Nuclear Fuel Assembly Hold Down Spring Design
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Solution Overview
Problem
Conventional hold down spring assemblies in nuclear reactor fuel assemblies are prone to stress corrosion cracking and lose resiliency over extended fuel cycles, failing to effectively counteract hydraulic forces and accommodate thermal expansion.
Innovation Solution
A hold down spring assembly with a primary spring member and secondary springs, where the primary spring has a straight leg attached at an acute angle and an arcuate transition portion, and the secondary springs interact with the transition portion to resist downward movement, with both springs having flat legs and radiused sections to distribute strain evenly.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hold down spring assemblies are used, then the fuel assembly can be restrained, but the springs are prone to stress corrosion cracking and lose resiliency over extended fuel cycles
Solution Approach 1:
The patent changes the geometric parameters of the spring assembly, specifically using a straight leg configuration with an acute angle attachment instead of conventional curved configurations. This parameter change reduces stress concentration factors and distributes strain more evenly, reducing maximum strain by approximately 29% and thereby improving resistance to stress corrosion cracking while maintaining service life
Solution Approach 2:
The patent applies radiused sections at critical stress points of the spring assembly. By introducing controlled curvature at the attachment points and transition zones, the design smooths stress transitions and eliminates sharp corners that would act as stress concentration sites, thereby preventing stress corrosion cracking initiation while maintaining the spring's restraining function over extended fuel cycles
2Force
If conventional hold down spring assemblies are used, then the fuel assembly can be restrained, but they fail to effectively counteract hydraulic forces and accommodate thermal expansion
Solution Approach 1:
The spring assembly is designed with straight leg portions that can deflect elastically in response to varying hydraulic forces and thermal expansion conditions. The acute angle attachment and radiused sections provide controlled flexibility, allowing the spring to dynamically adjust its restraining force to counteract hydraulic uplift while simultaneously accommodating dimensional changes due to thermal expansion during fuel cycle operation
Solution Approach 2:
The spring assembly is divided into distinct functional segments: straight leg portions for force transmission, radiused sections for stress distribution and flexibility, and attachment points for structural connection. This segmentation allows each portion to perform its specific function optimally, with the straight legs providing rigid force counteraction and the radiused portions providing adaptive movement for thermal expansion accommodation
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 new design reduces maximum strain by approximately 29% and enhances resistance to stress corrosion cracking, maintaining resiliency and effectiveness over extended fuel cycles.
Implementation Method 1
The primary spring member is oriented on the top end fitting so that the transition portion is at the vertical highest elevation, whereby movement of the end fitting and an upper plate of the reactor that the nuclear fuel assembly is designed to operate in, relatively towards each other, primarily loads the transition portion and deflects the first leg portion about the attachment to the end fitting
Data Source
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AI summary
A nuclear fuel assembly having a plurality of multi-leaf hold down spring sets extending from a top nozzle. Each spring set consists of a multiple number of springs leafs in order to provide a large working range of spring deflection. Each spring leaf has a straight, flat base section followed by a straight, flat tapered beam with a secondary spring set having a curvature at its peripheral end.