Thorium-Uranium Fuel Rods for Boiling Water Reactor Power Distribution
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Solution Overview
Problem
Fuel assemblies for nuclear boiling water reactors require burnable absorbers to control reactivity and achieve even power distribution, but this leads to internal power peaking and reactivity variations over time, necessitating varying uranium enrichment which complicates design and operation.
Innovation Solution
The fuel assembly varies the ratio of Thorium (Th) to Uranium (U) in different fuel rods, reducing the need for burnable absorbers and achieving even power distribution by transforming Th into fissile 233U, thereby minimizing reactivity differences between fuel assemblies over time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If burnable absorbers are used to control reactivity and achieve even power distribution, then power distribution uniformity is improved, but internal power peaking occurs and reactivity variations over time increase
Solution Approach 1:
The patent changes the chemical composition parameter by replacing burnable absorbers with Thorium, which transforms into fissile material over time. This parameter change resolves the contradiction by eliminating the harmful power peaking effect while maintaining power distribution uniformity through the in-situ generation of fissile 233U from Thorium-232 neutron capture.
2Reliability
If burnable absorbers are used to control reactivity, then initial reactivity control is improved, but reactivity differences between fuel assemblies over time increase
Solution Approach 1:
The patent applies the self-service principle by using Thorium-232 that automatically transforms into fissile 233U through neutron capture during reactor operation. This self-transforming property provides continuous reactivity compensation, eliminating the need for external burnable absorbers and maintaining stable reactivity characteristics throughout the fuel assembly lifecycle.
3Manufacturing precision
If varying uranium enrichment is used to compensate for burnable absorber effects, then power distribution is improved, but design and operation complexity increases
Solution Approach 1:
The patent applies homogeneity by using uniform uranium enrichment across all fuel assemblies and replacing the need for varied enrichment schemes with Thorium-containing fuel rods. This simplifies the design and operation while achieving power distribution uniformity through the consistent in-situ generation of 233U from Thorium, eliminating complex enrichment variation requirements.
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 approach reduces the need for burnable absorbers, achieves a more even power distribution, and simplifies uranium enrichment variations, leading to a more stable and efficient nuclear reactor core operation.
Implementation Method 1
transforming Th into fissile 233U
Implementation Method 2
The water also serves as a neutron moderator, i.e. the water slows down the neutrons to a lower speed
Implementation Method 3
the fuel rods are cooled mainly by water that is in a liquid phase
Implementation Method 4
the water that cools the fuel rods is evaporated such that the fuel rods are surrounded both by water in liquid phase and in a steam phase
Data Source
AI summary
The present invention concerns a fuel assembly for a nuclear power boiling water reactor. The fuel assembly comprises fuel rods. At least 95% of the fuel rods comprise nuclear fuel material in the form of U enriched in 235U. At least 20% of the fuel rods belong to a first set of fuel rods. The fuel rods in this first set comprise both U enriched in 235U and Th. The first set comprises a first and a second subset of fuel rods. The ratio, with regard to weight, between Th and U, in each fuel rod of said first subset, is higher than the ratio, with regard to weight, between Th and U, in each fuel rod of said second subset. The invention also concerns a nuclear power boiling water reactor and a manner of operating such a reactor.