NUCLEAR ENERGY FACILITY USES SIMULATION SOFTWARE TO ENSURE INTEGRITY OF CRITICAL WATER STORAGE TANK DURING A TORNADO

Nuclear energy facilities, like Entergy Operations, Inc.'s Arkansas Nuclear One facility shown here, must design many of their systems and structures to withstand hypothetical natural disasters like tornadoes. Entergy Operations recently used ALGOR's Accupak/VE Mechanical Event Simulation software to recreate the motion, impact and large deformation experienced in a laboratory tornado impact test.

July 1, 1999, Pittsburgh, Pennsylvania -- Nuclear energy facilities must design many of their systems and structures to withstand hypothetical natural disasters such as earthquakes, floods and tornadoes. Tornadoes are part of the design basis at the Arkansas Nuclear One (ANO) facility located in Russellville, Arkansas, a state that experiences an average of 19 tornadoes every year. ANO is operated by one of the United States' top nuclear energy facility operators, Entergy Operations, Inc., based in Jackson, Mississippi.

In 1985, Entergy Operations was required to assess the structural integrity of an important new water storage tank in tornado conditions. This assessment required a laboratory test in which engineers used an air cannon to fire a scaled-down model that represented a 4,000-lb automobile, a possible type of heavy tornado debris at ANO, at a scaled-down tank. This unique test required the services of a testing laboratory with facilities that could accommodate such a test at a considerable expense.

Entergy Operations has learned that it can now recreate the motion, impact and large deformation experienced in the laboratory tornado impact test with ALGOR, Inc.'s Accupak/VE Mechanical Event Simulation software. After using Accupak/VE to reevaluate the water tank's structural integrity, the company plans to use the software to simulate additional nuclear facility design scenarios.

Water Storage Tank is Important to Nuclear Power Plant's Safe Operation

Entergy Operations is a leader in the U.S. nuclear industry based on its safety record and operating performance. Entergy Operations, Inc., a subsidiary of New Orleans-based Entergy Corporation, the nation's fourth largest electricity generator, operates its parent company's five U.S nuclear power plants.

One of Entergy Operation's safety features at its ANO facility is a 30-foot-tall, 42-foot-diameter, 321,000-gallon-capacity emergency condensate storage tank (below) that contains demineralized water. The stainless steel tank is the primary water source for the emergency feedwater system, which is employed if the main feedwater system becomes unavailable due to malfunction or loss of off-site power. The emergency feedwater system ultimately uses the water to cool the nuclear reactor if the plant must be shut down.

Water for Arkansas Nuclear One's emergency feedwater system is stored in this 30-foot-tall, 42-foot-diameter, 321,000-gallon-capacity stainless steel tank. The tank is located near the containment structure, which houses the nuclear reactor and steam generators. In 1998, Entergy Operations, Inc. reevaluated the tank with ALGOR's Accupak/VE Mechanical Event Simulation software to determine if storing only 118,000 gallons of water affected its structural integrity in tornado conditions.

If the condensate storage tank was unavailable to supply water for the emergency feedwater system, plant operators would use water from an emergency water source such as a nearby lake. They prefer to avoid using the lake water because it contains contaminants that would have adverse effects on the steam generator, reducing its life span. To ensure the availability of the tank's water, the tank was designed to withstand many hypothetical situations including tornadoes, which are common in Arkansas. Entergy Operations (Entergy) needed to determine if tornado conditions could cause it to rupture near its base and lose its water supply.

1985 Calculations and Laboratory Test Verify Water Tank's Safety

When the tank was first designed in 1985, engineers verified the tank's structural integrity in tornado conditions using hand calculations and laboratory testing. First, they performed calculations addressing the potential damage caused by relatively small, flying debris from a tornado. Such objects have low mass but high velocity in tornado winds and could penetrate the tank in the localized region of impact. Engineers considered the effects of a steel pipe and a piece of timber, which could be small tornado debris at ANO. They anticipated that these objects could rupture the tank under severe tornado conditions, so a five-foot-tall, 18-inch-thick concrete barrier was constructed around the base to block it from potentially penetrating debris. If ruptured above the five-foot-tall barrier, the tank would retain enough water to supply the steam generator while plant operators prepared to use water from an alternate source.

After determining the effects of small tornado debris on the tank, they wanted to determine the possible global damage caused by relatively large, flying tornado debris. Although such objects have a relatively lower velocity in tornado winds, their impact could still rupture the tank's base because of their large mass. Engineers considered the effect of a 4,000-lb automobile that represented the size and weight of potential large tornado debris.

Entergy determined that calculating the possible damage caused by a car would be difficult and, at that time, computer software that could accurately simulate its effects was not readily available. Given the impractical nature of firing a full-scale car at a full-sized condensate storage tank, Entergy used an air cannon to fire a scaled-down model of a car at a scaled-down tank filled with water.

Entergy referred to its safety analysis requirements to determine that the propelled object would travel as fast as 50 mph and contact the tank as high as 25 feet above ground in a hypothetical tornado event. In this laboratory test, Entergy found that the car model caused permanent deformation at the point of impact, but would not rupture the tank, thereby safeguarding the critical water supply.

Tank's Safety Reevaluated after 13 Years

In 1998, Entergy determined that lowering the tank's water from a full, 30-foot level to an 11-foot level storing 118,000 gallons of water would give plant operators greater flexibility during maintenance and inspection and would still provide enough water to effectively operate the emergency feedwater system. The tank's structural integrity during a tornado would now have to be reevaluated due to the possible adverse impact of the lower water level. Entergy engineers saw an opportunity to avoid expensive laboratory testing involving the air cannon by replicating the laboratory tornado impact test conditions on the computer with ALGOR's Accupak/VE Mechanical Event Simulation software.

Mechanical Event Simulation Software Chosen to Recreate Tornado Conditions

Entergy's 1985 laboratory test simulated a piece of heavy tornado debris impacting the condensate storage tank. On a computer, Accupak/VE Mechanical Event Simulation software can recreate complete events involving motion and its consequences, including inertial effects, impact, permanent deformation and residual stresses.

"Linear stress analysis was not an option. The results would have been too conservative and not representative of the actual laboratory test results," said Bill Hovis, P.E., a senior design engineer with Entergy's Mechanical/Civil/Structural Engineering Department who performed the computer analyses with ALGOR's Accupak/VE software. "Mechanical Event Simulation more accurately describes the event's behavior and I can see the progress of stress and large deformation at each instant in time over the course of the event."

Mechanical Event Simulation expands upon traditional finite element analysis (FEA) because it combines kinematics, rigid/flexible body dynamics and stress analysis. The software uses the laws of physics to simulate motion in mechanical events, eliminating the need for engineers to calculate and input force values, steps typically required in traditional FEA.

Hovis decided to first try replicating the laboratory tornado test involving a car model striking a full tank of water to see if ALGOR software's analysis results correlated with the 1985 laboratory test results. This would enable him to verify the accuracy of his analysis setup and use the software's results as a benchmark for the Mechanical Event Simulation involving the tank filled with only 118,000 gallons of water.

Tank and Car Modeled with ALGOR's Superdraw III

Working on a PC running Windows NT 4.0, Hovis first designed a 3-D model of the scaled-down condensate storage tank using Superdraw III, ALGOR's precision finite element model-building tool and single user interface that accesses all of ALGOR's modeling and analysis tools. The tank was drawn to the same proportions, 1:12.4, as the laboratory tank model.

Hovis generated a finite element mesh made up of plate elements to best represent the tank's geometry and specified their thickness as that of 28-gauge steel, or 0.0151 inches. The real tank's thickness is actually tiered, with the thickest area at its base, but Hovis applied the real tank's thinnest measurement (scaled 1:12.4) to the entire tank model, as they did in the laboratory test, to simplify the analysis. He realized that he could expect more conservative stress and deflection results in areas of the real tank that are thicker and therefore stronger. He used solid brick elements to represent stiffening rings located in the area where the tank's cylindrical body meets its concave dome ceiling.

To reduce analysis time, Hovis modeled a 90-degree cross-section of the tank because the laboratory test showed that the buckled region around the point of impact included 54 degrees of the tank's cylindrical body. He added 18 degrees on either side in anticipation of additional displacement due to the lowered water level and for the ease of modeling symmetrical boundary conditions along both edges to represent their attachment to the remainder of the tank. Using the same logic, he omitted the tank's floor because it is bolted to a concrete foundation and therefore not expected to be significantly affected by the car model's impact. He constrained the bottom portion of the cylinder that connects to the floor with translational boundary conditions.

Prior to the availability of ALGOR's Accupak/VE Mechanical Event Simulation software, Entergy was required to perform a laboratory tornado impact test by firing a scaled-down model of a car, which represented the size and weight of potential large tornado debris, at a scaled-down condensate storage tank in order to demonstrate the tank's endurance in a tornado impact condition. This image shows the car model and the 90-degree cross-section of the scaled-down tank model that were created in ALGOR's Superdraw III precision finite element model-building tool and single user interface. The models were drawn to the same proportions, 1:12.4, as the laboratory test models.

Hovis specified stainless steel material properties for the tank, which he obtained from the American Society of Metals source book on stainless steel, and used a nonlinear material model based on the von Mises yield criterion with simplified average values used to represent the strain hardening portion on the stress-strain curve. He also selected the Total Lagrangian analysis formulation because the von Mises material model was based on the steel's engineering stress-strain curve. The Total Lagrangian option is also ideal for analyses involving large strain, which Hovis expected based on the large deformation found in the laboratory-tested tank at the point of impact.

Continuing on with ALGOR's Superdraw III model-building tool, Hovis created a 3-D solid brick element model that was scaled to the same proportions as the car model used in the laboratory test. Laboratory test engineers had used plywood material for the model because they were not concerned about a real car's stress or deflection in this event; their measurements were irrelevant to the tank's structural integrity. For the same reason, Hovis used wood's material properties, obtained from a Beer and Johnson's Mechanics of Materials handbook, for the ALGOR model. Because the car model's weight was critical to obtaining accurate analysis results, he then used ALGOR's Weight, Center of Gravity and Mass Moment of Inertia utility to quickly verify his weight calculation for the car model.

Hovis used Accupak/VE's new contact elements, a feature that was introduced with ALGOR's latest Release 12 version of software, between the car and the tank models to simulate their interaction, including the transfer of inertia from the car to the tank.

In his first analysis iterations, the car model passed through or stuck to the tank wall. But because Hovis was able to view the event's behavior on his computer screen during processing, he immediately recognized the need to halt the analyses and adjust the setup. He found that, in addition to adjusting the timestep increment, he had to adjust the contact elements' stiffness, cross-section area and distance at which contact begins, Accupak/VE software's three contact element settings, to obtain an accurate contact scenario.

Software Analysis Eliminates Need for Laboratory Test

Hovis applied an acceleration load curve for gravity and adjusted the car model's originating position to account for its drop due to gravity so that it would strike the tank at the same height as it did in the laboratory test. This was the maximum height at which a car would travel in a hypothetical tornado event. The car's velocity was set to 51.4 mph, the same speed measured for the laboratory test car model at the point of impact. He applied two directional velocities in the X- and Y- directions in order for the car model to travel this speed at a 45-degree angle to the global X- and Y- horizontal axes, striking the tank normal to its surface.

This image shows a scaled drawing of the condensate storage tank and the car model. Bill Hovis, a senior design engineer at Entergy, used ALGOR's Mechanical Event Simulation software to replicate the laboratory tornado test in which the car model impacted a full tank of water. This enabled him to verify the accuracy of his analysis setup and use the software analysis results as a benchmark for the Mechanical Event Simulation involving the tank filled only to the 11-foot level. The results of the latter simulation determined that Entergy could maintain the tank at the lower, more advantageous water level without compromising its structural integrity.

To determine the tank's behavior over time, Hovis set the duration of the simulation to 0.10 seconds. Based on the high-speed camera that had recorded 298 frames per second in the 1985 laboratory test, Hovis initially set the timestep increment, the number of moments at which analysis information is recorded, to 298 timesteps per second, or 0.0034 seconds. This setting was unsatisfactory because the results did not display the necessary details around the time of impact and the analysis did not meet his specified convergence tolerance requirements, the number of iterations required for the software to obtain a converged solution to the problem. Hovis found that he had to decrease the timestep increment to 0.00056 seconds to eliminate these problems. He further reduced the duration of the event to 0.084 seconds to save analysis time.

"From my first Mechanical Event Simulation setup, I learned that the timestep size and tolerance level can have a significant effect on the software's analysis results. They are important to meeting convergence requirements and minimizing analysis time," explained Hovis.

In the initial Mechanical Event Simulation scenario, he specified a hydrostatic pressure along the entire tank wall to replicate the effect of a full water level. He could then determine if the ALGOR software analysis results correlated to those in the 1985 laboratory test of the full tank.

After running the analysis, Hovis compared peak displacements in the laboratory tank and the ALGOR tank model. The software analysis found large deflection at the point of impact that measured within one-quarter inch of the deflection recorded in the laboratory test, which was acceptable. Also, as in the laboratory test, the tank's base did not experience permanent deformation.

Strain gauges were not used in the laboratory test because engineers had only been concerned with rupturing, so Hovis could not compare the analysis stress results to stress in the laboratory tank. Instead he used ALGOR's stress results as a guide to determine if the tank behaved as expected. For example, he did not expect the tank's base to experience stress near its yield point of 28,000 PSI because the base did not experience permanent deformation in the laboratory test. ALGOR's stress results agreed with his expectations that were based on the laboratory test.

In this image, Bill Hovis, P.E., a senior design engineer at Entergy Operation Inc.'s Arkansas Nuclear One facility in Russellville, Arkansas, has simulated the 1985 laboratory tornado impact test involving the full condensate storage tank with ALGOR's Accupak/VE Mechanical Event Simulation software (top) and compared the results to those from the actual laboratory test (bottom). The software results correlated closely with the laboratory test results, verifying the accuracy of Hovis' analysis setup and enabling him to use the results as a benchmark for the Mechanical Event Simulation involving the tank filled with only 118,000 gallons of water. The latter simulation provided assurance that the tank could be maintained at a lower, more advantageous water level without compromising its structural integrity.

After successfully replicating the laboratory test involving the full water tank, Hovis conducted a second Mechanical Event Simulation scenario in which he applied a hydrostatic pressure along the tank wall representing the lower water level. Again the analysis results found large deflection at the point of impact, but no permanent deformation near the tank's base. The software analysis results also revealed that stress did not exceed the steel's yield point near the tank's base. These results proved that the condensate storage tank could withstand this tornado impact load while storing only 118,000 gallons of water.

"I created a visual replay of the event that enabled colleagues to see a complete reenactment of the car flying towards the tank, striking it, causing permanent deformation and bouncing away toward the ground," said Hovis. The replay could be reviewed with any application capable of viewing an .avi file, such as the Windows NT, 95 and 98 utility "Media Player".

Additional Software Analyses Find that the Tank Can Withstand Tornado Winds and Pressure

As a result of Hovis' success using Mechanical Event Simulation to predict the effects of a large piece of tornado debris on the tank with a lower water level, Hovis decided to analyze the effects of severe tornado winds and pressure on the tank with a lower water level. Entergy engineers had calculated the effects by hand on a full tank around the time of the laboratory tornado test in 1985, but the effects on a tank with only 118,000 gallons of water were unknown.

Hovis took the previously entered data for the air cannon test and applied new loading conditions to represent pressures the tank would experience during 360-mph tornado winds. These loading conditions were determined by hand calculations and computer analyses conducted in 1985. First, he applied internal and external pressure load cases to the tank's cylindrical shell that represented an atmospheric pressure drop. He then combined these with pressures to the dome ceiling that represented uplift caused by the tornado's wind speed. Each analysis used a timestep of one-quarter second and ramped the pressure load over the first few seconds. The analysis results found high, but acceptable stress values where the dome ceiling connects to the tank's cylindrical walls. These results proved that the condensate storage tank could withstand this type of tornado loading while storing only 118,000 gallons of water.

Entergy to Use Mechanical Event Simulation for More Design Scenarios

Using Accupak/VE Mechanical Event Simulation software, Entergy was able to provide assurance that its condensate storage tank could be maintained at a more advantageous water level without compromising its structural integrity. The company also saved a considerable expense by simulating the tornado conditions on the computer rather than in a laboratory. The company expects to use Mechanical Event Simulation more in the future as a way to address additional design scenarios.