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In this work, a two-dimensional (2D) geometrically-accurate model of the TN-32 cask is generated in ANSYS/Fluent to investigate the effect of backfill gases and their pressures on the peak cladding temperature (PCT). This model is similar to the cask being used in high-burnup (HBU) spent fuel data project lead by the Electric Power Research Institute (EPRI). Helium, nitrogen, argon, and water vapor fill gases are investigated at pressures ranging from atmospheric (∼105 Pa) to 100 Pa. Steady-state computational fluid dynamics (CFD) simulations that include the effect of gas rarefaction (temperature-jump) at the gas-solid interfaces are conducted. The PCT as a function of heat generation rate and pressure is reported as well as the heat generation rate that brings the cladding temperature to the radial hydride formation limit. The results show that there are competing effects between the temperature-jump and the thermal conductivity of the gas to increase the fuel rods’ temperature. The low pressures increased the PCT, with the increase being most significant for the helium backfill.

In this work, a 3D, one-eighth homogenous fuel CFD model of the TN-32 UNF cask is constructed, and simulations are performed as part of the Extended Storage Collaboration Program (ESCP). The simulations are carried out using ANSYS/FLUENT1 package with geometry and boundary conditions recommended by ESCP. Simulation results, including external and internal cask temperatures, are compared with experimental data from the High-Burnup (HBU) project. A preliminary sensitivity analysis is conducted to determine the effect the different assumptions/simplification proposed by ESCP on the cask temperatures. This sensitivity analysis indicates that the cask internal natural convection has a significant effect on the prediction of internal temperatures when the drain holes at the base of the basket are modeled. Additionally, the results indicate that a small variation in the thickness of the peripheral basket-rail gap has a significant effect on the internal temperatures. The best simulation model is shown to include modeling of the drain holes, natural convection, a peripheral basket-rail gap of 4.78 mm (0.188 inch), and omission of the base inner liner-gamma shield gap.

Loop heat pipes (LHP) are passive thermal management systems widely used in electronics cooling, military applications, renewable energy, and spacecraft. These two-phase systems employ capillary forces instead of pumps to circulate the coolant. In these devices, the coolant evaporates and condenses in the evaporator and condenser, respectively. The condensed coolant liquid is driven toward the evaporator by capillary action in a wick structure located inside the evaporator. A mechanical pump can be added to the liquid line of the loop to reach the distributed heat loads and control the temperature to produce an isothermal surface. In this work, the porous wick of an evaporator in a mechanically pumped loop heat pipe was analyzed employing the Computational Fluid Dynamics (CFD) code ANSYS/Fluent. The Volume of Fluid (VOF) model in ANSYS/Fluent is modified using a User Defined Function (UDF) to calculate mass transfer between the liquid and vapor phases at the interface. This research focuses on effect of applied heat flux on the evaporator, liquid mass flow rate at the wick inlet, wick porosity, permeability, and material thermal conductivity, and value of gravitational acceleration on the overall performance of the system. The results illustrate effect of each parameter on overall system performance and flow patterns of two-phase working fluid inside the porous wick. Some design recommendations also are made to fabricate the wick of such a system for any precious thermal management application.

In many applications, the heat flux at the surface is known instead of the surface temperature. In addition, in some applications, like vacuum drying or high altitude flights, the pressure is below atmospheric pressure, so the rarefaction effects become important, and therefore, the Navier-Stokes-Fourier equations fail to predict gas thermal behavior. In this paper, a constant heat flux boundary condition is developed and implemented in the frame of the Shakhov model kinetic equation, with the possibility to simulate the diffuse-specular reflexion of the molecules from the surface. The developed technique is implemented for the simulation of gas heat transfer in a two concentric cylinders configuration, similar to vacuum drying of used nuclear fuel canisters. The numerical results obtained using developed approach are compared with experimental data of heat transfer through rarefied gas between two concentric cylinders.

The fire time of concern for a component within a used nuclear fuel transport package is the time after fire ignition when that component reaches its temperature of concern. In this work a legal we...

A temperature-jump thermal resistance, based on results from Lin & Willis 1979, was implemented at the gas/solid interfaces of a three-dimensional computational fluid dynamics model of a 7 × 7 array of heated rods within a square-cross-section pressure vessel filled with rarefied dry helium. This configuration is relevant to the vacuum drying of used nuclear fuel canisters. Simulations were performed for a range of rod heat generation rates, boundary temperatures, and gas pressures in the continuum and rarefied-gas-slip regimes. Experiments conducted by the current authors were used to measure rod and enclosure temperatures for the same conditions, to validate the simulations. For all measurement locations and experiments, the measured rod-to-boundary temperature differences varied from 12 °C to 102 °C. The simulated differences correlated linearly to the measured differences, closely following variations for different locations and experiments, but exhibited random variations from the best-fit line. In the slip regime, the predicted rod-to-boundary temperature differences were systematically 0.9 °C smaller than the measured values (less than half of the thermocouple uncertainty), and 95% of the simulated differences were less than 2.5 °C from the best-fit line (14% larger than the rod thermocouple uncertainty). The temperature-jump thermal resistance model will be useful for predicting temperatures during vacuum drying operations.

The goal of this work is to obtain experimental data that can be used to validate computational models that quantify the effect of gas rarefaction on heat transfer during vacuum drying of used nuclear fuel transport/storage canisters. An experimental apparatus is constructed, consisting of a 7 × 7 array of electrically heated rods held by spacer plates near their ends and contained within a square cross-section helium-filled pressure vessel. Thermocouples are used to measure heater rod, spacer plate, and enclosure temperatures for a range of rod heat generation rates, helium pressures in the continuum and rarefied-gas slip regimes, and different thicknesses of insulation outside the enclosure, which increases the apparatus temperatures. The results shows that the temperature difference between the enclosure and the rods increases by less than 4% when the pressure decreases within the continuum regime (from ~ 105 to 5700 Pa). However, it increases by up to 78% in the slip regime (~5700 to 65 Pa), due to the temperature-jump thermal resistance at the gas/solid interfaces. Random variation in the measured temperatures, caused by configuration and measurement errors, is less than 1.1℃, which makes this data well suited for benchmarking computational methods for calculating heat transfer and temperatures in used nuclear fuel canisters under vacuum drying conditions.

During vacuum drying of used nuclear fuel (UNF) canisters, helium pressure is reduced to as low as 67 Pa to promote evaporation and removal of remaining water after draining process. At such low pressure, and considering the dimensions of the system, helium is mildly rarefied, which induces a thermal-resistance temperature-jump at gas–solid interfaces that contributes to the increase of cladding temperature. It is important to maintain the temperature of the cladding below roughly 400 °C to avoid radial hydride formation, which may cause cladding embrittlement during transportation and long-term storage. Direct Simulation Monte Carlo (DSMC) method is an accurate method to predict heat transfer and temperature under rarefied condition. However, it is not convenient for complex geometry like a UNF canister. Computational Fluid Dynamics (CFD) simulations are more convenient to apply but their accuracy for rarefied condition are not well established. This work seeks to validate the use of CFD simulations to model heat transfer through rarefied gas in simple two-dimensional geometry by comparing the results to the more accurate DSMC method. The geometry consists of a circular fuel rod centered inside a square cross-section enclosure filled with rarefied helium. The validated CFD model will be used later to accurately estimate the temperature of an UNF canister subjected to vacuum drying condition.

In this work, a geometrically-accurate two-dimensional (2D) computational fluid dynamic (CFD) model of a used nuclear fuel cask, that can contain up to 32 pressurized water reactor (PWR) used nuclear fuel (UNF) assemblies, is constructed. This model is similar to the TN-32 cask employed in the ongoing high-burnup (HBU) Spent Fuel Data Project lead by the Electric Power Research Institute (EPRI). This model is used to predict the peak cladding temperature under vacuum drying conditions. Due to the symmetry of the cask, only one-eighth of the cross-section is modeled. Steady-state simulations that include the temperature-jump boundary conditions at the gas-solid interfaces are performed for different heat generation rates in the fuel regions and a range of dry helium pressures, from ∼105 to 100 Pa. These simulations include conduction within solid-gas regions and surface-to-surface radiation across all gas regions. The peak cladding temperatures are reported for various heat generation rates and rarefaction conditions, along with the maximum allowable heat generation that brings the cladding temperatures to the radial hydride formation limit. The results showed that the decrease of helium pressure significantly increased the temperature of the cladding material compared to the atmospheric pressure condition.

Vacuum drying of nuclear fuel canisters may cause the temperature of fuel assemblies to considerably increase due to the effect of gas rarefaction at low pressures. This effect may induce a temperature-jump at the gas-solid interfaces. It is important to predict the temperature-jump at these interfaces to accurately estimate the maximum temperature of the fuel assemblies during vacuum drying. The objective of this work is to setup a concentric cylinders experimental apparatus that can acquire data to benchmark rarefied gas heat transfer simulations, and determine the temperature-jump coefficient at the interface between stainless steel surface and helium gas. The temperature-jump is determined by measuring the temperature difference and heat flux across a 2-mm gap between the concentric cylinders that contains rarefied helium and compare the results to analytical calculations in the slip rarefaction regime.

Measurements of heat transfer from an array of vertical heater rods to the walls of a square, helium-filled enclosure are performed for a range of enclosure temperatures, helium pressures, and rod heat generation rates. This configuration is relevant to a used nuclear fuel assembly within a dry storage canister. The measurements are used to assess the accuracy of computational fluid dynamics (CFD)/radiation simulations in the same configuration. The simulations employ the measured enclosure temperatures as boundary conditions and predict the temperature difference between the rods and enclosure. These temperature differences are as large as 72 °C for some experiments. The measured temperature of rods near the periphery of the array is sensitive to small, uncontrolled variations in their location. As a result, those temperatures are not as useful for validating the simulations as measurements from rods near the array center. The simulated rod temperatures exhibit random differences from the measurements that are as large as 5.7 °C, but the systematic (average) error is 1 °C or less. The random difference between the simulated and measured maximum array temperature is 2.1 °C, which is less than 3% of the maximum rod-to-wall temperature difference.

During vacuum drying of used nuclear fuel canister, helium pressure is decreased to as low as 67 Pa to promote evaporation and removal of water remaining in the canister following draining operation. At low pressures associated with vacuum drying, there is a temperature jump (thermal resistance) between the solid surfaces and helium in contact with them. This temperature jump increases as the pressure decreases (rarefied condition), which contributes to the fuel assembly’s temperature increase. It is important to keep the temperature of the fuel assemblies below 400°C during vacuum drying to ensure their safety for transport and storage. In this work, an experimental apparatus consisting of a 7×7 array of electrically heated rods maintained between two spacer plates and enclosed inside a square cross-section stainless steel pressure vessel is constructed to evaluate the temperature of the heater rods at different pressures. This geometry is relevant to a BWR fuel assembly between two consecutive spacer plates. Thermocouples are installed in each of the 49 heater rods, spacer plates and enclosure walls. They provide a complete temperature profile of the experiment. Different pressures and heat generation relevant to vacuum drying conditions are tested. The results showed that the maximum temperature of the heater rods increases as the pressure decreases. The results from these experiments will be compared to computational fluid dynamics simulations in a separate work.

Computational fluid dynamics simulations of a 7×7 array of heated rods within a square-cross-section enclosure filled with rarefied helium are performed for heat generation rates of 50 W and 100 W and various helium pressures ranging from 105 to 50 Pa. The model represents a section of nuclear fuel assembly between two consecutive spacer plates inside a nuclear canister subjected to during vacuum drying process. A temperature jump model is applied at the solid-gas interface to incorporate the effects of gas rarefaction at low pressures. The temperature predictions from simulations are compared to measured temperatures. The results showed that when helium pressure decreased from 105 to 50 Pa, the maximum temperature of the heater rod array increased by about 14 °C. The temperatures of the hottest rod predicted by simulations are within 4°C of the measured values for all pressures. The random difference of simulated rod temperatures from the measured rod temperatures are 3.33 °C and 2.62 °C for 100 W and 50 W heat generation rate.

Steady-state heat transfer through a rarefied gas confined between parallel plates or coaxial cylinders, whose surfaces are maintained at different temperatures, is investigated using the nonlinear Shakhov (S) model kinetic equation and Direct Simulation Monte Carlo (DSMC) technique in the slip regime. The profiles of heat flux and temperature are reported for different values of gas rarefaction parameter δ, ratios of hotter to cooler surface temperatures T, and inner to outer radii ratio R. The results of S-model kinetic equation and DSMC technique are compared to the numerical and analytical solutions of the Fourier equation subjected to the Lin and Willis temperature-jump boundary condition. The analytical expressions are derived for temperature and heat flux for both geometries with hotter and colder surfaces having different values of the thermal accommodation coefficient. The results of the comparison between the kinetic and continuum approaches showed that the Lin and Willis temperature-jump model accurately predicts heat flux and temperature profiles for small temperature ratio T=1.1 and large radius ratios R≥0.5; however, for large temperature ratio, a pronounced disagreement is observed.

International audience; A two-dimensional computational model of a loaded used nuclear fuel canister filled with dry helium gas was constructed to predict the cladding temperature during vacuum-drying conditions. The model includes distinct regions for the fuel pellets, cladding, and helium within each basket opening, and it calculates the conduction heat transfer within all solid components, heat generation within the fuel pellets, and conduction and surface-to-surface radiation across the gas-filled regions. First, steady-state simulations are performed to determine peak clad temperatures as a function of the fuel heat generation rate, assuming the canister is filled with atmospheric pressure helium. The allowable fuel heat generation rate, which brings the peak clad temperature to its limit, is evaluated. The discrete velocity method is then used to calculate slip-regime rarefied gas conduction across planar and cylindrical helium-filled gaps. These results are used to verify the Lin-Willis solid-gas interface thermal resistance model for a range of thermal accommodation coefficients. The Lin-Willis model is then implemented at the solid-gas interfaces within the canister model. Finally, canister simulations with helium pressures of 100 and 400 Pa and 1, 0.4, and 0.2 are performed to determine how much hotter the fuel cladding is under vacuum-drying conditions compared to atmospheric pressure. For 0.4, the fuel heat generation rates that bring the clad temperature to its allowed limit for helium pressures of 400 and 100 Pa are reduced by 10% and 25%, respectively, compared to atmospheric pressure conditions. Transient simulations show that the cladding reaches its steady-state temperatures 20 to 30 h after water is removed from the canister.

Three large-scale fire tests were conducted in which a 2.4-m-(8-ft)-dia., 4.6-m-(15-ft)-long, 25-mm-(1-inch)-wall-thickness mild-steel pipe calorimeter was centered 1 m above a 7.9-m-dia. basin containing 7.57 m3 (2000 gal) of jet fuel. The wind conditions, calorimeter wall temperatures, and temperatures of foil radiant heat flux gages near the calorimeter were measured at several locations as functions of time during and after the fires. Video and still photography from several directions were used to monitor the calorimeter’s engulfment in flames. The objective of these tests was to determine how the fuel consumption rate, calorimeter coverage in flames and the calorimeter temperatures varied with wind conditions. These data can be used to benchmark computational and engineering models of heat transfer from large pool fires to thermally-massive objects. Those types of models are used to predict the response of rail-car-sized used-nuclear-fuel transport packages in severe accidents. The first two tests h...

An experimental apparatus was constructed, consisting of an 8×8 array of electrically-heated rods held in a square array by stainless-steel spacer plates near their ends. The rod/plate assembly was enclosed within a square-cross-section helium-filled aluminum pressure vessel and the rods were oriented vertically. The apparatus simulates the region between two consecutive spacer plates of a used nuclear fuel assembly within a vertical dry storage canister. Rod, spacer plate, and enclosure wall temperatures were measured using thermocouples in a matrix of nine experiments with total rod heat generation rates of 100, 300, and 500 W, and nominal helium pressures of 1, 2, and 3 atm. Steady-state simulations representing the experiment were performed, which include heat generation within the rods, conduction within the solid elements, as well as buoyancy-induced motion within, and natural convection and radiation heat transfer across, helium-filled regions. These were compared to the experimental results to assess the accuracy of the computational model for a range of boundary conditions. The comparison between the simulated and measured data showed that the simulations systematically under predict the hotter rod temperatures and over predict the cooler ones. Linear regression showed that 95% of the simulated temperatures are within 4.26°C of the correlation values.

Two- and three-dimensional thermal models of a Nuclear Assurance Corporation Legal Weight Truck (NAC-LWT) cask were constructed using the PATRAN commercial finite element package. The two-dimensional model included the effect of radial stiffeners in the package’s external neutron shield but the three-dimensional model did not. A normal conditions of transport (NCT) simulation using both models predicted the peak cladding temperature was roughly 210°C. The NCT package temperatures were used as initial conditions for transient fire/post-fire simulations. Different assumptions were used to determine when the neutron shield liquid drained from the tank and was replaced by air. When the liquid was assumed to remain within the tank during and after the fire, the peak cladding temperature was predicted to exhibit a temporal maximum of roughly 300°C, ∼6 h after the end of the fire. If the liquid drained from the tank during the fire, the cladding temperature did not exhibit a temporal peak. Rather, it eve...

Nuclear materials are placed in shielded, stainless steel packaging for storage or transport. These drum type packages often employ a layer of foam, honeycomb, wood or cement that is sandwiched between thin metal shells to provide impact and thermal protection during hypothetical accidents, as those prescribed in the Code of Federal Regulations (10 CFR 71·73). The present work discusses the modelling of the thermal degradation of polyurethane (PU) foam within an annular region during an 800°C fire. Measurements and analysis by Hobbs and Lemmon [M. L. Hobbs and G. H. Lemmon: ‘Polyurethane foam response to fire in practical geometries’, Polym. Degrad. Stab., 2004, 84, 183–197.] indicate that at elevated temperatures, PU foam exhibits a two-stage, endothermic degradation. The first stage produces a degraded solid and a combustible gas; the second stage reaction consumes the degraded solid and produces another combustible gas. As a result, during a prolonged fire, a gas filled void develops beside the...

Previously, the US Nuclear Regulatory Commission performed simulations using fire dynamics simulator (FDS) software to predict the response of spent nuclear fuel transport packages in severe naturally ventilated tunnel fires. The long-range objective of the authors' current project is to predict the response of such a package to those tunnel fires using different computational methods. The first stage of the project, which is the subject of this article, is to determine the accuracy of Container Analysis Fire Environment (CAFE) computer simulations in predicting gas speed and temperature measurements made in forced-ventilated and naturally ventilated fires from the Memorial tunnel test series performed for the Massachusetts Highway Department in the 1990s. The CAFE simulations accurately predict the average heat release rate in both types of tests. Gas speeds and temperatures are obtained from the simulations at the same locations as for the measurements. For the forced-ventilated test, CAFE predicts those quantities more accurately upstream of the fire than downstream. In addition, the predictions are less accurate for the naturally ventilated test than they are for the forced-ventilated experiment. However, the accuracy of the CAFE prediction in the naturally ventilated test is on par with that from FDS. Language: en

Experiments and computational fluid dynamics/ radiation heat transfer simulations of an 8 X 8 array of heated rods within an air-filled aluminum enclosure are performed. This configuration represents a region inside the channel of a boiling water reactor fuel assembly between two consecutive spacer plates. The rods are oriented horizontally or vertically to represent transport or storage conditions. The measured and simulated rod temperatures are compared for three different rod heat generation rates to assess the accuracy of the simulation technique. Simulations show that temperature gradients in the air are much steeper near the enclosure walls than they are near the center of the rod array. The measured temperatures of rods at symmetric locations are not identical, and the difference is larger for rods close to the wall than for those far from it. Small but uncontrolled deviations of the rod positions away from the design locations may cause these differences. The simulations reproduce the measured temperature profiles. For a total rod heat generation rate of 300 W, the maximum rod-to-enclosure temperature difference is 150°C. Linear regression shows that the simulations slightly but systematically overpredict the hotter rod temperatures but underpredict the cooler ones. For all rod locations, heat generation rates, and rod orientations, 95% of the simulated temperatures are within 11°C of the correlation values. For the hottest rods, which reside in the center of the domain where the air temperature gradients are small, 95% of the simulated temperatures are within 4.3°C of the correlation values. These results can be used to assess the accuracy of using simulations to design spent nuclear fuel transport and storage systems.

Experiments performed by Lovett (1991) measured the temperature of an 8 × 8 array of horizontal heated rods in air within a constant temperature enclosure. That apparatus was a scaled-down model of...

A two-dimensional finite volume mesh of a legal-weight truck cask cross section is constructed, including four pressurized water reactor fuel assemblies inside. Computational fluid dynamics (CFD) simulations calculate buoyancy-driven gas motion, natural convection and radiation heat transfer in geometrically accurate gas-filled fuel regions, and conduction within the solid components. Steady-state simulations are performed with the cask in a normal transportation environment for ranges offuel heat generation rate and cladding emissivity, with atmospheric-pressure helium or nitrogen cover gases. The cask thermal dissipation capacity is defined as the fuel heat generation rate that brings the fuel cladding temperature to its allowed limit. That capacity is 23% higher when helium is the cover gas than for nitrogen. Increasing the cladding emissivity by 10% increases the capacity by 4% for nitrogen, but only 2% for helium. Stagnant-gas simulations using the geometrically accurate mesh predict essentially the same cask thermal dissipation capacity as simulations that include gas motion. This indicates that buoyancy-induced gas motion is not strong enough to significantly enhance heat transfer for this configuration. Simulations employing effective thermal conductivities and homogenized (nongeometrically accurate) meshes in the fuel regions predict cask thermal capacities that are 3 to 8% lower than the geometrically accurate CFD simulations. Basket surface temperatures calculated in this work will be used as boundary conditions in future benchmark experiments.

Two-dimensional finite element thermal simulations of large rail casks designed to transport spent nuclear fuel assemblies were performed for normal conditions. Two different effective thermal conductivity models, developed by other investigators, were implemented within the basket openings that support the fuel assemblies. The effective thermal conductivity models affect the peak cladding temperature directly by influencing the temperature difference between the hottest cladding at the cask center and the walls that surround it. It also affects it indirectly by influencing the center basket wall temperature. The fuel assembly heat generation rates that cause the peak cladding temperature to reach the allowed limit were determined for both effective thermal conductivity models. At those generation rates the basket wall temperatures in the periphery of the package were highly nonuniform. The basket wall temperatures determined in this work will be used in future studies to develop improved thermal models of fuel assemblies.

The response of a truck package to a radiation fire model is simulated for a range of fire durations using three-dimensional finite element analysis. A model is developed to determine the cumulative seal degradation from its temperature-versus-time history. This model is used to determine the minimum fire duration that causes the seal to lose containment integrity. The fire durations that cause the cladding to reach its long-term creep deformation and burst rupture temperatures are determined and found to be longer than the durations that cause the seal to lose containment integrity. These simulations are repeated for package models without the compliant regions of the impact limiters, and for a package with the impact limiter completely removed. Those simulations quantify the level of thermal protection the impact limiters provide to the seals and cladding during simulated fires.

Heat transfer through a 1 mm gap between two concentric cylinders representing the gap between a fuel support basket and a canister is experimentally and numerically investigated. The objective of this work is to study rarefied gas heat transfer in a simple geometry, and to measure the thermal accommodation coefficient at the interface between stainless steel and rarefied helium. The thermal accommodation coefficient is used to characterize the interaction between gas molecules and wall at the molecular level. It is important to determine its value with precision for better determination of heat transfer at low pressure. The experimental procedure consists of measuring the temperature difference between the inner and outer cylinders as the pressure is decreased in the gap. By knowing the heat flux across the gap the thermal accommodation coefficient can be extracted from the theoretical expression relating the temperature difference to the radial heat flux. Three-dimensional simulations using the ANSYS/Fluent commercial code are conducted to assess on the design of the experimental apparatus. These simulations confirmed that the apparatus design is effective to study the heat transfer across rarefied gas and to determine the thermal accommodation coefficient for helium on stainless steel surface.Copyright © 2015 by ASME

This paper presents preliminary results of heat transfer simulations performed in geometrically-accurate-three-dimensional model of nuclear fuel canister filled with helium. The numerical model represents a vertical canister, which relies on natural convection as its primary heat transfer mechanism, containing 24 PWR fuel assemblies. The model includes distinct regions for the fuel pellets, cladding and gas regions within each basket opening. Symmetry boundary conditions are employed so that only one-eighth of the package cross-section is included. The canister is assumed to be filled with helium at atmospheric pressure. A constant temperature of 101.7°C is employed on the canister outer surfaces, assuming the canister to be surrounded with water. These conditions of pressure and temperature were considered, in this paper, for comparison purpose with previous work. The effects of buoyancy-induced gas motion and natural convection, along with radiation and conduction through gas regions and solid are considered. Steady state simulations using ANSYS/Fluent were performed for different heat generation rates in the fuel regions. Simulations that include the effect of natural convection and others that do not include this effect are conducted. The peak cladding temperature and its radial and axial locations are reported. The maximum allowable heat generation that brings the cladding temperatures to the radial hydride formation limit (TRH=400°C) is also reported. The results of the three dimensional model simulations were compared to two dimensional model simulations for the same heat generation rate. The results showed that the two-dimensional simulations overestimate the temperature in the canister by almost 70°C.Copyright © 2015 by ASME

Steady state heat transfer through a rarefied gas confined between two parallel plates or two coaxial cylinders maintained at different temperatures is investigated using the nonlinear S-model kinetic equation and the DSMC technique for a large range of gas rarefaction. The profiles of heat flux, density and temperature are reported for different values of gas rarefaction parameter and given values of temperature and aspect ratios. In the slip regime the results of the S-model and DSMC technique are compared to the simulations performed using the Lin and Willis temperature jump boundary conditions at the at the solid surface implemented in ANSYS/Fluent CFD simulations. The analytical expressions for density number, temperature and heat flux in the free molecular regimes are obtained for both parallel plates and coaxial cylinders geometries with hot and cold surfaces having different values of the thermal accommodation coefficient. The solutions of these analytical expressions are compared to the S-model kinetic equation and DSMC technique results in the free molecular regime.Copyright © 2015 by ASME

Risk assessment studies for hazardous material packages require fire response prediction tools that are both accurate and rapid. This article describes the theoretically based, semiempirical reaction chemistry and radiation heat transfer models for large, optically dense pool fires incorporated in the ISIS-3D CFD software. The chemistry model employs four separate reactions (two produce radiating soot). The heat transfer model divides the computational domain into the diffusely radiative fire and its nonparticipating environment. ISIS-3D simulations are performed on a 6-m square JP8 pool fire experiment in which the soot temperature and volume fraction are measured. The reaction rate and soot formation parameters of the chemistry model are determined based on a comparison of the simulation with the measured data. Simulations are then performed on an experiment that measures the temperature of a pipe calorimeter suspended over the leeside of a 19-m-diameter JP8 fuel pool fire with a 9.5 m/s crosswind. The ...

Federal regulations (10CFR71) require radioactive material transport packages to safely withstand a 30min fully engulfing fire. The three-dimensional Container Analysis Fire Environment (CAFE-3D) computer code was developed at Sandia National Laboratories to simulate the response of massive packages to large fires for design and risk studies. These studies require rapid and accurate estimates of the package temperature distribution for a variety of package designs and fire environments. To meet these needs CAFE-3D links a finite element model that calculates the package response to the Isis-3D CFD fire model. ISIS-3D combines computational fluid dynamics with reaction chemistry and thermal radiation models to rapidly estimate the heat transfer from a fire. In the current work, parameters used in the fire model were determined. Simulations were then performed of a test that modeled the conditions of a truck-sized nuclear waste package in a regulatory fire under light wind conditions. CAFE-3D underestimated the ability of the wind to tilt the fire and deliver oxygen to the region above the fuel pool. However, it accurately and rapidly estimated the total heat transfer to the test object. CAFE-3D will become a more useful tool for estimating the response of transport packages to large fires once it has been benchmarked against a larger range of fire conditions.

The effect of flow rate modulation on low Reynolds number heat transfer enhancement in a transversely grooved passage was numerically simulated using a two-dimensional spectral element technique. Simulations were performed at subcritical Reynolds numbers of Rem = 133 and 267, with 20% and 40% flow rate oscillations. The net pumping power required to modulate the flow was minimized as the forcing frequency approached the predicted natural frequency. However, mixing and heat transfer levels both increased as the natural frequency was approached. Oscillatory forcing in a grooved passage requires two orders of magnitude less pumping power than flat passage systems for the same heat transfer level. Hydrodynamic resonance appears to be an effective method of increasing heat transfer in low Reynolds number systems where pumping power is at a premium, such as micro heat transfer applications.

During the used nuclear fuel vacuum drying process, helium is evacuated to pressures as low as 70 Pa, to promote water vaporization and removal. At these low pressures the gas is rarefied to the extent that there is a temperature jump thermal resistance between the surface and gas. This occurs when the mean free path of a molecule becomes a comparable to the characteristic length of a system. In order to correctly apply this jump model to a nuclear transfer cask, a two dimensional model of parallel plates and concentric cylinders were created using ANSYS/Fluent package. Heat generation was plotted against a variety of relevant pressures. The results in these simple geometries are compared to kinetic model calculations, performed by other investigators, to determine the appropriate collision diameters to use in rarefied helium gas simulations within complex geometries. A two dimensional mesh of a transfer cask containing 24 pressurized water reactor used fuel assemblies is then constructed, and the rarefied gas model was implemented in the helium-filled regions between the fuel and basket support structures. Steady state simulations with a fuel heat generation rate of 710 W/m/assemble shows that the cladding is measurably hotter when the helium gas pressure is reduced from atmospheric conditions ∼105 Pa to 500 Pa. The heat generation rate that brings the peak cladding temperature to a hydride dissolution temperature of 400°C is as much as 10% lower when the gas is at 500 Pa than under atmospheric conditions.Copyright © 2014 by ASME

The objective of this work is to design an experimental apparatus that can acquire data to benchmark rarefied gas heat transfer simulations, and determine the thermal accommodation coefficient at the interface between the solid surfaces and the gas. The design consists of an aluminum cylinder with an electric heater at its centerline, and within a stainless-steel sheath, centered inside a cylindrical pressure vessel whose temperature is controlled using an external water jacket. There is 0.47-cm-wide helium-filled gap between the inner cylinder and vessel wall. For a given heat generation rate, the temperature difference across this gap will increase as the gas pressure decreases due to ratification. Thermocouples will be bonded to the vessel’s outer surface, and the inner surface of the sheath that surrounds the heated aluminum cylinder. Two, two-dimensional computational meshes of the apparatus (one cross sectional and the other cross sectional is offset) and one three-dimensional computational mesh are constructed. These models include heat generation within the electric heater, conduction within the solid and gas-filled regions, and radiation heat transfer across the gas, and rarefied gas thermal resistances at the solid/gas interfaces. These simulations show that the difference between the thermocouple temperatures and the surfaces of the helium filled gap are small compared to the temperature across the gap. This will allow this apparatus design to be used to effectively benchmark the ANSYS/Fluent simulations, and determine the thermal accommodation coefficient.Copyright © 2014 by ASME

In the current work, a geometrically-accurate two-dimensional model is developed of an isolated fuel assembly within isothermal compartment walls. Finite difference thermal simulations are performed to determine the cladding temperature for a range of compartment wall temperatures and assembly heat generation rates. The results for zero-heatgeneration-rate are used to determine a temperature-dependent effective thermal conductivity of the fuel region. The effective volumetric specific heat of the region is determined from a lumped capacity model. These effective properties are then applied to a twodimensional model of a legal weight truck cask with homogenized (smeared) fuel regions. Steady-state normal conditions of transport simulations are performed for a range of fuel heat generation rates. The generation rate that brings the zircaloy cladding to its radialhydride formation temperature, predicted by the homogenized model, is greater than that determined by simulations that employ an accurate-geometry fuel region model. Transient regulator fire accident simulations are then performed for a range of fire durations. The critical fire duration is defined as the minimum that brings the fuel cladding to its burst-rupture temperature. That duration is found to decrease as the fuel heat generation rate increases. The critical durations predicted by the homogenized fuel-region model are shorter than those predicted by the accurate-geometry model. [DOI: 10.1115/1.4026065]

Navier-Stokes simulations of three-dimensional flow and augmented convection in a flat passage downstream from a fully developed channel with symmetric, transverse grooves on two opposite walls were performed for 405 < Re < 764 using the spectral element technique. Unsteady flow that develops in the grooved region persists several groove-lengths into the flat passage, increasing both local heat transfer and pressure gradient relative to that in a steady flat passage. Moreover, the heat transfer for a given pumping power in the first three groove-lengths of the flat passage was even greater than the levels observed in a fully developed grooved passage.

Navier-Stokes simulations of three-dimensional flow and augmented convection in a channel with symmetric, transverse grooves on two opposite walls were performed for 180⩽Re⩽1600 using the spectral element technique. A series of flow transitions was observed as the Reynolds number was increased, from steady two-dimensional flow, to traveling two and three-dimensional wave structures, and finally to three-dimensional mixing. Three-dimensional simulations exhibited good agreement with local and spatially averaged Nusselt number and friction factor measurements over the range 800⩽Re⩽1600. [S0022-1481(00)00904-X]

Direct numerical simulations of three-dimensional flow and augmented convective heat transfer in a transversely grooved channel are presented for the Reynolds number range 140 < Re < 2000. These calculations employ the spectral element technique. Multiple flow transitions are documented as the Reynolds number increases, from steady two-dimensional flow through broad-banded unsteady three-dimensional mixing. Three-dimensional simulations correctly predict the Reynolds-number-independent friction factor behavior of this flow and quantify its heat transfer to within 16 percent of measured values. Two-dimensional simulations, however, incorrectly predict laminar-like friction factor and heat transfer behaviors.

The response of intact and damaged versions of a 125 ton Multi-Purpose Canister (MPC) rail package conceptual design to severe thermal events is simulated using finite element computer analysis. The ‘critical’ fire duration (the minimum duration that causes the fuel cladding temperature to reach 740°C) is calculated as a function of fire temperature and for different modelling assumptions. It is found that fire temperatures below 660°C cannot cause the cladding to reach 740°C, no matter how long they last. At higher fire temperatures, the critical duration versus fire temperature envelope is significantly affected by the presence of the external neutron shield (especially for fire temperatures > 720°C), and somewhat less affected by the assumed value for the fuel cladding temperature limit. It is insensitive to the fuel region thermal conductivity model. Due to the large mass of the MPC, significant safe margins exist between the conditions of the IAEA thermal event and all of the performance env...

Earlier experiments have shown that cutting transverse grooves into one surface of a rectangular cross-sectional passage stimulates flow instabilities that greatly enhance heat transfer/pumping power performance of air flows in the Reynolds number range 1000 < Re < 5000. In the current work, heat transfer, pressure, and velocity measurements in a flat passage downstream from a grooved region are used to study how the flow recovers once it is disturbed. The time-averaged and unsteady velocity profiles, as well as the heat transfer coefficient, are dramatically affected for up to 20 hydraulic diameters past the end of the grooved section. The recovery lengths for shear stress and pressure gradient are significantly shorter and decrease rapidly for Reynolds numbers greater than Re = 3000. As a result, a 5.4-hydraulic-diameter-long recovery region requires 44 percent less pumping power for a given heat transfer level than if grooving continued.

In the current work, a geometrically-accurate two-dimensional model is developed of an isolated fuel assembly within isothermal compartment walls. Finite difference thermal simulations are performed to determine the cladding temperature for a range of compartment wall temperatures and assembly heat generation rates. The results for zero heat generation rate are used to determine a temperature-dependent effective thermal conductivity of the fuel region. The effective volumetric specific heat of the region is determined from a lumped capacity model. These effective properties are then applied to a two-dimensional model of a legal weight truck cask with homogenized (smeared) fuel regions. Steady-state normal conditions of transport simulations are performed for a range of fuel heat generation rates. The generation rate that brings the zircaloy cladding to their radial hydride formation temperature predicted by the homogenized model is greater than that determined by an accurate geometry model. Transient regulator fire accident simulations are performed for a range of fire durations. The minimum fire durations that bring the fuel cladding to its burst rupture temperatures are estimated. These results are compared to simulations which employ cask models with geometrically-accurate fuel region models.

Two and three dimensional thermal models of a Nuclear Assurance Corporation Legal Weight Truck (NAC-LWT) cask were constructed using the PATRAN commercial finite element package. The two-dimensional model included the effect of radial stiffeners in the package’s external neutron shield but three-dimensional model did not. A normal conditions of transport (NCT) simulation using both models predicted the peak cladding temperature was roughly 210°C. The NCT package temperatures were used as initial conditions for transient fire/post-fire simulations. Different assumptions were used to determine when the neutron shield liquid drained from the tank and was replaced by air. When the liquid was assumed to remain within the tank during and after the fire, the peak cladding temperature was predicted to exhibit a temporal maximum of roughly 300°C, approximately 6 hours after the end of the fire. If the liquid drained from the tank during the fire, the cladding temperature did not exhibit a temporal peak. Rather, it eventually reached a maximum temperature of roughly 280°C, which is the steady state NCT peak temperature when air is in the neutron shield tank. This undergraduate project will be used to lay down a foundation for further research on NAC-LWT casks. Two and three dimension package of the cask will be constructed using ANSYS, and simulations will be run for NCT and fire/post-fire conditions. The models will also be linked to Container Analysis Fire Environment (CAFE) to predict response of the package in fire.

In the current work, two-dimensional spectral element simulations are used to investigate the heat transfer and fan power performance of the developing regions of finite-length, grooved channel passage arrays, including the accelerating and decelerating flows entering and exiting the arrays. The performance of the grooved channel arrays is compared with that of flat passage arrays with the same average wall center-tocenter spacing for Reynolds numbers ranging from 1000 to 3000. The simulations show that unsteadiness develops after a number of groove lengths and results in enhanced heat transfer. The unsteadiness improves the overall heat transfer compared with a flat passage array of equal average channel height by a factor of 1.46 at Re = 1000 and a factor of 2.75 at Re = 3000. The grooves also cause an increase in the required fan power by a factor of 8.56 at Re = 1000 and a factor of 18.10 at Re = 3000. Since past simulations have shown that threedimensional simulations are necessary to accurately predict heat transfer and fan power performance in transversely grooved passages, the current two-dimensional results will be used as a starting point for a three-dimensional model that will ultimately be used to predict heat transfer and friction factor performance in developing grooved channel flows.

Preliminary studies of used fuel generated in the US Department of Energy’s Advanced Fuel Cycle Initiative have indicated that current used fuel transport casks may be insufficient for the transportation of said fuel. This work considers transport of three 5-year-cooled oxide Advanced Burner Reactor used fuel assemblies with a burn-up of 160 MWD/kg. A transport cask designed to carry these assemblies is proposed. This design employs a 7-cm-thick lead gamma shield and a 20-cm-thick NS-4-FR composite neutron shield. The temperature profile within the cask, from its center to its exterior surface, is determined by two dimensional computational fluid dynamics simulations of conduction, convection, and radiation within the cask. Simulations are performed for a cask with a smooth external surface and various neutron shield thicknesses. Separate simulations are performed for a cask with a corrugated external surface and a neutron shield thickness that satisfies shielding constraints. Resulting temperature profiles indicate that a three-assembly cask with a smooth external surface will meet fuel cladding temperature requirements but will cause outer surface temperatures to exceed the regulatory limit. A cask with a corrugated external surface will not exceed the limits for both the fuel cladding and outer surface temperatures.

The temperature of spent nuclear fuel cladding within transport casks must be determined for both normal conditions of transport and hypothetical fire accident conditions to assure that it does not exceed certain limit conditions. In the current work a two-dimensional finite-element thermal model of a legal-weight truck cask is constructed that accurately models the geometry of the fuel rods and cover gas. Computational fluid dynamics (CFD) simulations are performed that include buoyancy induced motion in, and radiation and natural convection heat transfer across the cover gas, as well as conduction in all solid components. Separate simulations are performed using helium or nitrogen cover gas. Stagnant-gas CFD (SCFD) simulations are preformed and compared to CFD simulations to determine the effect of gas motion. For normal conditions of transport, the peak clad temperature is determined for a range of fuel heat generation rates to determine the thermal dissipation capacity based on peak cladding and surface temperature, QC and QS. These are respectively, the fuel heat generation rates that bring the peak cladding temperature to 400°C, or the peak surface temperature to 85°C (their allowed limits for normal transport). Transient fire/post fire simulations are then performed for a range of fire durations to determine the critical durations for cladding Creep Deformation or Burst Rupture, DCD or DBR . These are the fire durations that bring the cladding temperature to 570°C or 750°C, respectively. When the cladding temperature is used to select the fuel heat generation rate, the thermal dissipation capacity is 3265 W/assembly when helium is the cover gas, which is 30% higher when nitrogen is used (due to helium’s higher thermal conductivity). When nitrogen is the cover gas, the critical fire durations for creep deformation and burst rupture are, respectively, 3.3 and 7.2 hours. These durations are 18% and 14% shorter for helium (because the allowed fuel heat generation rate is higher for helium). When the fuel heat generation is chosen based on the package surface temperature, for helium, the thermal dissipation capacity is 1040 W/assembly, and the critical fire durations for creed deformation and burst rupture are, respectively, 4.7 and 11.6 hours. The values for nitrogen are all within 4% of these values. The CFD and SCFD simulations give essentially the same results. This indicates that gas motion does not significantly affect the cladding temperature, and the future calculations may not need to incur the increased computation expense required to model that motion.Copyright © 2010 by ASME

Experiments and computational fluid dynamics/radiation heat transfer simulations of an 8×8 array of heated rods within an aluminum enclosure are performed with nitrogen and helium as backfill gases in both horizontal and vertical orientations. This configuration represents a region inside the channel of a boiling water reactor fuel assembly between two consecutive spacer plates. The rods can be oriented horizontally or vertically to represent transport or storage conditions. The measured and simulated rod temperatures are compared for three different rod heat generation rates to assess the accuracy of the simulation technique. Simulations show that temperature gradients in the air are much steeper near the enclosure walls than they are near the center of the rod array. The measured temperatures of rods at symmetric locations are not identical, and the difference is larger for rods close to the wall than for those far from it. Small but uncontrolled deviations of the rod positions away from the design locations may cause these differences. The simulations reproduce the measured temperature profiles. For nitrogen experiment in horizontal orientation and a total rod heat generation rate of 500 W, the maximum rod-to-enclosure temperature difference is 138°C. The maximum measured heater rod and enclosure wall temperatures 375°C and 280°C, are measured in 2-inch insulated, nitrogen backfill vertical experiment for 1 atm internal pressure. Linear regression shows that the simulations slightly but systematically under predict the hotter rod temperatures but accurately predict the cooler ones. For all rod locations, heat generation rates, nitrogen and helium backfill gases, and apparatus orientations, 95% of the simulated temperatures are within 11°C of the correlation values. These results can be used to assess the accuracy of using simulations to design spent nuclear fuel transport and storage systems.Copyright © 2010 by ASME