Youssef Shatilla

Conformal mapping and hexagonal nodal methods. II: Implementation in the ANC-H code

The ANC-H code is the hexagonal geometry version of the Westinghouse three-dimensional advanced nodal code ANC. Together with PHOENIX-H, the hexagonal geometry version of the Westinghouse pressurized water reactor (PWR) lattice code PHOENIX-P, they provide the Westinghouse code package for designing VVER-type PWR cores of hexagonal geometry. The nodal theory of ANC-H is the net current nodal expansion method implemented with the technique of conformal mapping, which maps a hexagon to a rectangle while preserving the diffusion operator. The use of conformal mapping eliminates the root cause of singularities resulting from the conventional transverse integration. The intranode burnup gradient is accounted for by allowing spatially dependent nodal cross sections. The theory of ANC-H is qualified by benchmarking ANC-H against fine-mesh finite difference code solutions for a variety of benchmark problems. In all cases, the agreement has been excellent. The accuracy of ANC-H for hexagonal geometry cores is as good as ANC for Cartesian geometry cores.

A Fast-Spectrum Test Reactor Concept

Abstract The need for a new steady-state fast-neutron reactor has been the subject of numerous national meetings and discussions. This type of reactor will be able to open new frontiers of research for Generation IV reactors, the Space Propulsion Program, and the Advanced Fuel Cycle Initiative. With the confluence of these three programs’ fast-spectrum testing needs, we set out to conceptualize a new system by looking at previous successful reactor concepts. This paper presents a new concept for a fast-spectrum test reactor that is horizontal in orientation, with individual pressure tubes running the entire length of the scattering-medium tank filled with a liquid heavy metal. This approach for a test reactor will provide more flexibility in refueling, sample removal, and ability to completely reconfigure the core to meet different users’ requirements. Full core neutronic analysis of more than 14 combinations showed that a large hexagonal steam-cooled U-10Zr fuel, with a core power of 267 MW(thermal), produced a fast flux (>0.1 MeV) of 1.3 × 1015 n/cm2·s averaged over the whole length of the irradiation channel. A depletion run with an initial enrichment of 20 wt% 235U had a flat reactivity curve for the first 180 days of cycle due to in-core breeding. Although considerable neutronic optimization and a thermal-hydraulic analysis remain to be performed, it appears that a reactor core with this innovative geometry could meet future fast flux testing needs.

Investigating the effect of suspensions nanostructure on the thermophysical properties of nanofluids

The effect of fractal dimensions and Feret diameter of aggregated nanoparticle on predicting the thermophysical properties of nanofluids is demonstrated. The fractal dimensions and Feret diameter distributions of particle agglomerates are quantified from scanning electron and probe microscope imaging of yttria nanofluids. The results are compared with the fractal dimensions calculated by fitting the rheological properties of yttria nanofluids against the modified Krieger-Dougherty model. Nanofluids of less than 1 vol. % particle loading are found to have fractal dimensions of below 1.8, which is typical for diffusion controlled cluster formation. By contrast, an increase in the particle loading increases the fractal dimension to 2.0–2.2. The fractal dimensions obtained from both methods are employed to predict the thermal conductivity of the nanofluids using the modified Maxwell-Garnet (M-G) model. The prediction from rheology is found inadequate and might lead up to 8% error in thermal conductivity for a...

An experimental study of heat pipe performance using nanofluids

Heat pipe cooling is widely used in computer processors. Advances in microprocessor technology have resulted in reduced heat transfer surface area. Maintaining an efficient cooling process is therefore challenging. The main goal of this experimental study is to perform a parametric study on heat pipe performance using nanofluids. Nanofluids of 1 and 3 vol% of alumina nanoparticles of 20–50 nm diameters in deionized water versus deionized water as a base fluid were considered in the present study. The nanofluids are prepared in our laboratory using two-step method. The nanofluids thermal properties are measured to confirm the properties enhancement that could indicate a corresponding performance enhancement of the heat pipe. A 10 mm inner diameter, 200 mm long brass tube with 50 mm long evaporator, and 50 mm long water cooled condenser were used. Heat pipe wall temperature is reduced with nanofluids as is the temperature difference between the evaporator and condenser. The thermal diffusivity of the nanofluids is increased by 10%. The pipe pressure in case of deionized water was higher than the corresponding one for the nanofluids by 20–32%.

Effect of temperature on turbulent and laminar flow efficacy analysis of nanofluids

The present study investigates the temperature-dependent efficacy analysis of nanofluids for both laminar and turbulent flow applications. Characterizations for the thermo-physical properties of nanofluids are carried out at different temperatures for the viability analysis. Thermal conductivity of yttria nanofluids shows strong temperature dependence compared to copper nanofluids with an increase of 7.5% to 21% for a temperature range of 25 °C to 85 °C. Moreover, the effective viscosity of yttria nanofluids is found to decrease gradually with temperatures in spite of copper nanofluid insensitivity. A theoretical study is carried out to compute the efficacy of yttria and copper nanofluids for laminar and turbulent flow regimes over a range of temperatures with the help of their thermo-physical properties. Yttria nanofluids are found viable for both laminar and turbulent flow for temperatures above 37 °C and 45 °C, respectively. On the other hand, copper nanofluids are found applicable only in laminar flow condition for temperatures above 75 °C. The stability and superior figure of merit for yttria nanofluids with temperature make it a potential coolant for higher-temperature convective cooling systems working on both laminar and turbulent flows.

Actinide Transmutation in PWRs Using CONFU Assemblies

Abstract Expansion of domestic use of nuclear power to provide energy security and environmental sustainability requires minimization of the nuclear waste. To achieve this goal in the short term, transmutation of transuranic (TRU) elements in COmbined Non-Fertile and UO2 (CONFU) Generation-III pressurized water reactor (PWR) assemblies is evaluated. These assemblies are composed of a mix of standard UO2 fuel pins and pins made of recycled TRU in an inert matrix and are designed to fit in currently deployed PWRs. Previous studies have shown the feasibility of a CONFU-Equilibrium (CONFU-E) assembly design with a net TRU balance between production and destruction and a CONFU-Burndown (CONFU-B) assembly design with net destruction of TRU coming from several reactors. In this paper, a CONFU-self-Contained (CONFU-C) assembly is shown to achieve net TRU destruction in a self-contained TRU multirecycling system. Both the CONFU-B and CONFU-C designs are presented in this paper in detail. For these designs a detailed assembly-level neutronic analysis has been performed using CASMO-4 to investigate cycle length, TRU management performance, and key reactor reactivity parameters, along with detailed intraassembly power peaking factors (IAPPFs). Various fuel mixing schemes and cooling times were evaluated. Using the IAPPF results, a full core thermal-hydraulic analysis using VIPRE was performed to validate thermal margins, and a loss-of-coolant-accident event was assessed using RELAP5. Based on the TRU management characteristics of these designs, metrics were developed to reflect the material handling difficulties of the multirecycled fuel, along with its repository impact. These parameters were compared to a standard once-through UO2 cycle, along with other Pu or TRU multirecycling schemes [mixed oxide with enriched uranium (MOX-UE) and COmbustible Recyclage A ILot (CORAIL)]. Finally, an economic analysis has been conducted to compare the fuel cycle cost (FCC) associated with these designs. TRU management results of CONFU-B and CONFU-C showed a net TRU destruction of 2 to 20 kg/TW·h(electric) generated, with an FCC of 12 to 15 mills/kW·h(electric), depending on the mixing strategy and cooling time chosen. Reactor control parameters and thermal margins were found to be comparable to an all-UO2 assembly. While both designs offer significant repository benefits, the accumulation of minor actinides may limit the practicality of fuel multirecycling.

Mathematical Model of Cooling of a Stopped Pot and Its Validation

In aluminum reduction pot technology, the potshell is used for several generations. After each shut down the potshell is cooled by free convection and radiation. This cooling takes from five to nine days depending on the surrounding temperature. Cooling by spraying water on the potlining is used in some aluminum plants; this reduces the cooling time to less than one day but this method can be harmful for the potshell and for the environment.

Simulation of Flow Inside Heat Pipe: Sensitivity Study, Conditions and Configuration

Heat pipes are widely used as a heat transporting device in a variety of applications. From space satellites, large industrial appliances to a heat sink for cooling electronic components and packages. Heat pipes are extremely efficient because of their high effective thermal conductivity, compactness, low cost and reliability. Therefore, the designers of heat sinks are often required to optimize the performance of the heat pipe itself in order to improve the overall thermal management system of any particular equipment. However, the detailed internal modeling of a heat pipe presents a challenging problem for an engineer. It is a multi physics problem including two phase flow within porous media and with conjugate heat transfer adding the solicited high capillary and surface tension effect. In this present study, detailed modeling of the heat pipe considering the mentioned effects is pursued. A basic review of the governing equations describing the complete heat pipe operation is given. The commercially available simulation tool Fluent 6.3 is used to describe and solve these equations in a coupled conjugate heat transfer set up. The geometry of heat pipe is divided in two different regions which solve simultaneously. The first region is core region where only vapor flow is assumed. The second region consists of wall and wick structure through which the mass transfer due to wettability and heat dissipates through conduction. Water was used as flowing fluids through wick porous structure. Previous experimental as well as numerical models regarding the heat pipes have been studied and used for the verification of the present model along with a standard grid convergence study. The effects of different heat pipe length, heat fluxes, wall thickness, wall material and porosity are investigated. The pressure drop and wall temperature increase with the value of heat flux. Similarly, porosity and wall material affect the wall temperature distribution. The effect of wall thickness and heat pipe length was not significant. In addition, a theoretical model is developed for the pressure drop across the heat pipe in vapor region and the respective output was used for the simulation. Finally, the temperature distribution in wall and wick is shown and discussed.

Exergy Analysis of Nanofluids in Microchannel

Present study carries out an exergy analysis for nanofluids in microchannels. For this, two microchannels with diameter of 218 μm and 303 μm are chosen due to availability of experimental data.. The alumina nanoparticles with 45 nm average size are dispersed in DI water. The stability of these nanofluids is controlled by their pH. Three concentrations of 0.25 vol%, 0.5 vol% and 1vol% are chosen to observe the volume fraction effect. For the entropy generation analysis Bejan equations for internal flow are used. The order of magnitude method was used to simplify the equations. Initially, the analysis is carried out with the standard correlations (Dittus-Boelter and Blasius equations) for tube flow for laminar region. Frictional and thermal entropy generation rate ratios were found comparable and neither of them can be neglected. The entropy generation rate ratio was above unity for 218 μm channel while, it was below unity for 303 μm channel. The prediction of entropy generation rate ratio was higher for experimental correlations compared to that of theoretical correlations. The entropy generation number was higher for 303 μm channel and more prone to change with concentration. The thermal part of entropy generation was the major part of total entropy generation for 303 μm channel. Also, the absolute entropy generation was higher for 303 μm channel compared to 218 μm channel.Copyright

Heat pipe long term performance using water based nanofluid

Abstract The heat pipe is a passive cooling device that transfers heat from a hot source to a heat sink using fluids as a working medium. Working medium evaporation and condensation are key factors for designing an efficient heat pipe. Many researchers highlight nanofluids, a mixture of base fluid and nanoparticles, as a new working medium for more efficient heat pipes. The present research aimed to investigate heat pipe long-term performance using water-based nanofluids as working medium. Nanofluids with 1 and 3 vol% Al2O3 of 20–70 nm particle diameter in water were prepared and characterized. It has been seen in our previous study that the heat pipe performance is enhanced by an average of 26%; however, this enhancement was not sustained over long use and raised a concern about the long-life homogeneity of the nanofluid due to the liquid evaporation. Therefore, we investigated used nanofluid characteristics to determine whether it stays suspended in the base fluid as dispersed particles, or it agglomerates, then aggregates in bigger sizes and then precipitates. The dried heat pipe’s porous medium is cut-out after several uses and is scanned by electron microscope (SEM) at different operation heat loads. Some aggregated nanoparticles have been seen on the wick surface, which caused a capillary and thermal resistance. Also, a sample of the used nanofluid is dried and scanned by SEM, and it shows similar particles aggregation to those observed on the surface of the porous medium. This study showed heat pipe performance improvement because of the heat transfer enhanced features of nanofluids technology, and it justifies the performance decay after long-term use.