UO2 Fuel Pellets with Mn and Al Additives for Cladding Stress Reduction
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Solution Overview
Problem
High burnup nuclear fuel pellets experience increased internal pressure due to fission gases, leading to stress on the cladding tube, which can result in damage and safety concerns, especially during thermal expansion and power fluctuations.
Innovation Solution
Nuclear fuel pellets with large grain size and fast creep deformation rate are achieved by incorporating Mn and Al as additives, with a Mn/Al ratio of 1 to 100 by weight, mixed with UO2 powder and sintered at 1,600° C. to 1,800° C. in a reducing gas atmosphere, enhancing diffusivity and deformation properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high burnup fuel pellets are used to enhance economical efficiency and reduce spent fuel, then fuel efficiency is improved, but internal pressure increases due to fission gases leading to cladding tube stress and safety concerns
Solution Approach 1:
The patent changes the grain size parameter from conventional small grains (6-10 μm) to large grains (greater than 10 μm, preferably 20-50 μm) by controlling sintering conditions and using additive elements. This parameter change increases the distance for fission gas diffusion to grain boundaries, reducing fission gas release and internal pressure buildup while maintaining high burnup fuel efficiency
Solution Approach 2:
The patent uses composite material composition by incorporating additive elements (such as Gd, Dy, Er, Yb, or Lu at 100-1000 ppm) into the UO2 fuel matrix. These additives modify the microstructure to promote large grain formation and control fission gas behavior, creating a composite material system that maintains structural integrity under high burnup conditions
2Object-affected harmful factors
If grain size is increased to reduce fission gas release, then fission gas retention is improved, but creep deformation rate may be affected
Solution Approach 1:
The patent optimizes multiple parameters simultaneously: sintering temperature (1600-1800°C), sintering time (2-8 hours), and additive concentration (100-1000 ppm). These parameter changes create a balanced microstructure with large grains that retain fission gas while maintaining adequate creep deformation rate for pellet-clad interaction management
Solution Approach 2:
The patent creates local quality variations in the fuel pellet microstructure by distributing additive elements that promote grain growth in specific regions. This results in a heterogeneous microstructure with large grains in certain areas for gas retention and appropriate grain boundary characteristics in other areas to maintain creep deformation capability
3Shape
If sintering temperature is increased to enhance grain growth, then grain size is improved, but manufacturing complexity and energy consumption increase
Solution Approach 1:
The patent applies preliminary action by incorporating additive elements into the green pellet before sintering. These additives are pre-distributed uniformly in the compacted powder, so during sintering they automatically promote grain growth without requiring complex external intervention or multiple processing steps, simplifying the overall manufacturing process
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 pellets effectively retain fission products, reduce stress on the cladding tube, and enhance safety margins by allowing faster deformation, thereby preventing cladding tube failure and ensuring stable reactor operation.
Implementation Method 1
The nuclear fission gas is generated in a grain, moves to the grain boundary through diffusion so as to exist as bubbles in the grain boundary
Implementation Method 2
sintering the green pellet at 1,600° C. to 1,800° C. for two to eight hours in a hydrogen gas atmosphere
Implementation Method 3
The nuclear fission gas is generated in a grain, moves to the grain boundary through diffusion so as to exist as bubbles in the grain boundary
Data Source
AI summary
UO2 nuclear fuel pellets are fabricated by adding additive powder comprising Mn compound and Al compound into UO2 powder.