Water Rod Termination for Boiling Water Reactor Coolant Flow
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Solution Overview
Problem
In boiling water reactors, the pressure drop through fuel bundle assemblies increases in upper elevations due to reduced coolant passage area, leading to steam constriction and inhibited coolant flow, which restricts the passage of coolant and moderator fluids.
Innovation Solution
The water rods in the fuel bundle assembly are terminated below the upper tie plate, increasing the coolant passage area and allowing the liquid moderator to continue serving as a moderator and coolant, reducing pressure drop by expanding the available area for steam flow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If water rods extend to the upper tie plate, then moderator flow to upper elevations is improved, but pressure drop increases due to reduced coolant passage area
Solution Approach 1:
The water rod is segmented into two functional sections: an upper section that extends below the upper tie plate to provide moderator flow, and a lower section that is terminated above the lower tie plate to maintain coolant passage area. This segmentation allows each section to optimize its specific function without compromising the other.
Solution Approach 2:
Different portions of the water rod are given different functional characteristics: the upper portion is optimized for moderator delivery to the enriched fuel section, while the lower portion is optimized for maintaining open coolant passages. This local differentiation resolves the contradiction between moderator flow and pressure drop.
2Strength
If water rods are attached to upper tie plate, then structural support is improved, but coolant passage area is reduced
Solution Approach 1:
The water rod is extracted from the upper tie plate attachment, removing the obstruction that blocked coolant passages. The water rod is supported alternatively by spacers positioned along its length, which provide sufficient structural support without interfering with coolant flow paths.
Solution Approach 2:
Spacers serve as intermediary support elements between the water rod and the upper tie plate. These spacers provide the necessary structural support while maintaining open passages for coolant flow, eliminating the need for direct attachment to the upper tie plate.
3Quantity of substance
If water rods extend throughout the assembly, then moderator distribution is improved, but fluid circulation is inhibited
Solution Approach 1:
The water rod configuration is optimized for dynamic fluid circulation patterns in natural circulation reactors. By terminating the water rod above the lower tie plate, the design accommodates the natural upward flow of coolant and steam, allowing fluid circulation to adapt to the thermal-hydraulic conditions without mechanical restriction.
Solution Approach 2:
The water rod length parameter is changed from extending to the upper tie plate to terminating above the lower tie plate. This parameter modification optimizes the balance between moderator distribution and fluid circulation by aligning the water rod discharge with the natural flow patterns in the reactor core.
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 configuration balances moderator flow to the upper elevations of the fuel rods with increased coolant passage area, reducing pressure drop and enhancing fluid circulation through the fuel bundle assembly, particularly beneficial for natural circulation reactors.
Implementation Method 1
The water rods provide additional liquid water moderator flow through the interior of the fuel assembly
Implementation Method 2
reducing pressure drop by expanding the available area for steam flow
Implementation Method 3
allowing the liquid moderator to continue serving as a moderator and coolant
Data Source
Figure 1
Figure 2
Figure 3~6
AI summary
A nuclear reactor fuel bundle assembly has been developed that includes: a fuel bundle including an array of fuel rods attached to a lower tie plate, an upper tie plate and housed in walls of a channel, and a water rod, having an upper discharge end below and unattached to the upper tie plate, and the upper discharge end having an unobstructed opening.