*Nuclear Thermal Propulsion DOI: http://dx.doi.org/10.5772/intechopen.103895*

KANUTER [45] is unique among modern NTP designs because it uses HEU with an enrichment of 93%. The goals of the design are to maximize *I*sp, thrust-to-weight ratio, and allow for bimodal operation (e.g., thrust and electricity generation). The NTP design uses a tricarbide (U,Zr,Nb)C fuel matrix that was tested during the Russian NTP program. KANUTER uses an integrated fuel assembly concept; the fuel assembly depicted in **Figure 3(f)** contains both fuel and a ZrH moderator that are separated by carbon–carbon insulation. The fuel matrix is arranged in wafers and the coolant channels are square. In the core, the 37 fuel elements are arranged in a hexagonal lattice and held in place by cooled beryllium spaces.

Poston [32] investigated how the performance characteristics of NTP systems change when the fuel matrix is changed from GWDF to CERMET and the enrichment is changed from LEU (19%) to HEU (93%). The four variants discussed in Ref. [32] are thus HEU-composite (e.g., NERVA carbide composite fuel), LEUcomposite, LEU-CERMET, and HEU-CERMET. All concepts use hexagonal assemblies, but the assembly sizes differ: the HEU-composite uses the standard NERVA 19-hole element with a 1.91 cm flat-to-flat, the LEU-composite uses a 37-hole fuel element with a 2.77 cm flat-to-flat, the HEU-CERMET uses an element similar to the GE-710 designs with 91 holes and a 2.57 cm flat-to-flat, and the LEU-CERMET uses a 61 hole assembly with a 2.52 cm flat-to-flat. With the exception of the HEU-CERMET, all designs use the traditional fuel element/tie-tube concept of NASA albeit at different TTR (33% for HEU and LEU composite and 50% for LEU-CERMET). All concepts have an epithermal spectrum except for the HEU-CERMET. Moderation in the epithermal concepts is provided by the composite and by ZrH in the tie tubes; the LEU-CERMET requires more tie-tubes to increase the amount of moderator in the core. The CERMET in Poston's study is enriched to remove the highest absorbing isotopes from tungsten, molybdenum, rhenium, and zirconium; tungsten is used as a matrix material in the study. All designs use a Be radial reflector and the CERMET designs use a BeO top (cold-end) reflector. The performance difference and differences in the design parameters depend most heavily on 235U densities. The neutronics design ensures a 1% beginning of life reactivity margin and a shutdown margin of 5%; however, LEU-CERMET barely achieves the beginning of life margin.

In Ref. [33], Youinou evaluates alternative designs to the monolithic ZrH moderated, CERMET, or CERCER concepts of the early 2020s by NASA. While several different concepts of this report deserve attention, the most important design is an LEU, plate-fueled, fast design. This concept uses UN fuel plates of thicknesses 0.5–10 mm, MoW or W clad of thickness 0.25–0.5 mm, square assemblies of size 8 8 80 cm, and 7–49 fuel plates per assembly. There are 37 fuel elements in the core. The core has a power of 250 MW generating a thrust of 12,500 lbs. Youinou found that the smaller fraction of refractory metals in the plate design allow for fast LEU NTPs fueled with UN and clad with refractory metals.

The GE-710 NTP system is an example of an HEU, fast, CERMET-based concept that was developed concurrently with the graphite-based NERVA concepts [22]. The GE-710 program tested various CERMET matrix materials, including tungsten, tungsten-rhenium, tungsten-rhenium-molybdenum, and molybdenum-rhenium, among others [22]. All fuel elements investigated during the GE-710 are hexagonal and slightly larger than the NERVA fuel elements (e.g., 2.36 cm versus 1.91 cm flatto-flat). GE-710 elements contain significantly more coolant channels than the NERVA elements, which increases the pressure drop through the core, but decreases the difference between the coolant and the maximum fuel temperature. Overall, the GE-710 project demonstrated excellent thermal and mechanical stability during thousands of hours of testing [51].
