Nuclear Reactor Downcomer Flow Deflector Thermal Expansion Gap
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Solution Overview
Problem
The existing weld design for the flow deflector to core barrel interface in pressurized water reactors experiences high stresses and fatigue due to differential thermal expansion during direct vessel injection transients, which is exacerbated by increased transient occurrences in new reactor designs like the AP1000.
Innovation Solution
A new flow deflector design with a machined relief and protrusion affixed to the internal support structure, providing a gap and full penetration weld with a reinforcing weld, allows for flexibility and accommodation of thermal differential expansion between the flow deflector and core barrel.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a solid block flow deflector is used with a conventional weld design, then the structural integrity is maintained, but high stresses and fatigue occur due to differential thermal expansion during direct vessel injection transients
Solution Approach 1:
The flow deflector is segmented into a main body and a separate protrusion portion. The protrusion is affixed to the internal support structure with a weld, while the main body remains separate. This segmentation allows the main body to move freely with thermal expansion while the protrusion maintains a controlled connection to the support structure, reducing stress concentration at the weld joint.
Solution Approach 2:
The protrusion is designed with specific dimensional characteristics (length, width, height) that create a localized flexible connection zone. The gap between the protrusion and the internal support structure allows for controlled movement and stress distribution, while the weld area maintains sufficient structural integrity. This local quality optimization resolves the contradiction between strength and fatigue resistance.
2Stability of the object's composition
If the flow deflector is rigidly attached to the core barrel, then structural stability is maintained, but thermal strain and differential expansion cause high stresses in the weld
Solution Approach 1:
The connection between the flow deflector and internal support structure is made dynamic rather than rigid. The protrusion with the gap allows the flow deflector to move and adjust its position in response to thermal expansion and contraction. This dynamic connection accommodates dimensional changes while maintaining structural stability during normal operation.
Solution Approach 2:
The dimensional parameters of the protrusion (length, width, height) are specifically designed to provide appropriate flexibility. The gap dimension and protrusion size are optimized to allow controlled movement that accommodates thermal strain while maintaining sufficient structural stability. This parameter optimization resolves the contradiction between stability and stress.
3Reliability
If direct vessel injection transients occur frequently, then reactor safety is improved, but the weld joint experiences increased fatigue and stress
Solution Approach 1:
The protrusion with the gap is designed beforehand to cushion and absorb the stresses generated during direct vessel injection transients. This pre-designed flexible connection prevents stress concentration at the weld joint, protecting the weld from fatigue damage even when transients occur frequently. The cushioning effect is built into the structure before operation begins.
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 improved design enhances the weld joint's ability to manage thermal strain and relative expansion, reducing stress and fatigue, thereby extending the fatigue life of the fillet weld during direct vessel injection transients.
Implementation Method 1
differential thermal expansion during direct vessel injection transients
Implementation Method 2
manage thermal strain and relative expansion
Data Source
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AI summary
A nuclear reactor having a coolant flow deflector secured to a reactor core barrel in line with a coolant inlet nozzle. The flow deflector redirects incoming coolant down an annulus between the core barrel and the reactor vessel. The deflector has a main body with a front side facing the fluid inlet nozzle and a rear side facing the core barrel. The rear side of the main body has at least one protrusion secured to the core barrel so that a gap exists between the rear side of the main body adjacent the protrusion and the core barrel. Preferably, the protrusion is a relief that circumscribes the rear side of the main body.