Study for a toolbox probes for UGV platform in D&D operations.

The present thesis is a part of H2020 European project CLEANDEM. CLEANDEM plans to significantly improve Decommissioning and Dismantling (D&D) operations of Nuclear facilities by introducing some technological breakthrough while offering the possibility to deploy new methodologies. This thesis project work consisted of addressing the two key features of the CLEANDEM. These are gamma-ray spectrometry and neutron measurements using single detector for hotspot localization and radionuclide identification. It involves technologies which can be embedded on Unmanned Ground Vehicle (UGV) to enable the full remote non-destructive characterization in harsh environments, hence reducing human exposure to radiation. This included the development of probes addressing both the gamma spectroscopy and the gamma/neutron detection and source identification equipping the UGV platform. The systems were prepared and extensively tested with point-like gamma laboratory sources and Cf-252 gamma-neutron source. The study comprised of using two scintillators suitable for this purpose, a 2”x2” NaI(Tl) doped with 6 Li also known as NaIL, which is an inorganic scintillator and a 2”x2” EJ-309 which is an organic liquid scintillator both having with γ/ neutron discrimination capabilities. NaIL proved to be an excellent candidate for gamma spectroscopy owing to its resolution 6.5% at 662 keV and gamma/thermal neutron discrimination capability. A quantity of interest known as Figure of Merit (FoM) which represents how well the gamma and neutrons are separated was determined to be 4.10 for NaIL. A preliminary study was also done by coupling EJ-309 with a PMT and a SiPM in order to study its response. EJ-309 has the capability to detect the fast neutrons and can be used a counter. The FoM for EJ-309 coupled with PMT and SiPM was found out to be 2.34 and 1.36 respectively. After the characterisation, NaIL was used in the scope of operations of CLEANDEM. The first test consisted of studying it as a neutron counter using Cf-252 source with different materials like Graphite, Polyethylene and Teflon shielding the detector having different thicknesses for moderation. The best moderator with an optimal thickness of 6 cm was Polyethylene which can be used with NaIL in places having a high neutron contamination. The other major test performed was mounting the NaIL on a movable platform as a UGV prototype for remote localisation of the radioactivity hotspot and its identification. The mounted detection system was able to locate the radiation hotspot as well the identification of the source comprising the hotspot. As an extension of the project, NaIL was further used for ambient dose estimation from the gamma-ray spectra obtained from it. This included extensive use of Monte Carlo simulations package called Geant4. It was an attempt to address one of the other key features of CLEANDEM which is online dose rate monitoring. The idea behind it was the conversion of gamma spectra obtained from NaIL into gamma fluence using the stripping matrix. The stripping matrix is constructed using Geant4 simulations having the detector geometry defined exactly as available in the laboratory. A GM counter was used to measure the gamma dose which was used as a control to the estimation of the ambient dose from the NaIL gamma spectra.