Protective Grid Attachment for PWR Fuel Assemblies
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
In nuclear reactors, debris such as metal particles can cause fuel rod damage and intergranular cracking in protective grids due to differential thermal expansion between stainless steel nozzles and Alloy-718 grids, leading to fuel rod failures.
Innovation Solution
A spacer plate is introduced between the lower edge of the protective grid straps and the bottom nozzle, providing a small axial and lateral clearance to accommodate thermal expansion differences, allowing for a non-rigid connection that reduces stress and prevents cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the protective grid is rigidly connected to the bottom nozzle via a thimble screw, then the grid is securely attached and debris is effectively filtered, but differential thermal expansion causes stress buildup and intergranular cracking in the protective grid
Solution Approach 1:
The thimble screw connection is designed to allow relative movement between the protective grid and bottom nozzle during thermal expansion. The screw thread engagement permits axial displacement while maintaining radial constraint, transforming the rigid connection into a dynamic one that accommodates dimensional changes without generating excessive stress.
Solution Approach 2:
The connection parameters are modified to allow for thermal expansion by designing the thimble screw with specific clearance and engagement characteristics. The screw length, thread pitch, and engagement depth are optimized to maintain secure attachment at operating temperatures while preventing stress concentration that would lead to intergranular cracking.
2Strength
If the protective grid is spaced from the bottom nozzle to avoid contact with irregularities, then contact stress is reduced, but grid stability and debris filtration effectiveness are compromised
Solution Approach 1:
The thimble screw acts as an intermediary element between the protective grid and bottom nozzle, providing a controlled connection point that maintains grid stability while preventing direct contact between the grid straps and nozzle surface irregularities. This mediator allows the grid to be held in the optimal position for debris filtration without suffering from contact stress.
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 spacer plate design mitigates stress corrosion cracking and fuel rod failures by allowing for differential thermal expansion without substantial contact, maintaining grid integrity and preventing debris entrapment.
Implementation Method 1
differential thermal expansion between the bottom nozzle and the protective grid, since the bottom nozzle is made of stainless steel and the protective grid is made of alloy-718
Data Source
AI summary
A fuel assembly for a pressurized water reactor that has a protective grid attached to the bottom nozzle through a spacer insert captured between a control rod guide thimble end plug and the bottom nozzle. A thimble screw attaches the bottom nozzle to the control rod guide thimble end plug through a central opening in the spacer insert. The control rod guide thimble end plug is provided with a raised annular boss that encircles the thimble screw shank and rests against the upper surface of the bottom nozzle through the opening in the spacer insert. The opening in the spacer insert is large enough to provide both an axial and radial clearance between the spacer insert and the end plug to accommodate differences in thermal expansion.