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

#### **Figure 3.**

*An overview of the geometric arrangement of different NTP concepts. (a) Typical later NERVA fuel element layout. Six fuel elements are connected to a tie-tube [16]. (b) NERVA hexagonal fuel element layout with a different ratio of tie-tube and fuel elements [43]. (c) ANL-200, GE-711, and NERVA fuel assembly geometries [44]. (d) Hexagonal lattice of fuel elements typical for fast reactor designs like ANL-200 and GE-710 [32]. (e) Russian NTP concepts using a twisted ribbon fuel element in an encased assembly that is inserted into the reactor (picture (e) from left to right) [39] (length units are mm). (f) Korean integrated fuel assembly design with square flow channels [45]. (g) Recent "fuel assemblies under consideration for NASA's nuclear thermal propulsion reactor designs" by BWXT advanced technologies, LLC [46].*

In contrast to the tie tube design, the more recent NASA/BWXT design uses a monolith concept, as described in Ref. [46] and shown in **Figure 3(d)**. The monolithic core structure is made up of the metal hydride moderator and has borings that contain the fuel assemblies. The fuel elements are wrapped with insulator and structural support. The structural support is fastened to a support plate at the cold end of the core. Additional axial support at the cold end may be included in the design as well.

Finally, the KANUTER design, as shown in **Figure 3(f)**, arranges beryllium spacers between the integrated fuel assemblies. In contrast to the NASA/BWXT design, the integrated fuel assemblies contain a moderator where the core support structure is strongly moderating.

Reactors can also be classified by how the moderator and fuel are arranged. If the fuel and moderator are spatially separated, the reactor is *heterogeneous*; if the fuel and moderator are mixed, then the reactor is *homogeneous*. For this distinction, spatially separated means that there is sufficient distance on the order of a mean free path between the fuel and the moderator. Heterogeneous cores offer an advantage in reactivity over spatially homogeneous cores; the effect is sometimes referred to as fuel lumping. If the moderator is spatially separated from the fuel, then moderation happens away from the fuel, reducing the likelihood of resonance absorption during the slowing down process [48].

To the knowledge of the authors, the only truly *homogeneous* cores were early NERVA designs before PEWEE 1. In these designs, the moderator was the graphite matrix containing the fuel particles. Starting from PEWEE 1, the tie tubes were equipped with ZrH sleeves adding additional moderation to the system and making these designs essentially *mixed* moderation cores [17]. The Russian cores and KANUTER are *mixed* moderation cores due to the presence of graphite in the fuel compact (i.e., the homogeneous portion) and an additional moderator either in the fuel assembly or the structural components surrounding them. The recent NASA/ BWXT design is a *heterogeneous* core because the only significant amount of moderator is in the monolith outside of the fuel assemblies. Fast reactors do not fall into this classification because they do not contain a moderator.

The following section discusses a small selection of representative NTP reactor concepts and provides more detail on each design.

#### **3.4 Reactor concepts**

PEWEE-1 is a demonstration reactor tested in the NERVA program in 1968 toward the end of the program. It is a small reactor when compared with the preceding Phoebus tests with power reduced from 4000 MW in the Phoebus-A design to about 500 MW [17]. To offset the increased leakage from the smaller core size, ZrH sleeves were inserted into the standard tie-tube concept of the NERVA program; the tie-tube ratio (TTR)2 was increased and the reflector thickness was increased. The main objective of PEWEE-1 was to serve as a test bed for fuel elements and no attempt was made to maximize the outlet temperature [17]. Despite these differences to other tests within the NERVA project, PEWEE-1 is a good example of the technology used and resulting observed performance during NERVA.

In two different works [49, 50], Kotlyar focuses on studying the design space of thermal LEU-CERMET NTP concepts. These designs use the NERVA structural concept of fuel elements and tie tube/moderating elements without changing their size and shape (i.e., a hexagonal lattice with 1.905 cm flat-to-flat distance). However, the matrix is changed to LEU UO2 particles in W-CERMET [49] and LEU UN particles in Mo or MoW-CERMET [50]. In order to overcome the reactivity penalty of refractory metals, lower uranium enrichment, and the lack of moderation in the fuel compact, Kotlyar's core concepts include significantly more moderating elements (>50% depending on core size) than PEWEE-1 with more ZrH moderator and additional carbon per moderating element. The spectrum is more thermal than in the NERVA engines, but is significantly undermoderated for the optimal small, medium, and large NTP designs [49].

<sup>2</sup> The ratio of tie-tube elements to total number of elements.
