Patent Number: 
Section: claims

1. A nuclear thermal propulsion system comprising:a nuclear reactor core including:an array of fuel elements;an array of tie tubes adjacent the array of fuel elements, each tie tube including:a propellant supply passage to flow a propellant;an inner tie tube layer surrounding the propellant supply passage;a moderator sleeve surrounding the inner tie tube layer;a propellant return passage surrounding the moderator sleeve; andan outer tie tube layer surrounding the propellant return passage; anda burnable poison dispersed in the nuclear reactor core, wherein:the burnable poison includes Gadolinium (Gd),the Gd is dispersed in an alloy that forms the outer tie tube layer, andthe Gd dispersed in the outer tie tube layer alloy is in a quantity of 10 parts per million (ppm) to 70 ppm. 2. The nuclear thermal propulsion system of claim 1, wherein:the propellant is hydrogen. 3. The nuclear thermal propulsion system of claim 2, wherein:the array of fuel elements includes a graphite composite fuel formed of low-enriched uranium (LEU) having 20% or lower 235U; andthe moderator sleeve includes a solid hydride. 4. The nuclear thermal propulsion system of claim 2, wherein:the array of fuel elements includes a tungsten ceramic and metal matrix (CERMET) fuel formed of low-enriched uranium (LEU) having 20% or lower 235U, andthe moderator sleeve includes a solid hydride. 5. The nuclear thermal propulsion system of claim 2, wherein each tie tube further comprises:an inner gap between the inner tie tube layer and the moderator sleeve;a graphite insulation layer surrounding the outer tie tube layer;a medial gap between the outer tie tube layer and the graphite insulation layer;an outer gap surrounding the graphite insulation layer;a graphite sleeve surrounding the outer gap; anda coating formed of zirconium carbide or niobium carbide surrounding the graphite sleeve. 6. The nuclear thermal propulsion system of claim 5, wherein the graphite insulation layer includes zirconium carbide (ZrC), titanium carbide, silicon carbide, tantalum carbide, hafnium carbide, ZrC—ZrB2 composite, or ZrC—ZrB2—SiC composite. 7. The nuclear thermal propulsion system of claim 2, wherein:the propellant supply passage, the inner tie tube layer, the moderator sleeve, the propellant return passage, and the outer tie tube layer are radially arranged; anda respective radial wall thickness of the outer tie tube layer is less than the inner tie tube layer. 8. The nuclear thermal propulsion system of claim 7, wherein the array of fuel elements are interspersed with the array of tie tubes and each of the tie tubes is in direct or indirect contact with at least one fuel element. 9. The nuclear thermal propulsion system of claim 7, further comprising:a plurality of circumferential control drums surrounding the array of fuel elements and the array of tie tubes to change reactivity of the nuclear reactor core. 10. The nuclear thermal propulsion system of claim 1, wherein the Gd in the outer tie tube layer alloy is enriched in 157Gd isotope. 11. The nuclear thermal propulsion system of claim 10, wherein the outer tie tube layer alloy further includes nickel, chromium, and iron. 12. The nuclear thermal propulsion system of claim 11, wherein the outer tie tube layer alloy further includes molybdenum, niobium, cobalt, manganese, copper, aluminum, titanium, silicon, carbon, sulfur, phosphorus, and boron. 13. The nuclear thermal propulsion system of claim 10, wherein the outer tie tube layer alloy is also formed of zirconium or zirconium carbide.