Thermal Actuator Pump Seal for Nuclear Reactor Leakage
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Solution Overview
Problem
In pressurized water nuclear power plants, the reactor coolant pump shaft seal assemblies face challenges in maintaining high system pressure without excessive leakage, especially when temperatures rise due to loss of cooling, which can lead to reactor coolant leakage and potential core damage.
Innovation Solution
A thermally actuated shutdown seal with a split ring and a piston actuator that constricts to narrow the leakage annulus when the shaft slows or stops, using a temperature-responsive material to remove a spacer and block coolant leakage, and a pliable polymer seal ring to enhance sealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If a mechanical seal assembly is used to contain high system pressure, then pressure containment is improved, but leakage increases when temperature rises due to loss of cooling
Solution Approach 1:
The shutdown seal assembly is designed to automatically activate in response to temperature increase without requiring external control systems. The thermal expansion of components or phase change of thermal actuator material directly triggers the sealing action, allowing the system to self-protect against leakage when cooling is lost.
Solution Approach 2:
The shutdown seal assembly provides a backup sealing mechanism that activates beforehand to prevent catastrophic leakage. By having a redundant seal that engages when temperature rises, the system cushions against the harmful effect of coolant loss before it can lead to core damage.
2Ease of operation
If standard seal assemblies are relied upon for sealing, then normal operation is maintained, but reliability fails when power is lost and makeup pumping cannot occur
Solution Approach 1:
The shutdown seal assembly operates autonomously without requiring external power or control systems. When temperature increases indicate cooling loss, the thermal actuator automatically triggers the sealing action, ensuring reliability failsafe operation independent of power availability.
Solution Approach 2:
The seal assembly transitions from a normal operating state to an emergency sealing state through parameter changes triggered by temperature increase. The thermal actuator converts temperature parameter changes into mechanical displacement, closing the shutdown seal to prevent leakage when power is lost.
3Productivity
If the shaft seal allows controlled leakage during operation, then pressure is maintained, but leakage becomes excessive when temperature rises
Solution Approach 1:
The sealing system is segmented into two independent pathways: the primary seal for controlled leakage during normal operation and the shutdown seal for emergency sealing. This segmentation allows each seal to perform its specific function without interfering with the other, preventing excessive leakage when temperature rises.
Solution Approach 2:
The shutdown seal assembly dynamically transitions from an open state during normal operation to a closed state when temperature increases. The thermal actuator enables this dynamic response, allowing the system to adapt sealing characteristics based on operating conditions and prevent excessive leakage.
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
Effectively restricts coolant leakage through the shaft seal, providing a backup mechanism to prevent reactor coolant from escaping, even in the absence of power or cooling, thus ensuring the integrity of the reactor core.
Implementation Method 1
A material occupies at least a portion of the space within the cavity. The material expands upon an increase in temperature to exert a force on the piston that causes the piston to move
Implementation Method 2
The actuator is a phase change material actuator with a phase change material that undergoes a phase change at a predetermined temperature
Data Source
Figure 1~2
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AI summary
A thermal actuator for a rotating shaft shutdown seal that has a piston with a portion of its axial length enclosed within a chamber shell with a material that expands upon a rise in temperature. The portion of the actual length of the piston within the chamber has at least two different diameters with the larger diameter leading in the direction of travel of the piston. Upon a rise in temperature, expansion of the material surrounding the piston within the chamber creates a force on the piston in the desired direction of travel.