Nuclear Fuel Assembly Bottom Grid Debris Filtration
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Solution Overview
Problem
Current debris filtering systems in nuclear reactors face challenges such as pressure drop issues, stress corrosion cracking, and fatigue due to flow-induced vibrations, particularly in P-grids, which are inadequate for advanced fuel designs and fail to effectively trap smaller debris without increasing pressure drop or addressing fretting damage to fuel rod cladding.
Innovation Solution
A debris filter grid with an egg-crate lattice pattern formed by orthogonally arranged, parallel straps, featuring protrusions and dimples that extend into fuel rod cells to support rods and trap debris, while reducing fretting and vibration damage, made from nickel-plated alloy 718 straps brazed together and heat-treated with a low-temperature anneal to prevent stress corrosion cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional P-grids are used for debris filtering, then debris is trapped, but stress corrosion cracking and fatigue occur due to high stresses during manufacture and operation
Solution Approach 1:
The patent changes the material parameters by using alloy 718 with nickel plating instead of traditional materials, and applies low-temperature anneal heat treatment to modify the microstructure and reduce susceptibility to stress corrosion cracking while maintaining debris filtration capability
Solution Approach 2:
The grid employs a composite structure combining alloy 718 base material with nickel plating coating, creating a multi-layer composite that provides both mechanical strength and corrosion resistance to prevent stress corrosion cracking while maintaining filtration function
2Productivity
If advanced fuel designs are implemented, then productivity and efficiency improve, but pressure drop across the bottom nozzle increases
Solution Approach 1:
The patent employs an egg-crate lattice structure with strategically designed open cells that create a porous-like flow path, allowing coolant to pass through with minimal resistance while maintaining effective debris trapping capability, thus reducing pressure drop for advanced fuel designs
3Reliability
If smaller debris is trapped more effectively, then reliability improves, but pressure drop across the fuel assembly increases
Solution Approach 1:
The patent applies local quality by creating protrusions and dimples only in specific locations where debris trapping is most needed, rather than uniformly across the entire grid surface. This localized approach effectively traps smaller debris while maintaining open flow paths in other areas, minimizing pressure drop increase
4Reliability
If current debris filtering systems are used, then debris is removed, but fretting damage occurs to fuel rod cladding
Solution Approach 1:
The patent introduces protrusions and dimples as intermediary features between the grid structure and fuel rods. These features act as mediators that capture and hold debris particles, preventing direct contact between debris and fuel rod cladding, thus eliminating the fretting damage mechanism while maintaining debris removal capability
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 solution effectively traps debris without increasing pressure drop, reduces fretting and vibration-induced damage, and addresses stress corrosion cracking, providing improved support and debris filtration while maintaining the structural integrity and height of the fuel assembly.
Implementation Method 1
made from nickel-plated alloy 718 straps brazed together and heat-treated with a low-temperature anneal to prevent stress corrosion cracking
Implementation Method 2
A spring extends inwardly into the cell from the one wall above the protrusions and the dimple is sized to be spaced from the fuel rod
Data Source
AI summary
A protector bottom grid for a nuclear fuel assembly that includes three laterally staggered and horizontally oriented protrusions that extend into the fuel rod cell of a support grid below a vertically oriented spring. The three staggered protrusions extend into the cell a distance that maintains a space between the protrusions and the fuel rod. The vertically oriented spring biases the fuel rod against a dimple extending from the opposite cell wall that is at an elevation just above the spring. The protrusions below the spring trap incoming debris in the area of the fuel rod end cap and protect the fuel rod cladding from fretting.