Thermal Actuator Shutdown Seal for Nuclear Reactor Coolant Leakage
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Solution Overview
Problem
In pressurized water nuclear power plants, reactor coolant pumps face challenges in maintaining minimal leakage at high temperatures and pressures, and existing seal assemblies lack a reliable backup mechanism to prevent coolant leakage during loss of power or makeup pumping capacity, which could lead to reactor core damage.
Innovation Solution
A thermally actuated shutdown seal with a split ring and a piston actuator that constricts against the shaft when temperature rises, using a thermally activated material to remove a spacer and block coolant leakage, and includes backup seals and a safety lock to ensure reliable operation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a mechanical seal assembly is used to seal the shaft at high pressure and temperature, then the seal can maintain controlled leakage during normal operation, but the seal assembly lacks a backup mechanism to prevent excessive leakage during loss of power or cooling failure
Solution Approach 1:
The shutdown seal is pre-positioned and held in an inactive state by a retaining mechanism during normal operation. Upon detection of abnormal conditions (power loss, temperature rise), the retaining mechanism releases, allowing the shutdown seal to automatically activate and constrict against the shaft, blocking the flow annulus and preventing excessive coolant leakage without requiring external power or control systems.
Solution Approach 2:
The shutdown seal assembly serves as a backup safety mechanism that is prepared in advance to compensate for potential failures of the primary mechanical seal. It provides a secondary barrier that activates when the primary seal fails due to loss of power or cooling, ensuring continued containment of reactor coolant and preventing core damage.
2Reliability
If the shutdown seal constricts against the shaft to block coolant leakage, then coolant containment is improved, but the normal controlled leakage path is restricted
Solution Approach 1:
The shutdown seal is designed to be dynamically configurable between two states: an inactive state during normal operation where the flow annulus remains open for controlled leakage, and an active state during abnormal conditions where the shutdown seal constricts against the shaft to block the flow annulus and prevent excessive leakage. This dynamic transition ensures both normal coolant circulation and emergency containment.
Solution Approach 2:
The shutdown seal is extracted as a separate, independent component from the primary mechanical seal assembly. It operates as a distinct backup mechanism that can be independently activated without interfering with the normal operation of the primary seal, allowing controlled leakage during normal conditions while providing the capability to block leakage during emergencies.
3Reliability
If a thermal actuator is used to activate the shutdown seal, then the activation response is automatic and reliable, but the actuator structure and thermal material add device complexity
Solution Approach 1:
The thermal actuator is designed to automatically detect abnormal conditions (such as temperature rise or loss of cooling) and trigger the shutdown seal activation without requiring external power, control systems, or human intervention. The actuator uses thermal expansion or phase change of a thermal material to mechanically drive the shutdown seal into the constricted position, providing self-powered, reliable automatic activation based on physical parameter changes.
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 solution effectively restricts coolant leakage through the shaft seal, providing a reliable backup mechanism to prevent core damage by sealing the shaft during power loss or temperature increases, ensuring containment of high-pressure reactor coolant.
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 in a direction to remove the spacer from between the confronting ends when the material rises above a preselected temperature.
Implementation Method 2
The split ring has confronting ends that are maintained in spaced relationship by a spacer when the shaft is rotating during normal operation. When the shaft slows or stops rotating and the temperature in the housing rises, the spacer is removed from the confronting ends of the split ring and the split ring constricts against the shaft as the confronting ends of the split ring approach each other, which blocks a substantial portion of the leakage of coolant through the annulus.
Data Source
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. Below a preselected temperature the piston is positively locked with a passive release when the preselected temperature is reached.