The current knowledge of the performance characteristics of Centrifugal Tensioned Metastable Fluid Detectors is limited. While a theoretical treatment and experience with bubble chambers may be applied with some degree of success, they are no substitute for experimental and operational knowledge of real CTMFD systems. This research, as with other investigations into CTMFD systems in the past, applies theory and simulations. In addition, however, an experiment was conducted that for the first time attempts to determine the threshold energy for triggering a CTMFD system in a controlled manner. A CTMFD system works in a manner similar to classic bubble chambers. A liquid is brought to an unstable state in which it is favorable to form a volume of vapor; using centrifugal techniques similar to those employed in a Briggs apparatus, the pressure in the sensitive region can be brought to extremely low values, placing the liquid in a tensile state. In such states, the energy necessary to cause the formation of macroscopic bubbles can be vanishingly small, depending on the degree of tension. When such bubbles form in a CTMFD, if they have a size bigger than a critical value, they will grow until a large vapor column forms in the sensitive region of the CTMFD. The experiment developed for this research employed a carefully-controlled laser to fire pulses of known energies into the sensitive region of a CTMFD. By varying the laser power, the threshold values for the triggering energy of a CTMFD can be found. The experiment and simulation demonstrated the ability of the facilities to test CTMFD systems and the potential to extract their operational characteristics. The experiment showed a certain viability for the technique of laser-induced cavitation in a seeded fluid, and demonstrated some of the associated limitations as well. In addition, the CFD framework developed here can be used to cross-compare experimental results with computer simulations as well as with the theoretical models developed for this research.

<p>The primary focus of this thesis pertains to R&D results associated with deploying tensioned</p> <p>metastable fluid detector (TMFD) technology for monitoring of spent nuclear fuel</p> <p>(SNF) for actinide content from their neutron emissions while under extreme photon backgrounds</p> <p>(> 150 Gy/h), as may be expected within a hotcell. Traditional state-of-the-art</p> <p>neutron detectors such as 3He and BF3 based systems are well-known to be dysfunctional</p> <p>under such conditions, despite having pulse-shaped discrimination capabilities that allow</p> <p>them to differentiate photons vs. neutrons. The aim of this thesis was to test the ‘gamma</p> <p>blind’ ability of the centrifugally tensioned metastable fluid detector (CTMFD) based system,</p> <p>to monitor for actinide generated neutrons despite the anticipated high intensity gamma</p> <p>background, a goal which was successfully accomplished. Methods, designs, and experimental</p> <p>procedures are discussed for successful neutron monitoring from an Americium-Beryllium</p> <p>neutron source, as well as results showing no hindrance to neutron detection capability at</p> <p>modest negative pressure states through 150 Gy (15 kRad) accumulated gamma dose.</p> <p>A secondary focus was the ability of the TMFD based systems to perform alpha spectroscopy</p> <p>on closely separated (<10 keV) alpha particle emissions from 239Pu and 240Pu</p> <p>isotopes. Due to the closely spearated alpha decay energies, this feat could previously only</p> <p>be perfromed by tedious and expensive mass-spectrometry based systems. Instead, a wet</p> <p>chemistry apporach for detecting trace (? 10−3 Bq/mL) quantity alpha radiation with high</p> <p>alpha energy resolution (<10 keV) was developed and validated using the CTMFD system.</p> <p>Using this technique, mixtures containing samples of 239Pu:240Pu with activity concentrations</p> <p>ranging in ratio from 1:0 to 0:1 were able to be identified within ±12% accuracy.</p> <p>Lastly, successful assessments were conducted for detecting neutron emissions from a 1</p> <p>Ci Plutonium-Beryllium source under a variety of shielded configurations using a CTMFD</p> <p>and a 3He based Ludlum 42-49BTM detector. Concrete, lead, and water shielding materials</p> <p>of thicknesses ranging from 0 to ?30 cm were placed as shielding material, with the</p> <p>CTMFD configured only for fast energy neutron detection. Monte Carlo N-Particle Transport</p> <p>(MCNP) code-based simulations were performed for derivation of the neutron energy</p> <p>spectrum incident on the detectors to compute sensitivity estimates. At 0.6 MPa (6 bar) negative pressure, the CTMFD was determined to offer up to 7 times higher sensitivity vs the</p> <p>Ludlum 42-49B, though further increasing the negative pressure state to 1.1 MPA (11 bar)</p> <p>exponentially increases the sensitivity to offer 100+ times higher sensitivity for the CTMFD</p> <p>vs the Ludlum 42-49B.</p>

<p>This dissertation addresses a key 21st Century Grand Challenge – "Combatting Nuclear Terrorism”. A principal component associated with addressing this challenge pertains to timely and near real-time detection and tracking of small quantities of special nuclear materials (SNMs); the isotopes of uranium (U-235), and Plutonium (Pu) which constitute the key components of nuclear warheads. Such detection and tracking, especially for shielded U-235 using passive means is virtually impossible due to the extremely faint neutron-photon emission signals from radioactive decay which can be readily masked. Active photon and/or neutron interrogation methods are the only viable means for HEU detection but the field suffers from detector saturation in extreme 10⁴ R h^-1 radiation fields. Pu isotopes in multi-kg levels emanate neutrons from spontaneous fission that offer a means for passive interrogation with directionality, even at low levels assuming novel, high-efficiency detectors are available. Both U-235 and Pu isotopes also emit Rn gas (an alpha radiation emitter) at trace levels, during decay - which offers a possible novel means for identifying the presence of SNMs – from the faint multi Bq m^-3(pCi L^-1) alpha emitting gas and progeny in air - if only a real time sensitive enough detector were available. </p> <p> </p> <p>This thesis work was aimed at filling critical technology gaps, via researching and advancing the field of metastable fluid detector (TMFD) technology pertaining to novel/transformational passive and active (photoneutron) interrogation of SNMs. The results of R&D from this dissertation provide evidence for rapidly and conclusively monitoring for the presence of Rn-222 and progeny in air at ultra-trace (pCi L^-1) levels – even below the action levels mandated by the U.S. EPA by the development of protocols for sampling and detection using centrifugally tensioned metastable fluid detectors (CTMFD). </p> <p> </p> <p>For SNM neutron emission (either spontaneous or induced) based active and passive interrogation this dissertation presents evidence for advancing into novel designs, and schemes resulting in 100-1000x enhancements in detection efficiency for the acoustically driven ATMFD architecture in single and array forms. Novel drive modes: a direct (fixed and sweep) resonance mode, and radically novel indirect traveling wave mode were used to expand ATMFD capabilities and efficiencies beyond previous iterations of ATMFD technology. The experimentation work has been coupled with multi-physics theoretical modeling and simulations benchmarked against experimental data. ATMFDs in single and array-based architectures are being investigated for offering a novel, high-efficiency means for passive interrogation of SNMs. Coupled together with the Rn-alpha sensing approach, the ATMFD sensors for neutron monitoring enable a first-of-a-kind transformational dual mode architecture for monitoring both HEU (U-235) and Pu based SNMs.</p> <p> </p> <p>Successful results were also demonstrated for rapid and convincing 9 MeV (end point x-ray) photoneutron based active interrogation of 4.5 kg of depleted uranium in ultra-high gamma background of ~10⁴ R h^-1 using a single CTMFD or ATMFD sensor. Under such intense gamma backgrounds, conventional detectors are known to get saturated and have presented a major challenge. The research from this thesis offers a novel solution for both passive and active SNM interrogation. </p>

This thesis pertains to two R&D objectives associated with deploying TMFD sensor technology for meeting AARST-NRPP metrics for Radon (Rn) in-air detection, as well as for monitoring of ultra-trace actinides in air, amidst other Rn-progeny alpha emitting radionuclides. A challenge has persisted over the past 40+ years for detecting trace actinides in air amidst a 100-1000x higher Rn-progeny background. This thesis had a primary aim for addressing this challenge, and developing and assessing for a novel technology solution. Both objectives were successfully met. Methods, designs, and experimental effects of apparatus are discussed for successful Rn and progeny detection for 1-100 pCi/L concentration levels, as well as for Rn-progeny “blind” spectroscopic detection of 10-12 μCi/cc concentrations of actinides (Pu/U/Am) in air. The resulting CTMFD based technology was compared with the state-of-art “Alpha Sentry” CAM system and found to offer superior performance in multiple categories, and ~18x improvement in time to detect (e.g. at 0.02 DAC in 3 hrs vs ~70 hrs for state-of-art) for actinides while also remaining ~100% blind to ~102x higher Rn-progeny background; and, with 1 keV energy resolution vs ~300-400 keV for Alpha Sentry.