Floating Spacer Grid for Fuel Assembly Stress Reduction
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Solution Overview
Problem
Conventional spacer grids in fuel assemblies face issues with stress growth, reduced fuel rod straightness, low lateral rigidity, and increased bending susceptibility under normal and accident conditions, particularly due to fixed or floating connections with control rod guide tubes.
Innovation Solution
A spacer grid design featuring two grille units in an opposing vertical configuration with a movable clamp unit between them, forming a floating connection, which reduces stress on the guide tube and enhances lateral rigidity, while also improving the strength and stability of the fuel assembly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a fixed connection is used between the spacer grid and control rod guide tube, then the spacer grid is securely connected, but the guide tube is subjected to large tensile stress causing accelerated growth and reduced fuel rod straightness
Solution Approach 1:
The spacer grid employs a floating connection that allows dynamic adjustment between the spacer grid and control rod guide tube. The spacer grid can axially slide along the guide tube within a range defined by axial limiting devices, enabling the system to adapt to thermal expansion and growth differences, thereby reducing tensile stress on the guide tube while maintaining secure connection.
2Reliability
If a floating connection is used between the spacer grid and control rod guide tube, then the stress on the guide tube is reduced, but the lateral rigidity of the fuel assembly is low causing easy bending under normal operation
Solution Approach 1:
The spacer grid incorporates localized reinforcement features including protruding mixing vanes that extend into grille cells and intersect with guide tubes. These localized structural elements provide lateral support and rigidity at critical positions while maintaining the overall floating connection that allows axial movement, thus achieving both stress reduction and lateral stability.
3Stress or pressure
If the spacer grid is allowed to axially slide along the guide tube, then the stress concentration is reduced, but the fuel assembly becomes more susceptible to bending under impact conditions
Solution Approach 1:
The spacer grid design includes axial limiting devices that define the sliding range between the spacer grid and control rod guide tube. These limiting devices act as pre-positioned mechanical stops that prevent excessive axial displacement during impact events, providing beforehand cushioning that protects the fuel assembly from bending while still allowing normal thermal expansion movement.
Data Source
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AI summary
A spacer grid and a fuel assembly, the spacer grid comprising two grille units (10) and a clamp unit (20); the two grille units (10) are disposed in an opposing vertical configuration and the clamp unit (20) is inserted between the grille units (10), and can move vertically relative to the grille units (10). The spacer grid of the fuel assembly can reduce stress on a guide tube, reduce the fuel assembly growth and ensure lateral rigidity of the fuel assembly, prevent bending of the fuel assembly and reduce difficulties in inserting a control rod downwards during normal operations, increase the strength of the spacer grid and stability of the coolant circulation channel under accident conditions.