External Dashpot Tube for Control Rod Guide Stiffness
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Solution Overview
Problem
In nuclear reactors, the lack of stiffness in the dashpot region of thimble tubes leads to incomplete rod insertion events due to distortion, and existing solutions either compromise on stiffness or increase the number of parts and assembly steps.
Innovation Solution
A nuclear fuel assembly design featuring a guide tube with a reduced lower radius and an external dashpot tube around the lower portion, providing enhanced stiffness without adding internal diameter, and requiring fewer parts and assembly steps through a simplified installation process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the dashpot region of the thimble tube is made stiffer to prevent distortion and incomplete rod insertion, then the reliability of control rod insertion is improved, but the device complexity and number of parts increase
Solution Approach 1:
The external dashpot tube is nested around the lower portion of the guide tube, creating a tube-within-a-tube structure. This nested configuration provides enhanced stiffness to the dashpot region without requiring internal modifications to the guide tube, thereby improving reliability while adding only one external component rather than modifying the internal structure with multiple parts.
Solution Approach 2:
Instead of increasing stiffness by adding internal reinforcement elements within the guide tube, the solution adds an external tube surrounding the lower portion. This dimensional approach (moving from internal to external reinforcement) provides the necessary structural support in the dashpot region without complicating the internal guide tube structure, thus improving reliability with minimal increase in overall device complexity.
2Strength
If the dashpot region is reinforced with additional components to prevent distortion, then the strength and stiffness are improved, but the manufacturing and assembly complexity increase
Solution Approach 1:
The guide tube is divided into two functional segments: the upper portion maintaining the original diameter and the lower portion (dashpot region) with reduced diameter. The external dashpot tube is then applied only to the lower portion. This segmentation allows the reinforcement to be focused precisely where needed, simplifying both manufacturing (by limiting the reinforced zone) and assembly (by defining a specific installation region for the external tube).
Solution Approach 2:
The external dashpot tube is applied only to the lower portion of the guide tube where the dashpot function is needed, rather than reinforcing the entire guide tube. This local application of reinforcement provides the necessary stiffness exactly where distortion is most critical during control rod insertion, while leaving the upper portions simple and easy to manufacture and assemble.
3Strength
If the guide tube diameter is increased to accommodate reinforcement, then the stiffness is improved, but the thermal hydraulic performance deteriorates due to reduced flow area
Solution Approach 1:
The external dashpot tube is nested around the outside of the guide tube, rather than being placed inside. This external nesting configuration provides the necessary structural reinforcement and stiffness to the dashpot region without encroaching on the internal coolant flow area, thus improving strength while maintaining thermal hydraulic performance.
Solution Approach 2:
The reinforcement is moved from the internal dimension (which would reduce flow area) to the external dimension (which does not affect flow). By placing the external dashpot tube around the outside of the guide tube, the solution provides necessary stiffness without compromising the coolant flow path, effectively resolving the contradiction between structural strength and thermal hydraulic performance.
Data Source
AI summary
Methods of installing an external dashpot tube around a control rod guide tube in a nuclear reactor fuel assembly are disclosed herein. The nuclear reactor fuel assembly may include a top nozzle, a bottom nozzle, and a plurality of grids. The various methods may comprise inserting a guide tube into a skeleton of the nuclear reactor fuel assembly to a lower middle grid, the lower middle grid being second closest grid to the bottom nozzle of the plurality of grids. The various methods may also include installing an external dashpot tube over the guide tube after it has been inserted to the lower middle grid; inserting the guide tube with the installed external dashpot tube to the bottom nozzle; attaching the guide tube to the skeleton; and bulging the guide tube onto the external dashpot tube.