Research
Nuclear Packaging
Nuclear fuel assemblies consist primarily of square arrays of zircaloy cladding tubes that contain uranium dioxide fuel pellets. The pellets become highly radioactive and produce fission product gases while the assembly is used in a power reactor. After used nuclear fuel (UNF) assemblies are removed from a reactor, they are stored underwater while their radioactivity and heat generation rates decrease. The water also shields the surrounding from radiation and controls the fuel temperature. After sufficient time, assemblies are moved into gas-filled stainless-steel canisters. The canisters are then placed into thick-walled packages for onsite dry storage, or offsite transport. During normal storage and transport, the fuel must remain in its original configuration to allow for future processing or repackaging. The Code of Federal Regulations (10CFR71) also requires that transport packages ensure containment, shielding, and criticality safety after a series of hypothetical accident events. That series consists of a 9-m drop onto an unyielding surface, a 1-m drop onto a puncture bar, 30-minute engulfment in an 800°C fire, and then water emersion. A Safety Analysis Report of Packaging (SARP) is prepared for each system to demonstrate by analysis and/or testing that the package will meet all relevant requirements. Before the system may be used, the SARP must be assessed and approved by a regulatory authority.
Development and experimental validation of computational fluid dynamics (CFD) models to predict heat and mass transport under rarefied gas conditions within complex enclosures
We are currently developing and experimentally benchmarking computational fluid dynamics models of the conduction, convection, radiation heat, and mass transport within the interior of used nuclear fuel packages. Those calculations include the effect of gas rarefaction. Direct Simulation Monte Carlo (DSMC) calculations are being conducted and compared to experimental results to develop slip models for water vapor and a mixture of gases. The models will be used to assure the fuel cladding temperatures within these packages do not exceed safe limits under the pressurized conditions used during fuel storage and transport, and the rarefied conditions used during fuel drying and transfer operations.
The figure on the right shows an experimental apparatus consisting of a 7x7 array of heated rods within a stainless steel pressure vessel. The heater rods are instrumented with thermocouples. The temperature of each rod at different axial positions is obtained. The below-left figure shows a temperature contour from the CFD simulations of the experimental apparatus. It shows that the maximum temperatures are obtained in the axial center of the array. The below-central figure shows a comparison between the temperatures obtained from the experiment and simulations along the AB line at different pressure conditions (pressurized and rarefied). A good agreement is obtained. The below-right figure shows a 3D plot of the axial flow velocity between the rods colored with temperature. It shows that there is an upward stream at the center of the rods and a downstream at the periphery due to natural convection. The US Department of Energy funded this work.
Large Scale Simulations of Radiological Materials Staging Areas
We are currently developing large-scale computational fluid dynamics (CFD) simulations of facilities used to stage large numbers of radiological materials packages. The radiological materials within these packages generate heat, but the temperature of certain package components must not exceed specified limits. The computational models include the facility ventilation system, package heat generation, and airflow within the facility. These models will be used to predict the margin of safety between the package component temperatures and their allowed limits, for a range of ventilation temperatures and flow rates.
Development of sensing platforms to measure conditions within used nuclear fuel canisters and transmit the data wirelessly across the thick metal containment boundary
Our research group has been recently funded by the Nuclear Regulatory Commission (NRC) and the Nuclear Energy University Program (NEUP) to develop sensing platforms to measure conditions within used nuclear fuel canisters and transmit the data wirelessly across the thick metal containment boundary. These projects are in collaboration with Drs. Miles Greiner, Yan Wang from Mechanical Engineering, and Jihwan Yoon and Xiaoshan Zhu from Electrical Engineering. We are also collaborating with researchers from the Pacific Northwest National Lab (PNNL) and Orano Company. We are currently developing a proof-of-concept platform to measure conditions inside thick stainless-steel walled used nuclear fuel canisters, and safely transmit the data to a receiver outside. This work will help assure the safety of used nuclear fuel storage and transportation. The research will develop low-power magnetic resonance signals that can reliably transmit data across the canister walls. It will also develop and place thermoelectric devices that harvest heat generated by the used fuel to produce electric power. That energy will power the measurement and signal transmission devices. Our group will conduct CFD simulations to predict the power that the thermoelectric generator will receive and the best placement of the sensing platform within the canister.
Porous Media Heat Transfer and Flow
Transport of fluid and heat in porous media is important for many engineering applications, such as geothermal systems, underground spread of pollutants, nuclear waste materials storage, thermal insulation, electronic cooling, modeling of a petroleum reservoir, ceramic engineering, etc. Three different properties can define a porous media; (a) space filled with multiphase matter, consisting of at least the solid and fluid phases (fluid phase occupies the pore space around the solid matrix), (b) distribution of the solid phase throughout the porous medium, which affects the solid surface area, and (c) interconnection between the pores (interconnected pore space is called effective pore space and pores that are not connected are considered as a segment of the solid phase).
Due to the extensive use of porous media in different industries and by taking the complexity of modeling porous media into account, the main focus of this research is to model all the relevant physics of flow and phase-change in porous media both numerically and experimentally. This work was funded by NASA EPSCoR.
Computational Research
We are currently creating a solver in the object-oriented OpenFOAM architecture to model flow, heat transfer, and phase changes in porous media, by including all the relevant physics, such as capillarity and relative permeability. This solver is based on an existing solver where the IMPES (Implicit pressure explicit saturation) method is employed to solve the pressure and saturation equations for flow in porous media. The solver is then customized with the energy equation and the phase-change models (empirical and interface equilibrium models). Hydrodynamic and thermal coupling of flow and phase change in porous media is complex numerically, and, this complexity is achieved in the present study by coupling between all the governing equations and the phase-change models using the VOF (Volume of Fluid) approach. Temperature-dependent properties for density, thermal conductivity, and viscosity and different effective thermal conductivity models for the three phases (solid, liquid, and vapor) are included in the solver for more accurate results.
We have performed both 1D and 2D simulations. A simple 1-D simulation case is chosen to demonstrate the capability of the newly developed phaseChangeImpesFOAM solver. The simulation domain consists of a 10 cm long porous media with a porosity of 0.38, a permeability of 2×10-12 m2, and thermal conductivity of 2.48 W/m-K. The Leveret model for capillarity and Carman-Kozeny model for relative permeability and the empirical rate parameter model for phase change are employed in this simulation. Figures on the left show the 1D simulation results where an S-shaped profile was found in the two-phase zone for saturation profile due to the capillarity. Moreover, a flat temperature profile in the two-phase zone was obtained from the numerical analysis.
For 2D analysis, a two-phase flow and phase-change simulation in a small segment of the porous domain of an LHP evaporator is conducted. In an LHP evaporator, liquid advances through the porous media and evaporates at the interface between the porous media and the casing/pillar (where heat is provided). The produced vapor leaves through the vapor grooves between the pillars. Figure 4.8a shows a simple schematic of a typical flat evaporator and Fig. shows the simulation domain considered in this study. The simulation domain shown here consists of a 1×1 cm2 porous media with a porosity of 0.38 and a permeability of 10-12 m2. The thermal conductivity of the porous media is 2.4 W/m-K. From the temperature contour, we obtained that the heated plate is at 5K above the saturation temperature and the inlet temperature is at 5K below it. This causes a temperature gradient within the domain, with the regions surrounding the heater plate being above saturation. We expect evaporation to occur in those regions From the saturation contour we found that when the simulation starts, vapor begins forming close to the heated plate and more vapor is produced as regions farther from the heated plates increase in temperature. A minimum saturation of 0.74 for liquid (maximum of 0.26 for vapor) is reached close to the heated plate. The liquid close to the heated plate does not completely change into vapor, because of the low heat flux provided to the domain as the heated plate temperature is very close to the saturation temperature.
Experimental Research
An experimental apparatus consisting of a thin porous wick sandwiched between two plates is developed to study heat transfer and phase change in porous media (see right figure). The top edge of the wick is in contact with a stainless steel plate heated by a cartridge heater, and the bottom edge is in contact with a liquid reservoir. The liquid flows through the wick and evaporates at the interface with the heated stainless steel plate. The resulting vapor is pushed by capillary action to the vapor chambers on both sides of the heated plates, where it accumulates and leaves through vapor lines. The rate of liquid evaporation is measured by condensing the vapor leaving through the vapor lines and measuring the weight change of the resulting liquid. Various porous wicks with different permeabilities, porosities, and materials are tested. These experiments are then used to validate CFD simulations using the OpenFOAM solver.
The figures at the bottom show some qualitative results of the experiment. At the porous media and heated plated contact vapor was formed as the contact temperature was greater than the saturation temperature. The IR camera was used to capture the temperature contour of the porous media and heated plate. To obtain the temperature of the porous media, a copper plate is placed in front of the porous media. The heated plate and the copper plate are painted black to measure the temperature accurately. Temperature contour from the IR camera showed that the temperature in the contact is higher than the saturation temperature which results in vapor formation and the formed vapor finally leaves through the vapor groove as shown in the figure.
Focusing Aerodynamic Lenses
Aerosol mass spectrometers (AMS) are instruments used to measure the chemical composition and physical properties of atmospheric air. AMSs have critical applications in climate science, atmospheric science, and health science; for example, they are used to quantify particulate matter content in regions of the atmosphere, measure and monitor carbon dioxide emissions and other airborne pollutant emissions, describe cloud properties, and calculate radiation transmissivity of atmosphere. Research efforts at the University of Nevada, Reno, aim to improve the aerosol detection mechanism by expanding the boundaries of the particle sampling range to encapsulate a greater particle size range.
Researchers at UNR are investigating several components of aerodynamic focusing principles. Of which, a key area of interest is the aerodynamic focusing lens (AFL) system – a device that separates bulk fluid flow from aerosol flight paths. Computational fluid dynamics (CFD) software is used in conjunction with theoretical numerical analysis to simulate the two-phase flow through an AFL system. Effects of geometry, rarefaction, turbulence, and shock wave propagation, are currently being studied.
This is an ongoing project at UNR involving collaborations with the Desert Research Institute and the Pacific Northwest National Laboratory.
