Boiling Water Reactor Fuel Assembly with Segmented Water Channels
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Solution Overview
Problem
Nuclear boiling water reactors face challenges in achieving optimal reactivity during operation and shutdown conditions, as well as uniform fission power distribution and reduced pressure drop, due to variations in water density and steam formation affecting moderation and cooling efficiency.
Innovation Solution
A fuel assembly design featuring a combination of full-length and shorter fuel rods, with strategically positioned large water channels to create an overmoderated region at shutdown and enhanced moderation during operation, ensuring uniform fission power distribution and reduced pressure drop.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large amount of water is used for cooling fuel rods, then cooling efficiency is improved, but reactivity during operation is reduced due to over-moderation
Solution Approach 1:
The patent introduces water channels with different cross-sectional areas at different axial positions. The upper part has larger water channels providing over-moderation for shutdown margin, while the lower part has smaller water channels maintaining reactivity during operation. This local differentiation resolves the contradiction between cooling efficiency and reactivity.
Solution Approach 2:
The fuel assembly is divided into upper and lower regions with different water channel configurations. The segmentation allows each region to optimize for its specific function: upper region for shutdown margin and lower region for operational reactivity, thereby resolving the overall contradiction.
2Power
If water channels are positioned closer to the central axis, then moderation is improved, but fission power distribution becomes less uniform
Solution Approach 1:
Water channels are positioned at different radial distances from the central axis depending on axial location. In the lower region, channels are closer to the axis for better moderation, while in the upper region, channels are positioned to achieve uniform power distribution. This local quality differentiation resolves the contradiction.
3Stress or pressure
If the cross-sectional area of water channels is increased, then pressure drop is reduced, but moderation capability is decreased
Solution Approach 1:
The water channel system is segmented into upper and lower parts with different cross-sectional areas. The upper part has larger channels reducing pressure drop, while the lower part has smaller channels providing better moderation capability. This segmentation resolves the contradiction between pressure drop and moderation 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 design achieves improved cold shutdown margin, high reactivity during operation, and uniform fission power distribution across the fuel assembly, while reducing pressure drop and enhancing thermal-hydraulic stability.
Implementation Method 1
a cooling medium, usually water, flows up through the fuel assembly. This water fulfils several functions. It functions as a cooling medium for cooling the fuel rods such that they will not be overheated.
Implementation Method 2
The water also serves as a neutron moderator, i.e. the water slows down the neutrons to a lower speed. Thereby, the reactivity of the reactor is increased.
Implementation Method 3
Another requirement is that the cooling of the fuel rods is sufficient such that a so-called dry-out does not occur. The water functions in fact also as a neutron absorber.
Data Source
AI summary
A fuel assembly for a nuclear power boiling water reactor, including: a fuel channel defining a central fuel channel axis, fuel rods, each having a central fuel rod axis, at least 3 water channels for non-boiling water, each water channel having a central water channel axis and each water channel having a larger cross-sectional area than the cross-sectional area of (the average) fuel rod. The fuel rods comprise a first group of full length fuel rods and a second group of shorter fuel rods. The fuel assembly comprises at least 5 fuel rods which belong to said second group and which are positioned such that the central fuel rod axis of each of these at least 5 fuel rods is closer to the central fuel channel axis than any of the water channel axes of the water channels.