aircraft is crossing. The HPGe detector has been specially designed to take into account the particular application and the geometrical constraints imposed by the installation in the aircraft.

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Detailed description of hardware and software for control and acquisition can be found in [9]. 4 SNIFFER sampling unit on the SKY ARROW platform

The project has been thought since the beginning in such a way that the instrumentation weight, power consumption and space allocation would be compatible with its use on either a manned (for far field) or an unmanned air-vehicle (UAV) (for near field). The sampling probe has to be put in a location in which aerodynamic perturbation induced by the advancing of the platform is negligible. This requirement calls for a fixed wing aircraft. Safety conditions for the flights must be satisfied at an altitude range from some tens of meters to a few kilometres, whilst take off and landing operations should be possible in a grass type airstrip of a few hundreds meters (Short Take Off and Landing – STOL type aircraft).

Figure 7 Installation of the SNIFFER in the front part and of the NaI(Tl) detector on the rear part of the Sky Arrow aircraft.

For the first installation the Sky Arrow 650, manufactured by Iniziative Industriali Italiane S.p.A. (Rome – Italy), has been chosen. The sampling unit with the HPGe detector is located in the front part of the aircraft while in the rear part the NaI(Tl) detector for combined measurements is installed (Fig.7). The filter disk can be removed by opening the removable cover without the necessity to dismount the whole sampling

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unit. Filters used in the mission can be removed and analyzed in laboratory for further investigations concerning the chemical composition and the size of the collected aerosols. After a mission, possible radioactive contamination can be removed by external washing of the aircraft and forcing air flow through the sampling line. In fig. 8 the Sky Arrow equipped with the sampling unit and HPGe detector is shown.

Figure 8 Sky Arrow with all equipment installed

5 Status of the project and future plans

Tests of all the instrumentation with the final cabling outside the aircraft have been completed and are now underway with the installed equipment. Operative test flight missions are foreseen starting soon after the completion of the procedure to obtain from the control authority the authorization to fly with the aircraft modified fuselage and the new on board instrumentation. In a sequence of flight mission the operability and suitability of the instrumentation will be tested and comparison between test spectra acquired in flight and in laboratory will be made and deterioration of performances established.

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With the present project, data are stored in a flash memory and analyzed at ground after the flight. Next step of the program must foresee communication operability between the aircraft and a ground emergency control room. The expected transmission rate remains within the limit of a narrowband (≤ 56 kbits), while a large geographical coverage to have instrumentation control and data transmission to and from a remote site, wherever allocated, is a cogent requirement. The far field version of the equipment is under discussion and study with Galileo Avionica, manufacturer of the FALCO UAV (fig 9). In emergency near field operations, near the plume release area, the level of in-plume radiation could be incompatible with a man operated system, whatever the operator function could be.

Figure 9 FALCO UAV on the right shown for dimensional comparison with an helicopter.

The SNIFFER parameters are already compatible with the maximum loads defined for FALCO’s pay-loads but a SNIFFER design review, using up-to-date instrumentation, could certainly reduce both weight and power absorption, with even better performances in terms of operability. The FALCO will be equipped with the basic avionics that includes navigation, command-control and link equipment. Due to the reduced

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bandwidth required, all payload controls and data collected can be transferred to the ground based operators using the existing command-control data link without any additional equipment. The application on the Falco UAV would also take advantage of the endurance performance that this platform offers, that exceeds 12 flight hours in this payload configuration. This UAV can be managed in flight without any piloting skill, thus allowing system operations also to scientists and engineers more familiar with payload issues instead than aeronautics.

Acknowledgement

This activity has been partially funded in the framework of collaborative agreements among Istituto Superiore di Sanità, Italian Ministry of Environment and Italian Department for Civil Protection.

References

[1] Commission of the European Community: Atlas of Chernobyl deposition on Europe after the Chernobyl Accident, EUR Report 16733, EC Office of Publication, Luxembourg, (1996)

[2] Gummer, W.K., Campbell, F.R., Knight, G.B. and Ricard, J.L. Cosmos 954. The Occurrence and Nature of Recovered Debris. Atomic Energy Control Board. (INFO-0006). AECB, Canada. (1980)

[3] Kettunen, M. and Nikkinen, M. Gammajet – Fixed-wing Gamma Survey for the Detection of Radioactive Material. Stuk-Yto-Tr 185 Report. (2002)

[4] Bertocchi, A., Crateri, R., Frullani, S., Gaddini, M., Garibaldi, F., Giuliani, F., Gricia, M., Maselli, R., Monteleone, G., Mazzini, F., Naddeo, C., Salusest, B., Santavenere, F., Tacconi, O. and Vischetti, M.,: Remote Sensing of Fission Products Famma Rays Radioactive Contamination. In: Proceedings of the Conference: Physics in Environmental and Biomedical Research. Rome, November 26-29, 1985 S. Onori & E. Tabet (Eds.) World Scientific Publisher, Singapore. pag.361-366 (1985)

[5] OCDE-NEA. Chernobyl. Ten years on radiological and health impact. Paris. November 1996.

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[6] A. Bertocchi et al. Aerial monitoring of radioactivity. Annali dell’Istituto Superiore di Sanità vol. 23, 1987 pag.409-430 (S. Risica ed.) (in Italian).

[7] Cisbani, E. et al. : Aerial Platform Equipped for Nuclear Emergency Measurements, in Proceedings of the 1966 International Congress on Radiation Protection (IRPA9), Vol 2 p. 696-671 (1996). http://www.irpa.net/irpa9/cdrom/VOL.2/V2_238.PDF

[8] Frullani, S. et al. : In-plume Sampling and On-line Measurements During Emergency Early Phase, in Proceedings of the 2004 International Congress on Radiation Protection (IRPA11) (2004). http://www.irpa11.com/new/pdfs/7a10.pdf

[9] Castelluccio, D. et al: Aerial Platform for In-Plume Sampling and On-Line Measurements in the Emergency Early Phase, in Proceedings of the International Conference on Monitoring, Assessments and Uncertainties for Nuclear and Radiological Emergency Response, Rion de Janeiro 21-15 November 2005 to be published