N. R. Chalasani

Benchmark of Computational Fluid Dynamics Simulations Using Temperatures Measured Within Enclosed Vertical and Horizontal Arrays of Heated Rods

Abstract Experiments and computational fluid dynamics/radiation heat transfer simulations of an 8 × 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.

Natural convection/radiation heat transfer simulations of enclosed array of vertical rods

Abstract Experiments performed by Arya and Keyhani (1990) measured the temperature of 12 vertical heated rods within a constant temperature, internally finned cylindrical enclosure. Measurements were performed with air and helium in the enclosure for ranges of rod heat generation rate and gas pressure. In the current work, steady three-dimensional computational fluid dynamics simulations of conduction, natural convection and radiation heat transfer within the experiment were conducted to benchmark the simulation techniques. In the computational model, different thermal conductivities were applied to a spacer plate between a plate that held the heaters, and one of the enclosure endplates. This was done to model a range of contact resistance between the plates. This was necessary because the experimental endplate conditions were not completely documented. The calculations accurately reproduced the local and average temperatures when a high contact resistance was modelled. These results emphasise that conditions far from data measurement locations can affect experimental results. Those conditions must be well documented if they are to be used to benchmark computational methods.

Thermal Protection Provided by Impact Limiters to a Containment Seal Within a Truck Package

The Container Analysis Fire Environment (CAFE) computer code is used to simulate the response of a generic legal-weight-truck package to 3 h fires. Simulations are performed with the package centered 1 m above a 7.2-m-diameter JP-8 fuel pool. They are also performed with the package horizontally offset from that location by I m and 2.5 m. Simulations without impact limiters are performed to quantify the level of thermal protection they provide. The minimum fire duration that causes the seal to reach its temperature of concern is determined for each configuration. When the center of the no-impact limiter package is within 2.5 m of the pool center, 0.7 h fires are capable of causing the seal to reach its temperature of concern. By contrast, the intact package protects the seal in fires that last more than 2 h. These results help risk analysts understand the effect of package position and the role of impact limiters on accident consequences.

Natural Convection/Radiation Heat Transfer Simulations of an Enclosed Array of Vertical Rods

Experiments performed by others measured the temperature of twelve heated vertical rods within a constant temperature, internally finned cylindrical enclosure. Measurements were performed for a range of air and helium pressures and a range of rod heat generation rates. In the current work, three-dimensional computational fluid dynamics simulations of natural convection and radiation heat transfer within this domain were conducted to benchmark the simulation techniques. These calculations accurately reproduced the local and average temperatures when the heat generation rate was sufficiently low that the velocity field is steady. Future simulations will be used to design experiments that model spent nuclear fuel within non-isothermal cells of storage packages.Copyright

Validation of Container Analysis Fire Environment (CAFE) Code for Memorial Tunnel Fire Ventilation Test Program

The Container Analysis Fire Environment (CAFE) computer code was developed at Sandia National Laboratories to predict the response of spent nuclear fuel (SNF) transport packages in large fires. CAFE’s fire model has been benchmarked using measurements from large, unconfined outdoor fires. In the current work CAFE simulations are benchmarked using data acquired in two fires from the Memorial Tunnel test series. The Memorial Tunnel, a decommissioned highway tunnel in West Virginia, is 850 m (2,800 ft) long, 4.38 m (14.3 ft) wide, and has a 3.2% slope. In both fires, the time-dependent air temperature and speed were measured at several locations throughout the tunnel during 50 MW fires. The first test used forced ventilation and the upper portal of the tunnel was sealed. Shortly after the fire started, air was forced into the tunnel at a location between the sealed portal and the fire, forcing the air flow toward the lower portal. The second test used natural ventilation, in that both portals were open and there was no forced flow. However, wind outside the tunnel appeared to cause a net flow inside, even before the fire started. While the Memorial Tunnel Fire test conditions and results were well documents, some details were not available to the current authors. This necessitated the used of some assumptions. CAFE simulations accurately reproduced many of the characteristics of the temporal and spatial variation of the measured air speed and temperature. The maximum simulated temperatures for the forced and naturally ventilated tests were, respectively, 26°F (14°C) and 201°F (94°C) below the corresponding measured values. This work will be used to assess the accuracy of CAFE in predicting the likely response of SNF packages in historic transportation tunnel fires.Copyright

Thermal Protection Provided by Impact Limiters to Containment Seal Within a Truck Package

The Container Analysis Fire Environment computer code is used to simulate the response of a truck package designed to transport one PWR fuel assembly to 7.2-m-diameter pool fires. Simulations are performed with the package centered over the fire, and offset axially from that location by 1 and 2.5 m. In all simulations the package body is 1 m above the fuel pool. The simulations predict the package containment seal exceeds its temperature of concern for all three package locations. Simulations of a no-impact-limiter version of the package are also performed to quantify the level of thermal protection provide by the limiter. The minimum fire duration that causes the seal to reach its temperature of concern is determined for each configuration. When the center of the no-impact limiter package is within 2.5 m of the pool center, fires shorter than 0.7 hour are capable of causing the seal to reach its temperature of concern. By contrast, the intact package protects the seal in fires that last roughly 2 hours. These results will help risk analysts better understand the effect of package position and the role of the impact limiters on accident consequences.Copyright

Validated Simulations of Heat Transfer From a Vertical Heated-Rod Array to a Helium-Filled Isothermal Enclosure

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. [DOI: 10.1115/1.4037493]

Benchmark of Computational Fluid Dynamics Simulations Using Temperatures Measured Within Enclosed Vertical and Horizontal Array of Heater Rods

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