Analyzing Debris Transport in a Nuclear Reactor Containment Building following a Loss of Coolant Accident
This application note, describing how FLOW-3D was used to model the performance of containment pools in nuclear facilities, was contributed by Tim Sande & Joe Tezak of Alion Science and Technology1.
In a pressurized water reactor nuclear power plant, the water circulated through the reactor core is enclosed in a primary piping system that is maintained at a pressure and temperature of roughly 2,080 psi and 585°F.
Due to the high water pressure, a break in the piping would result in the generation of multiple debris types within containment. This would occur due to insulation being blown off the equipment and piping in the area surrounding the break. Some typical examples of different types of debris that could be generated are shown (right).
Emergency Core Cooling System (ECCS)
Following the pipe break, the emergency core cooling system (ECCS) would be activated. Containment sprays would be turned on to lower the containment building pressure and remove radioactive material from the atmosphere. Water would be injected into the core to remove decay heat and prevent a meltdown. This water would subsequently spill out of the pipe break.
The water from both the containment spray and the decay heat removal is then pumped into containment by ECCS pumps from an outside tank. The volume of water pumped into containment via the spray and break flow collects on the containment floor and forms a pool.
Sump Strainers and Debris
After the water from the outside tank has been depleted, the suction to the ECCS pumps would be switched over to one or more sumps in the containment building (an example of two sump strainers is shown to the left). The function of the sump(s) is to recirculate water from the containment pool to the pump suction.
Each sump has a strainer system in place to prevent debris from being sucked into the ECCS pumps causing blockage or damage.
However, debris that accumulates on the strainer may cause head losses that exceed the net positive suction head (NPSH) required by the pumps, causing them to fail, and preventing a safe shutdown of the plant. This is the crux of the Nuclear Regulatory Commission’s Generic Safety Issue (GSI) 191.
FLOW-3D Applied to Evaluate Performance
FLOW-3D is used to model the containment pool and determine the quantity of debris that could arrive at the strainer(s).
The pipe break, direct spray areas (regions where the spray enters the pool like rain), and runoff spray area (regions where spray water runs off floors at a higher elevation and enters the pool like a waterfall) are modeled as regions populated with mass-momentum source particles, which are assigned an appropriate flow rate and velocity. The latter depends on the freefall distance to the pool surface. The strainer areas are modeled as mass sinks, which draw the water from the containment pool.
Tumbling and settling velocities
The model is run with a free surface to identify significant water level changes (due to sump suction or choke points in the pool), and the RNG model is activated to predict turbulence in the pool.
The ability of the destroyed insulation to travel through the containment pool is a function of its settling velocity (ability to travel in suspension) and tumbling velocity (ability to travel across the floor). The settling velocity correlates to the amount of kinetic energy needed in the pool to keep the piece of insulation in suspension. These settling and tumbling velocities are determined through flume and tank testing and are values calculated by the FLOW-3D model.
After the model reaches steady-state conditions, the FLOW-3D results are post-processed to determine the areas where the velocities are high enough to tumble the various debris types across the floor of the pool (shown in red) or areas where the turbulence is high enough to carry debris in suspension (shown in yellow).
The velocity vectors are then used in conjunction with the red and yellow areas to determine whether the flow would carry the debris toward the strainer(s). These areas are then compared to the initial debris distribution areas to determine the transport fractions for each type and size of debris.
By combining debris transport testing with CFD modeling, the debris loads that the ECCS strainers must be able to withstand can be significantly reduced from the overly conservative values that must otherwise be assumed. CFD has also proved to be valuable in identifying containment pool water level changes, flow patterns in the vicinity of the ECCS strainer to support head loss testing, and plant design modifications.
1Alion Science and Technology is a consulting engineering company with the ITS Operation comprised of engineering professionals skilled at developing and completing diverse projects vital to power plant operations. Alion ITSO provides engineering, program management, system integration, human-systems integration, design review, testing, and analysis for nuclear, electrical and mechanical systems, as well as environmental services. Alion ITSO has developed a meticulous Quality Assurance Program, which is compliant with 10CFR50 Appendix B, 10CFR21, ASME NQA-1, ANSI N45.2 and applicable daughter standards. Alion ITSO has provided a myriad of turnkey services to customers, delivering the highest levels of satisfaction for almost 15 years.
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