MODEL CALIBRATION OF AN AIRBORNE GAMMA SPECTROMETER FOR SEARCHING OF RADIONUCLIDE SOURCES USING UAV.

A study of a small airborne gamma spectrometer usability, carried by unmanned aerial vehicle (UAV), during a search for radionuclide sources and estimating their activity was performed. Model calculations of calibration coefficients for Georadis D230A spectrometer (with two NaI (Tl) 2″ x 2″ detectors), 241Am, 57Co, 137Cs, 192Ir, 60Co and 152Eu radionuclides and set of flight altitudes from 1 up to 25 m were done. The results were compared with the results of experimental measurements for 137Cs and 60Co sources using vertical heights up to 10 m above the source on the ground. Simulated and discussed is the possible significant influence of the source construction and housing and the position/orientation of the source (due to non-isotropic emission) to estimate the activity of an unknown source. The influence of the natural background and estimation of flight line spacing requirements when planning a UAV route to ensure source localization depending on its activity are discussed.

ESTIMATION OF RADIATION DOSES IN THE AREA OF THE MAIN PRODUCTION BLOCK OF THE NUCLEAR POWER PLANT AFTER A REACTOR ACCIDENT.

The aim of this work is to propose methodology for estimations the characteristics of the radiation fields (and derived quantities from the field of dosimetry and radiation protection) in knowledge of a distribution of radionuclides released into the main production unit (or just a containment) area after a nuclear reactor accident. For such task stochastic Monte Carlo method has been chosen. Because of dimensions and thick shielding (concrete) barriers in the facility, application of the variance reduction techniques has been necessary. Monte Carlo code Monaco in sequence MAVRIC (from package SCALE 6.2.3) with variance reduction techniques using CADIS methodology has been employed for designing the proposed methodology. Procedure has been tested on a simulation model of a main production unit described by inspiration from the block of a nuclear power plant with a VVER-1000 reactor (installed in the Czech Republic, Central Europe).

DOSE RATE MAPPING INSIDE THE STRAZ POD RALSKEM CALIBRATION FACILITY-MONTE CARLO APPROACH.

The gamma spectrometry is one of the fundamental methods used in nuclear technology with well-established procedures of data acquisition and evaluation. Yet with expanding possibilities of detection and data processing, the question of accurate calibration very often rises, since the traditional approach to calibration might not always be the most efficient. In our work, we focus on field spectrometry and present a way of utilizing an already established and well working calibration facility and expanding its scope for new types of radiation detectors.

MODELING THE SPATIAL RESOLUTION AND FLIGHT ALTITUDE DEPENDENCE OF THE RESPONSE IN RADIOMETRIC MONITORING USING UAV.

The result interpretation of a terrain airborne radiometric monitoring depends, among others, on the spatial resolution of the measurement (i.e. the area from which the value of the measured quantity is averaged) and on the altitude dependence of the response of the measuring device. For the needs of monitoring using unmanned aerial vehicles, the approximate values of these parameters were determined by stochastic calculations for selected model conditions. Calculations were of natural background components (K, U and Th) and the surface contamination with radionuclides emitting energies was in the range of 0.3-3 MeV. The dependence on the source radius was modeled in the range of 2-450 m and flight altitude dependence was modeled in the range of 1-30 m. The modeled dependencies are used to refine the interpretation of experimental data, or to optimize the planning of monitoring the flight routes, according to specified monitoring objectives.

THE USE OF DECONVOLUTION TECHNIQUE FOR THE ANALYSIS OF GAMMA SPECTROMETRY DATA FROM FIELD MONITORING USING UNMANNED AERIAL VEHICLES.

Airborne gamma spectrometry is an effective tool for prompt monitoring and mapping of large areas contaminated after NPP accident, radionuclides leakage cases, an impact of uranium ore mining and processing, etc. Airborne spectrometry data analysis using deconvolution technique enables to calculate air kerma rates and/or radionuclides concentrations as well as identification of radionuclides. Application of this technique on the airborne data (from manned as well as an unmanned survey using drones) is rather specific due to the requirements for short time of one scan data acquisition, a relatively long distance from the source and small detector size, due to the limited payload of the usually used drones. Application of deconvolution techniques for analysis of spectra with very poor statistics, methods and possibilities to improve the processing of such spectra are discussed.

SIMULATION OF DOSE DISTRIBUTION IN VICINITY OF CLOUD OF CONTAMINATED AIR LEAKAGE FROM SEVERE NPP ACCIDENT.

Method of Monte Carlo simulation of gamma radiation fields in the vicinity of the cloud of air contaminated by the radionuclides from emergency leakage from nuclear power plant was designed and tested. Air kerma rates distributions as well as gamma field spectral distributions were calculated for the Gaussian cloud model, different atmospherical conditions and emergency scenarios source terms. Based on this model, the radiation doses in the aerial vehicle (helicopter) and its shielding properties in the radiation fields in cloud vicinity were evaluated with an aim to prepare a method providing data for planning adequate radiation protection of the personnel during airborne monitoring/interventions.

Air Kerma Rate estimation by means of in-situ gamma spectrometry: a Bayesian approach.

Bayesian inference is used to determine the Air Kerma Rate based on in-situ gamma spectrum measurement performed with an NaI(Tl) scintillation detector. The procedure accounts for uncertainties in the measurement and in the mass energy transfer coefficients needed for the calculation. The WinBUGS program (Spiegelhalter et al., 1999) was used. The results show that the relative uncertainties in the Air Kerma estimate are of about 1%, and that the choice of unfolding procedure may lead to an estimate systematic error of 3%.

MCNP-based computational model for the Leksell gamma knife.

We have focused on the usage of MCNP code for calculation of Gamma Knife radiation field parameters with a homogenous polystyrene phantom. We have investigated several parameters of the Leksell Gamma Knife radiation field and compared the results with other studies based on EGS4 and PENELOPE code as well as the Leksell Gamma Knife treatment planning system Leksell GammaPlan (LGP). The current model describes all 201 radiation beams together and simulates all the sources in the same time. Within each beam, it considers the technical construction of the source, the source holder, collimator system, the spherical phantom, and surrounding material. We have calculated output factors for various sizes of scoring volumes, relative dose distributions along basic planes including linear dose profiles, integral doses in various volumes, and differential dose volume histograms. All the parameters have been calculated for each collimator size and for the isocentric configuration of the phantom. We have found the calculated output factors to be in agreement with other authors' works except the case of 4 mm collimator size, where averaging over the scoring volume and statistical uncertainties strongly influences the calculated results. In general, all the results are dependent on the choice of the scoring volume. The calculated linear dose profiles and relative dose distributions also match independent studies and the Leksell GammaPlan, but care must be taken about the fluctuations within the plateau, which can influence the normalization, and accuracy in determining the isocenter position, which is important for comparing different dose profiles. The calculated differential dose volume histograms and integral doses have been compared with data provided by the Leksell GammaPlan. The dose volume histograms are in good agreement as well as integral doses calculated in small calculation matrix volumes. However, deviations in integral doses up to 50% can be observed for large volumes such as for the total skull volume. The differences observed in treatment of scattered radiation between the MC method and the LGP may be important in this case. We have also studied the influence of differential direction sampling of primary photons and have found that, due to the anisotropic sampling, doses around the isocenter deviate from each other by up to 6%. With caution about the details of the calculation settings, it is possible to employ the MCNP Monte Carlo code for independent verification of the Leksell Gamma Knife radiation field properties.