Ahti Suo-Anttila

Radiation Heat Transfer and Reaction Chemistry Models for Risk Assessment Compatible Fire Simulations

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 ...

Validation of the Isis-3D Computer Code for Simulating Large Pool Fires Under a Variety of Wind Conditions

The Isis -3D computational fluid dynamics/radiation heat transfer code was developed to simulate heat transfer from large fires. It models liquid fuel evaporation, fuel vapor and oxygen transport, chemical reaction and heat release, soot and intermediate species formation/destruction, diffuse radiation within the fire, and view factor radiation from the fire edge to nearby objects and the surroundings. Reaction rate and soot radiation parameters in Isis -3D have been selected based on experimental data. One-dimensional transient conduction modules calculate the response of simp le objects engulfed in and near the flames. In this work, Isis -3D calculations were performed to simulate the conditions of three experiments that measured the temperature response of a 4.66-m-diameter culvert pipe located at the leeward edge of 18.9-m and 9.45-m diameter pool fires in crosswinds with average speeds of 2.0, 4.6 and 9.5 m/s. The measured wind conditions were used to formulate time-dependent velocity boundary conditions for a rectangular Isis -3D domain with 16,500 nodes. Isis -3D accurately calculated characteristics of the time-dependent temperature distributions in all three experiments. Accelerated simulations were also performed in which the pipe specific heat was reduced compared to the measured value by a factor of four. This artificially increased the speed at which the pipe temperature rose and allowed the simulated fire duration to be reduced by a factor of four. A 700 sec fire with moderately unsteady wind conditions was accurately simulated in 10 hours on a 2.4 GHz LINUX workstation with 0.5 GB of RAM.

Measurements of Heat Transfer to a Massive Cylindrical Calorimeter Engulfed in a Circular Pool Fire

A large-scale experiment was performed to measure heat transfer to a massive cylindrical calorimeter engulfed in a 30 minute circular-pool fire. This test simulated the conditions of a truck-sized nuclear waste transport package in a severe fire. The calorimeter inner surface temperature and the flame environment emissive power were measured at several locutions us functions of time. An inverse heat conduction technique was used to estimate the net heat flux to the calorimeter. Tall porous fences surrounded the test facility to reduce the effect of wind on the fire. Outside the fences, 2.9 m/s winds blew across the calorimeter axis at the beginning of the test but decreased with time. The wind tilted and moved the fire so that the initial flame environment emissive power was substantially less on the windward side than the leeward side. The calorimeter became more uniformly engulfed as the winds decreased. The maximum heat flux to the calorimeter was 150 MW/m 2 on the leeward side at the beginning of the fire, and generally decreased with time. The local variations of calorimeter temperature and heat flux were closely related to the local flame environment emissive power.

Benchmark of a Fast-Running Computational Tool for Analysis of Massive Radioactive Material Packages in Fire Environments

Federal regulation.; (IOCFR71) require radioactive material transport packages to safely withstand a 30 min 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-SB 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-JD underestimated the ability of the wind to tilt the fire arid deliver oxygen to the region above lie fuel pool. However, it accurately and rapidly estimated the total treat 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 again-it a larger range of fire conditions.

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.

Fast Running Pool Fire Computer Code for Risk Assessment Calculations

The Isis -3D computational fluid dynamics/radiation heat transfer code was developed to simulate heat transfer in fires. It models liquid fuel evaporation, fuel vapor and oxygen transport, chemical reaction and heat release, soot and intermediate species formation/destruction, diffuse radiation within the fire, and view factor radiation from the fire edge to nearby objects and the surroundings. Reaction rate and soot radiation parameters in Isis -3D have been selected based on experimental data. One-dimensional transient conduction modules are used to calculate the response of simple objects engulfed in and near the flames. Moderate-resolution Isis -3D simulations (less than 60,000 nodes) are relatively fast running and may be used for transportation risk assessment studies. In this work, Isis -3D calculations were performed to simulate the conditions of an experiment that measured the temperature response of a 4.66-mdiameter culvert pipe located at the leeward edge of an 18.9-mdiameter pool fire in a 10-m/s crosswind. The nearby wind conditions were also measured, and the fire lasted for 11 minutes. The wind conditions were used to formulate timedependent velocity boundary conditions for a rectangular Isis 3D domain with 16,500 nodes. The specific heat used in the one-dimensional model of the culver pipe was reduced compared to the measured specific heat of the pipe steel by a factor of twelve. This artificially increased the speed at which the pipe temperature rose, such that a 2.4 GHz LINUX workstation with 0.5 GB of RAM completed a simulation of the fire in 3.5 hours. Simulations that employed the true pipe specific heat required twelve times the computational time with minimal improvements in the predicted heat transfer to the pipe. The parameters of the soot model were adjusted by comparing simulation results with the experimental data. Isis -3D simulations that employed the appropriate parameters accurately reproduced both the total heat transfer to the pipe and the local temperature distributions.

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

Development of an Active Detector for the Characterization of the Late-Time Radiation Environment from a Reactor Pulse

This paper discusses the use of a commercially available 235U fission chamber, with a matching compensating ion chamber, originally sold as a single-ended detector with the signal conducted over the shield of a coaxial cable. The authors designed an aluminum housing that isolates the two detectors and converts the signals to full differential mode as a noise-reduction technique. The signals are processed using the switched resistor technique to extend the signal range to longer times from the peak of the pulse [Luker, S. M., Griffin, P. J., King, D. B., and Suo-Anttila, A. J., “Improved Diagnostics for Analysis of a Reactor Pulse Radiation Environment,” 13th International Symposium on Reactor Dosimetry, Akersloot, Netherlands, May 25, 2008, pp. 4–6.]. The newly configured fission chamber assembly has been used at the annular core research reactor at Sandia National Laboratories to provide a high-fidelity characterization of the neutron time profile from a pulsed operation.

Application of a Silicon Calorimeter in Fast Burst Reactor Environments

Frequently in experiments at fast burst reactors (FBRs), it is necessary to know the dose and peak dose rate absorbed by a material in terms of dose to silicon. The dose to silicon at a given point in an irradiation cannot be reliably measured by a passive dosimeter retrieved at late times from a mixed field environment, so we rely on the silicon calorimeter as the true standard. A silicon calorimeter has been developed for applications in a water-moderated pulsed reactor. In this paper, the authors investigate the application of this silicon calorimeter in an FBR environment. Tests have been conducted at the White Sands Missile Range (WSMR) FBR, also known as MoLLY-G, to develop techniques to use this silicon calorimeter for a measure of rad(Si) during and soon after a pulsed operation. This calorimeter can be coupled with the response of a diamond photoconductive detector (PCD) in order to derive a dose rate monitor suitable for application during an FBR operation [1].