Fuel Assembly Void Reduction via Partial Rod Placement
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Solution Overview
Problem
In boiling-water nuclear reactors, fuel assemblies with low enrichment of plutonium face challenges in maintaining reactor shutdown margin and enhancing the worth of burnable poisons due to reduced neutron absorption effects and increased void reactivity coefficients, which can lead to reduced safety margins during reactor operation and shutdown.
Innovation Solution
A fuel assembly configuration is introduced where full-length fuel rods without burnable poisons, partial-length fuel rods without burnable poisons, and fuel rods with burnable poisons are arranged within the channel box, with two partial-length fuel rods positioned in the outermost periphery to reduce void fraction and enhance neutron moderation, thereby increasing the worth of burnable poisons and improving reactor shutdown margin.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If fuel assemblies with low enrichment of plutonium are used, then cost is reduced and enrichment is simplified, but neutron absorption effect decreases and void reactivity coefficient increases
Solution Approach 1:
The patent applies local quality by positioning specific fuel rod types (those with burnable poisons) in the outermost periphery of the fuel assembly where they can locally enhance neutron absorption in regions with higher void fractions. This localized arrangement optimizes the neutron economy without requiring high enrichment across the entire assembly.
Solution Approach 2:
The patent changes parameters by introducing burnable poisons (gadolinium or erbium) into specific fuel rods, which fundamentally alters the neutron absorption characteristics. This parameter change compensates for the reduced neutron absorption capacity of low-enrichment fuel, maintaining reactor shutdown margin while using cost-effective low-enrichment uranium.
2Reliability
If burnable poison concentration is increased to enhance neutron absorption, then reactor shutdown margin improves, but thermal conductivity of fuel rod decreases
Solution Approach 1:
The patent uses local quality by introducing burnable poisons only in specific fuel rods located in the outermost periphery, rather than uniformly throughout the entire fuel assembly. This localized approach enhances neutron absorption where most needed (in high void fraction regions) while minimizing the negative impact on overall thermal conductivity.
Solution Approach 2:
The patent employs porous materials by incorporating burnable poison particles (gadolinium or erbium) as discrete phases within the fuel matrix. This porous structure allows neutron absorption functionality while maintaining sufficient thermal conductivity pathways through the fuel rod, balancing both requirements.
3Reliability
If partial-length fuel rods are positioned in outermost periphery to reduce void fraction, then worth of burnable poisons increases, but fuel assembly complexity increases
Solution Approach 1:
The patent applies segmentation by dividing the fuel assembly into different radial zones with distinct fuel rod types: full-length fuel rods in the center, partial-length fuel rods in the outermost periphery, and intermediate fuel rods in between. This segmented arrangement optimizes neutron moderation and burnable poison effectiveness while maintaining manageable assembly complexity through systematic zonation.
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 significantly increases the worth of burnable poisons and enhances reactor shutdown margin by reducing void fraction and increasing thermal neutron generation, leading to improved safety and operational stability during reactor operation and shutdown.
Implementation Method 1
enhance neutron moderation, thereby increasing the worth of burnable poisons
Implementation Method 2
The burnable poison and the control rod absorb neutrons which are excessively generated by nuclear fission
Implementation Method 3
nuclear fuel material (for example, uranium oxide)... nuclear fission of the nuclear fuel material
Data Source
AI summary
A fuel assembly includes full length fuel rods which contain a plutonium fissile (Puf) but do not contain a burnable poison, full length fuel rods which contain the fissile uranium and the burnable poison, and partial length fuel rods which contain Puf but do not contain the burnable poison in a channel box. The plutonium enrichment is decreased in an order of the full length fuel rods. The concentration of the burnable poison of the full length fuel rod is higher than the concentration of the full length fuel rod. In each side of a rectangular outermost periphery adjacent to the inner surface of the channel box in a horizontal cross-sectional view of the fuel assembly, two partial length fuel rods are adjacently disposed, and the full length fuel rod containing the burnable poison is disposed to be adjacent to each partial length fuel rod.