Monocrystal UO2 Fuel Pellets via Additive Recrystallization
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Solution Overview
Problem
Uranium dioxide (UO2) nuclear fuel pellets exhibit low heat conductivity, leading to significant temperature gradients and thermal stress, which limits their service life and increases the risk of fission gas release and cladding damage, compromising reactor safety and efficiency.
Innovation Solution
A method to prepare monocrystal UO2 nuclear fuel pellets by granulating and pelleting UO2 powder, coating with monocrystal growth additives, and activated-sintering to form a core-shell structure, promoting recrystallization and increasing crystal grain size, thereby enhancing heat conductivity and radiation stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If UO2 fuel pellets are used with conventional polycrystal structure, then the fuel can be manufactured with standard processes, but the heat conductivity is low (2.8 W/m·k at 1000°C) leading to large temperature gradients and thermal stress
Solution Approach 1:
The patent changes the crystal structure parameter from polycrystal to monocrystal, and controls crystal grain size to be greater than 100 μm (preferably 1-5 mm). This fundamental parameter change in the microstructure transforms the heat conduction mechanism, eliminating grain boundary scattering and achieving heat conductivity 2-5 times higher than conventional fuel, thereby resolving the contradiction between heat conductivity and fuel integrity
Solution Approach 2:
The patent uses composite powder comprising UO2 particles (3-10 μm) and crystal grain growth promoters (CaO, SrO, BaO, or their mixtures in specific ratios). This composite material approach allows the UO2 to form monocrystal structure during sintering while the promoters facilitate grain growth, achieving both high heat conductivity and structural reliability
2Temperature
If the crystal grain size of UO2 is increased to improve heat conductivity, then the heat conductivity increases, but the manufacturing process becomes more complex requiring controlled sintering conditions
Solution Approach 1:
The patent introduces crystal grain growth promoters (CaO, SrO, BaO) as intermediary substances that facilitate monocrystal formation and grain growth during sintering. These promoters act as mediators that lower the energy barrier for grain boundary migration and facilitate the transformation to large-grain monocrystal structure, simplifying the manufacturing process while achieving the desired crystal structure
Solution Approach 2:
The patent performs preliminary mixing of UO2 powder with crystal grain growth promoters before sintering, creating a pre-prepared composite powder with optimal composition and distribution. This preliminary action ensures uniform promoter distribution throughout the UO2 matrix, which facilitates controlled and uniform monocrystal formation during the subsequent sintering process, reducing manufacturing complexity
3Ease of manufacture
If conventional sintering is used to manufacture UO2 pellets, then the manufacturing process is simple, but the crystal grain size remains small leading to low heat conductivity
Solution Approach 1:
The patent applies specific sintering parameters: temperature of 1800-2200°C, holding time of 1-10 hours, and controlled atmosphere (vacuum or inert gas). These parameter changes transform the sintering process from a simple consolidation operation to a controlled crystal growth process, enabling monocrystal formation with grain sizes exceeding 100 μm while maintaining manufacturing feasibility
Solution Approach 2:
The use of composite powder containing UO2 and crystal grain growth promoters enables the sintering process to simultaneously achieve particle consolidation and monocrystal formation. The promoters facilitate grain boundary migration and crystal growth during sintering, allowing the production of large-grain monocrystal pellets through a relatively simple one-step sintering process rather than requiring complex multi-stage processing
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 method significantly improves high-temperature radiation stability, heat conductivity, and fission product retention, reducing the interaction between fuel pellets and cladding, enhancing reactor safety and economic benefits by increasing the size of UO2 crystal grains and reducing crystal boundaries.
Implementation Method 1
liquefying monocrystal growth additives on the surfaces of the core-shell structure particles at a high temperature
Implementation Method 2
diffusing into the UO2 pellets, dissolving the UO2 in the liquid monocrystal growth additives
Implementation Method 3
recrystallizing the UO2 to form the monocrystal UO2 nuclear fuel pellets
Implementation Method 4
activated-sintering the core-shell structure particles at a high temperature
Data Source
AI summary
The application discloses a preparation method of monocrystal uranium dioxide nuclear fuel pellets, comprising: granulating and pelleting UO2 powder to obtain UO2 pellets; then coating surfaces of the UO2 pellets with monocrystal growth additive micro powder to form core-shell structure particles; and activated-sintering the core-shell structure particles at high temperature, liquefying the monocrystal growth additive on the surface of the core-shell structure particle at high temperature and then diffusing into UO2 pellets, dissolving the UO3 in the liquid monocrystal growth additive, and recrystallizing the UO2 to form the monocrystal UO2 nuclear fuel pellets.