Porous nuclear fuel element for high-temperature gas-cooled nuclear reactors
Porous nuclear fuel elements for use in advanced high temperature gas-cooled nuclear reactors (HTGR's), and to processes for fabricating them. Advanced uranium bi-carbide, uranium tri-carbide and uranium carbonitride nuclear fuels can be used. These fuels have high melting temperatures, high thermal conductivity, and high resistance to erosion by hot hydrogen gas. Tri-carbide fuels, such as (U,Zr,Nb)C, can be fabricated using chemical vapor infiltration (CVI) to simultaneously deposit each of the three separate carbides, e.g., UC, ZrC, and NbC in a single CVI step. By using CVI, the nuclear fuel may be deposited inside of a highly porous skeletal structure made of, for example, reticulated vitreous carbon foam.
1. A nuclear fuel element, comprising a porous, open-celled, foam material comprising
a three-dimensional, reticulated network of interconnected hollow tubular structural ligaments, with interconnected pores disposed throughout the network in-between the ligaments;
wherein each hollow tubular ligament comprises:
a continuous ceramic shell made of a uranium tri-carbide nuclear fuel alloy, and
a hollow, empty central core space disposed inside of the shell substantially along the entire length of the ligament;
wherein the alloy comprises a solid-solution alloy of UC, ZrC, and XC, where X is selected from the group consisting of Nb, Ta, Hf and W;
wherein the nuclear fuel element has a total porosity greater than or equal to about 70%; and
wherein no underlying reticulated vitreous carbon foam skeleton exists inside of the ligaments.
2. The fuel element of claim 1 , wherein the structural ligaments have a triangular-shaped cross-section.
3. The fuel element of claim 1 , wherein X is Nb.
4. The fuel element of claim 1 , wherein the uranium metal mole fraction in the solid-solution alloy is less than or equal to 10%.
5. The fuel element of claim 1 , further comprising a ZrC barrier coating covering the external surface of each ligament.
6. The fuel element of claim 1 , wherein the uranium tri-carbide solid-solution alloy has a stoichiometry equal to (U W Zr X Nb Y )C Z ; wherein:
0.04<W<0.12,
0.45<X<0.90,
0.00<Y≦0.45, and
0.92<Z<1.00.
7. The fuel element of claim 6 , wherein the uranium tri-carbide alloy has a stoichiometry equal to (U 0.1 Zr 0.77 Nb 0.13 )C 0.95 .
8. The fuel element of claim 6 , wherein the uranium tri-carbide alloy has a stoichiometry equal to (U 0.1 Zr 0.675 Nb 0.225 )C.