Fuel element with multi-smear density fuel
A fuel element has a ratio of area of fissionable nuclear fuel in a cross-section of the tubular fuel element perpendicular to the longitudinal axis to total area of the interior volume in the cross-section of the tubular fuel element that varies with position along the longitudinal axis. The ratio can vary with position along the longitudinal axis between a minimum of 0.30 and a maximum of 1.0. Increasing the ratio above and below the peak burn-up location associated with conventional systems reduces the peak burn-up and flattens and shifts the burn-up distribution, which is preferably Gaussian. The longitudinal variation can be implemented in fuel assemblies using fuel bodies, such as pellets, rods or annuli, or fuel in the form of metal sponge and meaningfully increases efficiency of fuel utilization.
1 . A method of manufacturing a fuel element, the method comprising:
providing a cladding;
disposing a first fissionable composition within a first end section of an interior volume of the cladding and in thermal transfer contact with an interior surface of the cladding, the first fissionable composition having a first average smear density;
disposing a second fissionable composition within a central section of the interior volume and in thermal transfer contact with the interior surface of the cladding, the second fissionable composition having a second average smear density less than the first average smear density; and
disposing a third fissionable composition within a second end section of the interior volume of the cladding and in thermal transfer contact with the interior surface of the cladding, the third fissionable composition having a third average smear density greater than the second average smear density;
wherein the first average smear density is different from the third average smear density.
2 . The method of claim 1 , wherein the smear density of the first fissionable composition varies according to a decreasing step function within the first end section and the smear density of the third fissionable composition varies according to an increasing step function within the second end section.
3 . The method of claim 1 , wherein the disposing steps are performed such that the first end section and the second end section are of equal length and the first average smear density is greater than the third average smear density.
4 . The method of claim 1 , wherein a smear density profile defined by the smear density of the first fissionable composition, the second fissionable composition, and the third fissionable composition varies continuously along a longitudinal axis of the fuel element and the smear density profile approximates an inverted Gaussian shape.
5 . The method of claim 1 , wherein the first average smear density, the second average smear density, and the third average smear density collectively vary according to a step function.
6 . The method of claim 1 , wherein disposing a first fissionable composition within the first end section of in the interior volume of the cladding comprises disposing a fissionable metal sponge.
7 . The method of claim 1 , wherein disposing a first fissionable composition within the first end section of in the interior volume of the cladding comprises disposing fuel pellets.
8 . The method of claim 1 , wherein the first average smear density is between about 70% and about 85%.
9 . The method of claim 1 , wherein the second average smear density is between about 50% and about 65%.
10 . The method of claim 1 , wherein the first average smear density is at least 10% greater than the second average smear density.
11 . The method of claim 1 , further comprising:
determining an expected neutron flux profile along a longitudinal axis of the fuel element; and
selecting the first average smear density, the second average smear density, and the third average smear density based on the expected neutron flux profile,
wherein the second average smear density is selected to be lower than the first average smear density and the third average smear density to accommodate increased fuel swelling in a region of higher expected neutron flux.
12 . The method of claim 11 , wherein the expected neutron flux profile is asymmetric along the longitudinal axis of the fuel element, and wherein the first average smear density is selected to be greater than the third average smear density to account for a lower expected neutron flux at the first end section relative to the second end section.
13 . The method of claim 1 , wherein the first end section is configured to be positioned proximate a coolant entry region of a nuclear reactor during operation, and wherein the first average smear density is greater than the third average smear density.
14 . The method of claim 1 , wherein the second average smear density is selected to accommodate a burnup of at least 20% FIMA in the central section.
15 . The method of claim 1 , wherein the first average smear density, the second average smear density, and the third average smear density are selected such that cladding strain at any location along the fuel element remains below a predetermined strain limit during operation to a target burnup.
16 . The method of claim 1 , wherein the first end section comprises a plurality of zones, each zone having a different smear density, and wherein the smear density decreases from a zone proximate a first end of the cladding toward a zone proximate the central section.
17 . The method of claim 16 , wherein the plurality of zones in the first end section comprises at least three zones, each having a successively lower smear density from the first end of the cladding toward the central section.
18 . The method of claim 1 , wherein at least one of the first fissionable composition, the second fissionable composition, and the third fissionable composition comprises a uranium-zirconium alloy.
19 . The method of claim 1 , wherein disposing the second fissionable composition within the central section comprises disposing an annular fuel body having a central void.