Detection and Characterization of Short-lived Fission Fragments of Th-232 Using Radiochemical Separation Techniques PDF Download

Author: Maksat Kuatbek
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Languages : en
Pages : 0

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Book Description
Accurate knowledge of actinide fission yields is a prerequisite for numerous applications, such as nuclear forensics, nuclear safeguards, nuclear waste management, and sub-critical fission/fusion reactor kinetics. The experimental measurement of long-lived fission fragments of various actinides has been an active area of interest in the nuclear community over several decades. However, fission yields of the shorter-lived (half-life 3 days) radionuclides were typically obtained through modelling and extrapolation of the available data, causing relatively high uncertainties (up to 64%). The lack of experimental measurements for short-lived fission products is associated with a combination of challenges related to the complexity of the sample (a mixture of hundreds of radionuclides), controllability of the experiment (knowledge of neutron source), time management (pace of radioactive decay), and counting statistics (significant gamma-ray interferences). The experimental fission yield data limitations are particularly pronounced for the fast neutron (0.1 MeV) energy spectrum because of the limited availability of research nuclear reactors with hard neutron spectra. This research aims to fill this gap in nuclear data by measuring and characterizing short-lived (half-lives from 10 minutes to 3 days) fission fragments of Th-232, using rapid radiochemical separation techniques to remove interfering neutron activation products and using the Penn State Breazeale Reactor's (PSBR Fast Neutron Irradiator (FNI) fixture as a source of fast neutrons. As a part of this work, the neutron spectrum in the FNI was fully characterized using the multi-foil activation technique, Monte Carlo software predictions, and the Pacific Northwest National Laboratory's STAYSL neutron flux adjustment software. A high purity germanium (HPGe) gamma-ray spectrometer was fully characterized by using the GEANT4 Cascade Summing Correction (G4CSC) code, and the simulation results were validated by comparing them with experimental measurements of known standard sources. In addition, the HPGe detector was fully calibrated using a customized multi-nuclide multi gamma-ray emitting calibration source. The results of the FNI fixture characterization were used to determine the optimal experimental parameters for achieving approximately 108 fissions in the sample, and the expected gamma-ray spectra were simulated using the GEANT4 model of the HPGe detector. A 27.18 mg thorium sample was irradiated at the FNI fixture with the reactor power at 200 kW for 15 minutes. Then, the mix of fission products and Th-233 (an activation product) was measured using the HPGe (Blue) detector for 15 minutes, after 22.3 minutes from the end of irradiation. Next, the bulk of the fission products were isolated from the Th-233 using ion-exchange chromatography. The sample containing thorium fission products was repetitively counted sixteen times, ranging in durations from five minutes to 12 hours. The first 5-minute measurement was conducted 85.3 minutes after the completion of the irradiation. Finally, the experimentally determined fission yields were compared to the reported values in the Evaluated Nuclear Data Files (ENDF) and the Joint Evaluated Fission/Fusion Files (JEFF) libraries. The evaluation revealed that in this study, the cumulative fission yields of nine thorium fission products were determined with improved uncertainties compared to those reported in JEFF 3.3. The measurement uncertainties of ten fission products were also found to be lower than the uncertainties presented in ENDF/B-VIII.0. Furthermore, the scope of this research includes performing the necessary calculations and simulations for subsequent measurements of short-lived fission fragments of U-233, U-235, and U-238. Consequently, this work provides all the experimental parameters required for studying the uranium isotopes, as well as the estimated gamma-ray spectra at various decay times.