Numerical Evaluation of Micro-Pocket Fission Detectors

Micro-pocket fission detectors (MPFDs) are miniature fission chambers suitable for in-core neutron measurement that have been under development at Kansas State University for over one decade. Current-generation devices have been used at a number of university reactors (Kansas State, Wisconsin, and MIT) and as part of the first experiments performed during the recent restart of TREAT. Ongoing research aims to improve understanding of the existing MPFDs and to optimize designs for future deployment. To aid in this development, the dynamic response of a prototypic MPFD was evaluated using Garfield++, Elmer, Gmsh, and Stopping and Range of Ions in Matter (SRIM). Specifically, the finite-element code Elmer was used to calculate the electric field on a mesh generated by Gmsh. SRIM was used to compute the energy loss tables of the fission fragments in the gas. With output from Elmer and SRIM, Garfield++ was used to simulate the ionization process, the resulting electron drift, and the induced signal. This particular Garfield++ application was developed with hybrid parallelization based MPI and OpenMP. The performance of the MPFDs subjected to different temperatures and applied voltages was evaluated. The preliminary results indicate the fission fragment deposits a few MeV of energy in the gas, consistent with previous estimates. The pulses in the MPFDs can be formed in the nanosecond scale, thus accommodating high count rates and, hence, high neutron-flux levels. Ongoing work aims to extend this model and validate it against existing and planned experimental data

Strong flame acceleration and detonation limit of hydrogen-oxygen mixture at cryogenic temperature

A series of experiments were carried out in a closed tube at cryogenic temperature (77 K) for hydrogen-oxygen mixtures. Flame propagation speed and overpressure were measured by optical fibers and pressure sensors, respectively. The first and second shock waves were captured in the cryogenic experiments, although the shock waves always precede the flames in all cases indicating the absence of stable detonation. However, strong flame acceleration was observed for all situations, which is consistent with the prediction by expansion ratio and Zeldovich number. Besides, the tube diameter and length are also critical for flame acceleration to supersonic. All the flames in this work accelerate drastically reaching the C-J deflagration state. But at 0.4 atm, only fast flame is formed, while at higher initial pressures, the flame further accelerates to a galloping mode manifesting a near-limit detonation, which could be indicated by the stability parameter χ

Modeling and simulation of neutron detectors for the transient reactor test facility

Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringJeremy A. RobertsThe Transient REActor Test (TREAT) facility was restarted and will be used to test accident-tolerant fuels to improve nuclear reactor safety. In this work, alternative neutron detectors for use in core and with the hodoscope at the TREAT facility were modeled and simulated using different computational tools to understand the underlying physics. The Hornyak button scintillation detector used in the original TREAT hodoscope to detect fast neutrons and its variants were evaluated using Geant4 to simulate the coupled nuclear and optical physics. The Hornyak-button model predicted an intrinsic efficiency of 0.35% for mono-directional fission neutrons and strong gamma-induced Cherenkov noise, which agree relatively well with the reported experimental observations. The proposed variants use silicon photomultipliers to reduce Cherenkov noise and have optimized layered or homogenized scintillation volumes. The layered and homogenized variants with 5-cm length were predicted to achieve neutron-detection efficiencies of 3.3% and 1.3%, respectively, at a signal-to-noise ratio of 100. Another candidate devices for the hodoscope are the actinide and hydrogenous microstructured semiconductor neutron detectors (MSNDs) evaluated using Geant4 and MCNP. With a sufficient rejection of the gamma noises, the U235 -filled and the hydrogenous MSNDs were predicted to yield neutron-detection efficiencies of 1.2% and 2.5%, respectively, at the length of 2 cm. The micro-pocket fission detectors (MPFDs) were developed to detect in-core neutrons, and the electron collection process in such devices was evaluated using Garfield++-based computational routine. The high-performance Garfield++ application was developed using the built-in, optimized element-search techniques and a hydrid MPI and OpenMP parallelization scheme. The preliminary results indicated that the averaged deposited energy per fission fragment was 7.15 MeV, and the induced current occured within 400 ns

Employing a fan to improve the wintery heating performance of solar air collectors with phase-change material: An experimental comparison

Solar Air Collectors integrated with Phase-Change Material (SAC-PCM) were widely used for wintery heating with the superiority of overcoming the mismatch shortcoming between supply and demand time from solar heating. However, the traditional SAC-PCM had the low-efficiency heat output due to the low air flow of natural convection. Based on this, a fan was proposed to enhance air flow and thereby, increase the heat output from SAC, so an experiment was built to compare the thermal performance between SAC-PCM and SAC-PCM with a fan (SAC-PCM-F). The experimental result showed that the fan enhanced the outlet wind speed from 0.8 m/s-1.55 m/s to 2.15 m/s-2.3 m/s and by employing the fan, the heat output was easier from SAC-PCM and the average temperatures of outlet air were increased 4.18 °C in all day with the average temperature difference of outlet and inlet air increased by 1.17 °C. Employing the fan could improve indoor average temperature by 1.17 °C, while the daily heat output was increased by 146.02% by employing the fan and the heat collecting efficiency was increased from 24.92% to 61.32

Boosting Memory with a Persistent Memory Mechanism for Remote Sensing Image Captioning

The encoder–decoder framework has been widely used in the remote sensing image captioning task. When we need to extract remote sensing images containing specific characteristics from the described sentences for research, rich sentences can improve the final extraction results. However, the Long Short-Term Memory (LSTM) network used in decoders still loses some information in the picture over time when the generated caption is long. In this paper, we present a new model component named the Persistent Memory Mechanism (PMM), which can expand the information storage capacity of LSTM with an external memory. The external memory is a memory matrix with a predetermined size. It can store all the hidden layer vectors of LSTM before the current time step. Thus, our method can effectively solve the above problem. At each time step, the PMM searches previous information related to the input information at the current time from the external memory. Then the PMM will process the captured long-term information and predict the next word with the current information. In addition, it updates its memory with the input information. This method can pick up the long-term information missed from the LSTM but useful to the caption generation. By applying this method to image captioning, our CIDEr scores on datasets UCM-Captions, Sydney-Captions, and RSICD increased by 3%, 5%, and 7%, respectively

Influence of nocturnal thermal insulation on thermal performance improvement of solar air collector with phase-change material

Solar air collectors (SAC) are widely used for the wintery heating, but they have a mismatch shortcoming between supply and demand periods of heat quantity. Integrating phase change material (PCM) into solar air collectors was usually employed to overcome this mismatch, but it was proved that the heat loss through the glass covers reduced utilization efficiency largely in the actual application. According to this, the study employed PCM and nocturnal thermal insulation (NTI) to overcome the mismatch between supply and demand time and improve the nocturnal thermal insulation performance, respectively. Three air collectors (traditional SAC, collector integrated with phase change material (SAC-PCM), collector integrated with both phase change material and nocturnal thermal insulation (SAC-PCM-NTI)) were built to analyze the influence. Experimental results showed PCM could change the heat output time by the latent storage, but the low thermal insulation of the glass plate caused the much heat loss and reduced the thermal efficiency obviously. Employing NTI was highly efficient for collectors which PCM by reducing the heat loss, while it could increase the average temperature of indoor air by 2.49 °C. Employing PCM could reduce the heat collecting efficiency by 13%–21% due to the much heat loss during the heat storage and release processes, while using NTI could increase heat collecting efficiency from 24.46% to 32.63%

Current and Future Land Use Characters of a National Central City in Eco-Fragile Region—A Case Study in Xi’an City Based on FLUS Model

Land use change plays a key role in terrestrial systems and drives the process of ecological pattern change. It is important to investigate the process of land use change, predict land use patterns, and reveal the characteristics of land use dynamics. In this study, we adopted the Markov model and future land use (FLUS) model to predict the future land use conditions in Xi’an city. Furthermore, we investigated the characteristics of land use change from a novel perspective, i.e., via establishment of a complex network model. This model captured the characteristics of the land use system during different periods. The results indicated that urban expansion and cropland loss played an important role in land use pattern change. The future gravity center of urban development moved along the opposite direction to that from 2000 to 2015 in Xi’an city. Although the rate of urban expansion declined in the future, urban expansion remained the primary driver of land use change. The primary urban development directions were east-southeast (ENE), north-northeast (NNE) and west-southwest (WSW) from 1990 to 2000, 2000 to 2015, and 2015 to 2030, respectively. In fact, cropland played a vital role in land use dynamics regarding all land use types, and the stability of the land use system decreased in the future. Our study provides future land use patterns and a novel perspective to better understand land use change