Pivot Dimpled Grids for Nuclear Fuel Rod Protection
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Solution Overview
Problem
Conventional nuclear reactor fuel assembly grids face challenges in maintaining adequate rod support and coolant mixing while minimizing fuel rod scratching and galling, due to dimensional changes and vibrations during irradiation, which can lead to corrosion fretting and heat transfer inefficiencies.
Innovation Solution
The improved grid design features space, parallel, elongated straps in an egg-crate pattern with 'dog bone' shaped cutouts and radius-coined edges to reduce stiffness and contact stresses, allowing for smoother fuel rod loading and reduced wear, while promoting coolant mixing and heat transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional rigid dimples and springs are used in grid straps, then fuel rod support strength is improved, but fuel rod scratching and galling occur due to high contact stresses during vibration and dimensional changes
Solution Approach 1:
The patent changes the geometric parameters of the dimple by introducing a dog-bone shaped cutout that removes material from the dimple base. This reduces the dimple's stiffness and contact stress while maintaining its support function, directly addressing the contradiction between support strength and rod surface damage
Solution Approach 2:
The patent applies radius coining to create curved, rounded edges on the dimple contact surfaces instead of sharp or flat edges. This curvature distributes contact stresses more evenly and reduces stress concentration points that would otherwise cause scratching and galling of the fuel rod cladding
2Stability of the object's composition
If stiff dimples are used to maintain rod positioning, then rod support stability is improved, but corrosion fretting occurs due to continuous vibration and relative motion between rod and support surfaces
Solution Approach 1:
The dog-bone cutout modifies the dimple's structural parameters to reduce stiffness, allowing the dimple to flex with vibrations rather than resisting them rigidly. This reduces relative motion between the rod and support surface, minimizing corrosion fretting while maintaining adequate positioning stability
3Ease of manufacture
If traditional dimple designs are used, then manufacturing simplicity is maintained, but rod support adequacy deteriorates due to rapid spring force relaxation and dimensional changes during irradiation
Solution Approach 1:
The dog-bone cutout is designed with specific geometric parameters that can be integrated into existing stamping and forming processes. The shape allows for controlled material removal that adjusts the dimple's mechanical properties without requiring fundamentally new manufacturing equipment or processes
4Manufacturing precision
If abrupt dimple edges are used, then manufacturing precision is easier to achieve, but fuel rod loading causes scratching due to high contact stresses during insertion
Solution Approach 1:
Radius coining creates smoothly curved edges on the dimple contact surfaces. This spherical/curved geometry eliminates sharp edges that would act as stress concentrators and scratching points during rod insertion, while the curvature can be achieved through standard forming operations
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 design enhances fuel rod support and coolant mixing, reduces the risk of scratching and galling, and minimizes manufacturing costs by using a softer dimple design that pivots during loading, maintaining effective heat transfer and pressure drop balance.
Implementation Method 1
radius-coined edges to reduce stiffness and contact stresses, allowing for smoother fuel rod loading and reduced wear
Implementation Method 2
Under the effect of axial flow and crossflow induced by thermal and pressure gradients within the reactor and other flow disturbances, such as standing waves and eddies, the fuel rods, which are slender bodies, are continuously vibrating with relatively small amplitudes
Implementation Method 3
The grids, as is known in the relevant art, are used to precisely maintain the spacing and support between the fuel rods in the reactor core, provide lateral support for the fuel rods and induce mixing of the coolant
Data Source
AI summary
A soft pivot dimple nuclear fuel assembly grid that utilizes a “dog bone” shaped window cutout and radius coining of edges perpendicular to coolant flow, to reduce the susceptibility of fuel rod leaking during the reactor operation. Radius coining allows the fuel rod to smoothly transition over the radiused edge to the flat rod contact section of the dimple. The symmetric “dog bone” shape enables the dimple to pivot during rod loading resulting in improved alignment between the dimple and the fuel rod, thereby minimizing scratching. The “dog bone” shape also allows for a large contact area dimple to be softer than a typical dimple which reduces contact stresses and fretting wear during reactor operations.