Nuclear Reactor Spacer Grid Spring Direction Switch
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Solution Overview
Problem
Conventional spacer grids in nuclear fuel assemblies weaken the outer straps due to the presence of springs, leading to structural weakness and potential fuel rod bow, despite attempts to compensate with thicker straps and mechanical features.
Innovation Solution
The spacer grid design includes a switch point for spring direction that is not at the center, with intersecting straps having springs and dimples arranged to engage fuel rods, where the outer straps contain only dimples and no springs, enhancing structural strength and minimizing fuel rod bow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If springs are placed in outer straps to provide rigid foundation, then resistance to inward-acting forces is improved, but structural strength of outer straps deteriorates due to spring cutouts
Solution Approach 1:
The patent applies local quality by differentiating the spring placement strategy between outer and interior straps. Outer straps contain only dimples without springs, preserving their structural integrity, while interior straps contain both springs and dimples to provide fuel rod engagement. This localized differentiation resolves the contradiction by optimizing each region's properties for its specific function.
Solution Approach 2:
The patent inverts the conventional arrangement by placing springs in interior straps rather than outer straps. This inversion allows the outer straps to maintain their full structural strength while the interior straps provide the necessary spring engagement forces, effectively swapping the functional roles to resolve the strength-weakness contradiction.
2Strength
If thicker outer straps are used to compensate for weakness, then structural strength is improved, but spring rates become different from interior springs leading to fuel rod bow
Solution Approach 1:
The patent applies local quality by creating distinct functional zones: outer straps with only dimples for structural support, and interior straps with springs and dimples for fuel rod engagement. This eliminates the need for uniform thickening while maintaining both strength and alignment stability through localized property optimization.
3Strength
If spring cutouts are removed from outer straps, then structural strength and material integrity are improved, but the ability to engage fuel rods in outer row may be affected
Solution Approach 1:
The patent extracts the spring features from the outer straps, removing the source of structural weakness while preserving the dimple features for fuel rod engagement. This extraction resolves the contradiction by eliminating the harmful element (springs causing cutouts) while retaining the useful element (dimples for engagement).
Solution Approach 2:
The dimples in the outer straps serve multiple functions: they provide structural reinforcement through their geometry and maintain fuel rod engagement capability without requiring springs. This multi-functionality allows the outer straps to achieve both strength and reliability without the conflicting spring cutouts.
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
This design enhances the structural strength of the spacer grid by eliminating spring cutouts on the outer straps, reducing the risk of fuel rod bow and maintaining the material's integrity during lateral impacts.
Implementation Method 1
The springs are located at or near the mid-plane of the spacer grid, and are sized so that an interference condition exists when a fuel rod is inserted into the grid cell. This interference causes the springs to deflect backwards towards the cell walls on which they are located, preloading the fuel rod in two orthogonal directions against the opposing dimple pair and clamping it in position.
Data Source
Figure 1
Figure 2
Figure 3~5
AI summary
A spacer grid includes intersecting straps defining cells with springs and dimples arranged to hold fuel rods passing through the cells. The direction of the springs switches at a switch point in the spacer grid that is not at the center of the spacer grid. The intersecting straps may include a first set of mutually parallel straps including a first transition strap and a second set of mutually parallel straps including a second transition strap, with the second set intersecting the first set. The springs of the first set of mutually parallel straps face away from the first transition strap, and the springs of the second set of mutually parallel straps face away from the second transition strap. The outer straps in some embodiments include dimples but not springs.