Releasable Fasteners for BWR Fuel Assembly Bottom End-Piece
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Solution Overview
Problem
In boiling water nuclear reactors, the upward force exerted by coolant on the bottom end-piece can cause it to lift off from the transition piece, especially under conditions of uprated core-power and core mass flow, leading to instability in the fuel assembly.
Innovation Solution
The nuclear fuel assembly incorporates releasable complementary fasteners between the bottom end-piece and the transition piece, allowing for secure attachment and easy detachment without damaging the components, using a 'bundle in basket' design with a centering and positioning device and leaf springs to counteract lift-off forces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the bottom end-piece rests freely on the transition piece, then the assembly is simple and easy to manufacture, but the bottom end-piece may lift off under upward coolant force
Solution Approach 1:
The clamp automatically engages with the fastening pin through elastic deformation when the bottom end-piece is positioned on the transition piece. The elastic element provides automatic retention without requiring additional fastening operations, maintaining manufacturing simplicity while ensuring reliable connection against upward coolant forces
Solution Approach 2:
The elastic element changes its physical state from deformed to recovered, utilizing elastic deformation to create a retaining force. This parameter change allows the clamp to transition from an open to a closed position, automatically securing the bottom end-piece to the transition piece while maintaining ease of assembly
2Reliability
If downward pressing forces are increased to prevent lift-off, then connection stability is improved, but device complexity and cost increase
Solution Approach 1:
The elastic clamp automatically generates the necessary retaining force through its elastic properties, eliminating the need for external downward pressing forces or complex restraint mechanisms. The system self-regulates to prevent lift-off without adding structural complexity
Solution Approach 2:
The connection system is divided into discrete functional elements: the fastening pin on the transition piece, the clamp on the bottom end-piece, and the elastic element providing the retaining force. This segmentation allows each component to perform its specific function simply, avoiding the need for complex integrated structures
3Reliability
If permanent fastening is used to prevent lift-off, then connection stability is improved, but ease of repair and maintenance deteriorate
Solution Approach 1:
The connection system transitions from a static permanent fastening to a dynamic reversible connection. The elastic clamp can automatically engage and disengage from the fastening pin, allowing the bottom end-piece to be easily removed and reinstalled while maintaining stable connection during operation
Solution Approach 2:
The clamp is pre-positioned on the bottom end-piece and the fastening pin is pre-positioned on the transition piece. When assembled, they automatically engage without requiring additional fastening actions, and can be easily separated by simply removing the bottom end-piece, facilitating maintenance while ensuring operational stability
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 solution reliably prevents the lifting-off of the bottom end-piece from the transition piece during reactor operation, simplifies assembly and maintenance, and reduces costs by eliminating the need for increased downward pressing forces, while ensuring stable operation and easy servicing.
Implementation Method 1
The clamp (34) is provided with an elastic element (46) which is deformable under the effect of a force applied to an outer surface (65) of the clamp (34) in a direction (A) substantially parallel to the longitudinal axis (A) of the clamp (34), from a closed position in which the clamp (34) retains the fastening pin (35) towards an open position in which the clamp (34) is disengaged from the fastening pin (35)
Data Source
Figure 1
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AI summary
The present invention relates to a nuclear fuel assembly (1) for a nuclear boiling water reactor comprising : - a bottom end-piece (3), a top end-piece (4), a bundle of fuel rods (5) extending along a longitudinal direction (L) between the bottom end-piece (3) and the top end-piece (4); - a fuel channel (10) surrounding the bundle of fuel rods (5) ; and - a transition piece (16) extending the fuel channel (10) longitudinally downwardly for fitting into a lower core plate (2) of the nuclear boiling water reactor, characterized in that the bottom end-piece (3) and the transition piece (16) comprise releasable complementary fasteners and are releasably fastened to one another through the releasable complementary fasteners.