RWST Standpipe Flow Staging for 72-Hour Passive Core Cooling
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Solution Overview
Problem
In a loss of coolant accident (LOCA), the refueling water storage tank (RWST) drains faster than expected, reducing the capacity to remove decay heat in the nuclear reactor core, falling short of the designed 72-hour operation duration.
Innovation Solution
Implementing a standpipe with multiple orifices or cross-connection pipes in the RWST to control the flow of water into the reactor pressure vessel, tailoring the flow rate over time to match the decay heat profile, using float valves or orifices at different elevations to maintain adequate water levels.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the RWST drains water rapidly to remove decay heat, then the heat removal efficiency is improved, but the operation duration falls short of the designed 72-hour period
Solution Approach 1:
The standpipe is divided into multiple segments with orifices at different elevations. Each segment activates at different water levels, providing staged flow control. This segmentation allows the system to maintain high flow rates when needed while extending the overall operation duration by progressively engaging different segments as the water level drops.
Solution Approach 2:
The system transitions from a static drain configuration to a dynamic one where the effective drainage area changes with water level. As the water level drops, different orifices become exposed or submerged, automatically adjusting the flow rate to match the declining heat generation from the reactor core, thereby extending operational duration while maintaining effectiveness.
2Device complexity
If the RWST uses a simple drain configuration, then the device complexity is reduced, but the flow rate cannot be tailored to match the decay heat profile
Solution Approach 1:
The standpipe with multiple orifices at different elevations provides self-regulating flow control based on water level. As the water level changes, different orifices are naturally exposed or submerged, automatically adjusting the drainage rate without requiring external control systems. This self-service mechanism achieves flow rate tailoring while minimizing device complexity.
Solution Approach 2:
Different sections of the standpipe have different local properties - orifices at different elevations provide different flow characteristics. The upper portions have different drainage capabilities than the lower portions, allowing the system to provide locally optimized flow control at different water levels, matching the decay heat profile at each stage.
3Measurement precision
If float valves are added to control flow through orifices, then the flow rate control precision is improved, but the device complexity increases
Solution Approach 1:
The float valve system operates autonomously based on water level position. The float automatically rises and falls with the water level, opening or closing orifices without requiring external control signals or complex actuation systems. This self-service operation achieves precise flow control while minimizing the addition of complex control infrastructure.
Solution Approach 2:
The float acts as an intermediary mechanism that translates water level position into flow control actions. Rather than directly controlling complex valve mechanisms, the simple float intermediary automatically opens or closes orifices based on its position, providing precise flow rate control with minimal added complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The tailored flow system ensures the reactor core remains immersed in water for the designed 72-hour period, effectively removing decay heat and preventing fuel rod damage.
Implementation Method 1
Depressurization valves allow gravity-driven flow (or injection) of water from the RWST into the reactor
Implementation Method 2
a float valve configured to regulate flow through one of the two or more orifices, the float valve including a float disposed in the standpipe
Implementation Method 3
Boiling heat transfer removes decay heat generated in the fuel assemblies and the resulting steam is subsequently vented through depressurization lines
Implementation Method 4
The required RWST volume can be computed based on the latent heat capacity of water (i.e., the amount of thermal energy that is removed per liter of liquid water converted to steam)
Data Source
AI summary
A nuclear reactor comprises a pressure vessel containing a nuclear reactor core. A reactor core cooling system comprises a standpipe including a plurality of orifices in fluid communication with a refueling water storage tank (RWST) to drain water from the RWST into the standpipe, and an injection line configured to drain water from the standpipe to the pressure vessel. In some embodiments the standpipe is disposed in the RWST, while in other embodiments the standpipe is disposed outside of the RWST and cross-connection pipes connect the plurality of orifices with the RWST. The reactor core cooling system may further comprise a valve configured to control flow through one orifice of the plurality of orifices in fluid communication with the RWST based on water level in the standpipe. The valve may comprise a float valve having its float disposed in the standpipe.