Active - passive spent fuel interrogation using neutrons and photons

“This dissertation consists of three main parts. The first part is devoted to the comprehensive dead-time calculations with different detectors and conditions using different dead-time models as well as computer simulations. The minimum time that must separate two detectable events is called the counting system’s dead-time. If events take place during the system’s dead-time, they will not be recorded and will be lost. Such lost information is very important in many applications including high-intensity spectroscopy and nuclear spent fuel interrogations. The second part, a multitude of fission products identified as candidates have been scrutinized for their suitability of burnup analysis and spent fuel analysis for irradiated Mixed Oxide (MOX) fuels. Best isotopes obtained for analysis by investigating half-life, photon energy, fission yield, branching ratios, production modes, thermal neutron absorption cross section and fuel matrix diffusivity. 132I and 97Nb are identified as good isotope candidates for MOX fuel on-line burnup analysis. The third and most important part, in terms of time spent and effort, deals with spent fuel analysis using non-destructive (NDA) delayed fast neutron measurement technique for safeguard purposes. The spent fuel investigation experiment was held in Missouri University of Science and Technology Research Reactor (MSTR) which is a swimming pool type reactor and licensed to operate at 200 kilowatts power. The core of the reactor consists of 15 fuel elements with low-enriched Uranium-235. Using the NDA technique, the reactor fuel burnup and 235U - 239Pu conversion values calculated. The fast neutron measurements were taken with a liquid scintillator detector which its dead-time value calculated to be 101.2 μs for paralyzing dead-time model and 254.8 μs for non-paralyzing model”--Abstract, page iv

Experimental Evaluation of the Deadtime Phenomenon for GM Detector: Deadtime Dependence on Operating Voltages

A detailed analysis of Geiger Mueller counter deadtime dependence on operating voltage is presented in the manuscript using four pairs of radiation sources. Based on two-source method, detector deadtime is calculated for a wide range of operating voltages which revealed a peculiar relationship between the operating voltage and the detector deadtime. In the low voltage range, a distinct drop in deadtime was observed where deadtime reached a value as low as a few microseconds (22 µs for 204Tl, 26 µs for 137Cs, 9 µs for 22Na). This sharp drop in the deadtime is possibly due to reduced recombination with increasing voltage. After the lowest point, the deadtime generally increased rapidly to reach a maximum (292 µs for 204Tl, 277 µs for 137Cs, 258 µs for 22Na). This rapid increase in the deadtime is mainly due to the on-set of charge multiplication. After the maximum deadtime values, there was an exponential decrease in the deadtime reaching an asymptotic low where the manufacturer recommended voltage for operation falls. This pattern of deadtime voltage dependence was repeated for all sources tested with the exception of 54Mn. Low count rates leading to a negative deadtime suggested poor statistical nature of the data collected for 54Mn and the data while being presented here is not used for any inference

Simultaneous Experimental Evaluation of Pulse Shape and Deadtime Phenomenon of GM Detector

Analysis of several pulse shape properties generated by a Geiger Mueller (GM) detector and its dependence on applied voltage was performed. The two-source method was utilized to measure deadtime while simultaneously capturing pulse shape parameters on an oscilloscope. A wide range of operating voltages (600-1200 V) beyond the recommended operating voltage of 900 V was investigated using three radioactive sources (204Tl, 137Cs, 22Na). This study investigates the relationship between operating voltage, pulse shape properties, and deadtime of the detector. Based on the data, it is found that deadtime decreases with increasing voltage from 600 to 650 V. At these low voltages (600–650 V), the collection time was long, allowing sufficient time for some recombination to take place. Increasing the voltage in this range decreased the collection time, and hence deadtime decreased. It is also observed that rise and fall time were at their highest at these applied voltages. Increasing the voltage further would result in gas multiplication, where deadtime and pulse width are observed to be increasing. After reaching the maximum point of deadtime (~ 250 µs at ~ 700 V), deadtime started to exponentially decrease until a plateau was reached. In this region, it is observed that detector deadtime and operating voltage show a strong correlation with positive pulse width, rise and fall time, cycle mean, and area. Therefore, this study confirms a correlation between detector deadtime, operating voltage, and pulse shape properties. The results will validate our hypothesis that deadtime phenomena at different operating voltages are phenomenologically different

Neutron Flux Characterization of the Beam Port of the Missouri University of Science and Technology Reactor

Neutron activation analysis was used to determine the thermal and epithermal neutron flux and its spatial distribution at the beam port of the Missouri University of Science and Technology Research Reactor. Gold foil irradiations were conducted and the determined flux was compared to Monte Carlo radiation transport predictions using the Monte Carlo N-particle code. The average experimental thermal (E \u3c 0.55 eV) and epithermal (0.55 eV \u3c E \u3c 100 keV) fluxes were (5.7 ± 0.3) x 106 cm-2 s-1 and (7.3 ± 0.3) x 105 cm-2 s-1, respectively and were found to be in good qualitative agreement with the MCNP simulations

Neutron Reflector Analysis for the Beam-Port of the Missouri S&T Reactor

A preliminary neutron reflector material selection and feasibility study of an inexpensive reflector replacement for the neutron beam-port at the 200 kW Missouri University of Science and Technology Research Reactor (MSTR) was conducted using Monte-Carlo techniques. The Monte-Carlo N-Particle transport code (MCNP6.1) was used to model the neutron beam-port of the Missouri S&T Reactor in order to study the effects of adding different reflector materials, in terms of the neutron flux reaching the radiography/tomography facility in front of MSTR\u27s neutron beam-port. Aluminum, beryllium, titanium, nickel, nickel-58, lead, bismuth, tungsten and stainless steel reflectors were modeled to find the best neutron reflector for the beam-port. After examining reflector materials, it was concluded that none of them were an improvement over the current design. Experimental thermal flux was measured to be 1.0 x 107 ± 3.16 x 103 cm-2 s-1 at the exit of beam port for current version of beam-port. The current ratio of beam port inlet and outlet obtained from simulations was found to be 3.37 x 104. The flux of beam port inlet was determined on the order of 1.0 x 1011 cm-2 s-1 which is consistent with previous findings