Patent Number: 
Section: claims

1. An apparatus comprising:a fuel element for a fuel assembly, the fuel element having a tubular interior volume and storing a fissionable composition within at least a portion of the tubular interior volume, the fissionable composition in thermal transfer contact with an interior surface of the fuel element and defined by a smear density profile that includes at least five different smear densities that selectively vary with position along a longitudinal axis of the fuel element and includes:at least one region of locally increased smear density positioned to correspond to at least one region of locally decreased neutron flux, andat least one region of locally decreased smear density positioned to correspond to at least one region of locally increased neutron flux. 2. The apparatus of claim 1, wherein the smear density profile includes multiple regions of locally increased smear density corresponding to regions of locally decreased neutron flux. 3. The apparatus of claim 1, wherein the smear density profile approximates an inverted Gaussian shape. 4. The apparatus of claim 1, wherein the smear density profile varies according to a step function. 5. The apparatus of claim 1, wherein the smear density profile is higher at a first end of the fuel element than at a second opposite end of the fuel element. 6. The apparatus of claim 5, wherein the first end of the fuel element is proximate a coolant entry point within the fuel assembly and the second opposite end of the fuel element is proximate a coolant exit point of the fuel assembly. 7. The apparatus of claim 1, wherein the smear density profile of the fuel element is lower in a central section of the longitudinal axis than at either a first end or a second end of the longitudinal axis. 8. The apparatus of claim 1, wherein the fuel element comprises at least three sections, a first section proximate a first longitudinal end of the fuel element, a third section proximate a second longitudinal end of the fuel element, and a second section between the first and third sections, wherein an average smear density of the first section is greater than an average smear density of the second section, and wherein an average smear density of the third section is greater than the smear density of the second section. 9. The apparatus of claim 8, wherein the average smear density of the third section is less than the average smear density of the first section. 10. The apparatus of claim 7, wherein the smear density profile of the fuel element is not symmetrical around the central section. 11. The apparatus of claim 10, wherein the smear density profile is higher at a first end of the fuel element than at a second opposite end of the fuel element. 12. The apparatus of claim 1, wherein the fissionable composition comprises a fissionable metal sponge. 13. The apparatus of claim 1, wherein the fissionable composition comprises fuel pellets. 14. An apparatus comprising:a fuel element having an interior volume with a fissionable composition within at least a portion of the interior volume, the fissionable composition in thermal transfer contact with an interior surface of the fuel element and having a smear density profile that includes, longitudinally arranged in order from a first end to a second end of the fuel element:a first section at the first end having a first average smear density,a second section having a second average smear density different than the first average smear density,a third section having a third average smear density less that the first average smear design and the second average smear density,a fourth section having a fourth average smear density greater than the third average smear density, anda fifth section having a fifth average smear density greater than the fourth average smear density. 15. The apparatus of claim 14, wherein the smear density profile approximates an inverted Gaussian shape. 16. The apparatus of claim 14, wherein the smear density profile varies according to a step function. 17. The apparatus of claim 14, wherein the fifth average smear density is greater than the first average smear density.