Elongate SiC Fuel Element Swelling Management
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Solution Overview
Problem
The use of silicon carbide (SiC) cladding in nuclear fuel rods faces challenges due to mechanical interaction with fuel pellets, leading to cladding failure and hermeticity loss, particularly with low thermal conductivity fuels like UO2, which results in increased fuel centerline temperatures and reduced uranium loading.
Innovation Solution
An elongate shaped fuel element with elongate SiC cladding and specific gap dimensions between the fuel and cladding allows for minimal swelling-induced deformation of the cladding, preventing strain and maintaining hermeticity, while enabling higher uranium loading and improved heat transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a large pellet-cladding gap is left to prevent PCMI in SiC cladding, then cladding hermeticity is maintained, but fuel centerline temperature increases significantly and uranium loading is reduced
Solution Approach 1:
The invention changes the geometric parameters of the fuel element by using an elongate fuel geometry with aspect ratio greater than 2:1, which fundamentally alters the swelling behavior and stress distribution patterns, allowing the fuel to swell anisotropically along its length rather than radially, thus maintaining contact with SiC cladding without excessive gap requirements
Solution Approach 2:
The invention transitions from traditional cylindrical fuel rods to an elongate fuel geometry, adding a longitudinal dimension to the swelling accommodation strategy. The fuel is allowed to swell primarily along its length rather than radially, which changes the dimensionality of the problem and enables different gap management approaches
2Quantity of substance
If UO2 fuel is used in SiC cladding with minimal gap, then thermal conductivity issues arise leading to high fuel temperatures, but increasing the gap reduces uranium loading and increases centerline temperature further
Solution Approach 1:
The invention changes the fuel geometry parameters (aspect ratio > 2:1) which fundamentally alters the thermal and mechanical behavior, allowing UO2 fuel to be used with SiC cladding at high loading densities without the traditional temperature penalties by enabling longitudinal swelling accommodation
3Strength
If fuel pellets are allowed to swell freely, then mechanical interaction with SiC cladding causes cladding failure, but constraining the fuel reduces swelling accommodation
Solution Approach 1:
The invention changes the fuel geometry (aspect ratio > 2:1) which fundamentally alters the swelling mechanics, allowing the fuel to accommodate swelling primarily through longitudinal expansion rather than radial expansion, thus protecting SiC cladding integrity while maintaining swelling accommodation capability
Solution Approach 2:
The invention redirects the swelling accommodation from the radial dimension (traditional approach) to the longitudinal dimension (new approach), allowing fuel to expand along its length rather than pushing radially against the cladding, which changes the dimensionality of the swelling problem
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 design reduces fuel temperatures, minimizes fission gas release and swelling, and decreases tensile stress on the cladding, allowing for the use of UO2 fuel without compromising accident tolerance or economic viability, while accommodating fuel expansion without mechanical failure.
Implementation Method 1
when the fuel swells, the swollen fuel swells beyond the first distance and up to but not beyond the second distance to deform the elongated walls of the cladding
Implementation Method 2
the swollen fuel swells beyond the first distance and up to but not beyond the second distance to deform the elongated walls of the cladding without deforming the end walls of the cladding and without straining the cladding more than 0.1%
Implementation Method 3
enabling higher uranium loading and improved heat transfer
Data Source
AI summary
An elongate fuel element is described that has a silicon carbide cladding enclosing a fuel, such as UO2, wherein the fuel is dimensioned relative to the cladding to define gaps at each lateral end of the enclosure sufficiently large such that upon swelling in use, the fuel does not increase the strain on the cladding beyond the limits of the claddings strain tolerance. The lateral gaps at the ends of the fuel allow lateral expansion during swelling that reduces the strain on the cladding.