**3.3 Geometrical arrangements**

This section introduces different criteria to distinguish and classify geometries in NTP reactors. The criteria we use to distinguish these reactors are the fuel element geometry (e.g., hexagonal, annular, plates), the structural concept (tie tube or monolith), and if the moderator is heterogeneous or homogeneous. Here we compare U.S. NTP designs to concepts evaluated in the Soviet Union and the Republic of Korea. The Soviet Union began at about the same time as the Rover program, but ended in 1989 with the collapse of the Union of Soviet Socialist Republics (USSR). The Korean concept is still under active development, beginning in 2013.

NTP reactors are distinguished by their fuel element layout. The original NERVA design used hexagonal fuel elements arranged in a hexagonal lattice, as shown in **Figure 3(a)** and **(b)**. A group of six fuel elements is connected to a tie tube. The tie tube is relevant both for moderation and structural integrity as discussed below. The ratio of the number of fuel elements and tie tubes in the lattice is an important parameter for NERVA-type designs.

Fast reactor concepts originating from the ANL and GE projects also use hexagonal fuel elements, as observed in **Figure 3(c)**, arranged in a hexagonal lattice, as seen in **Figure 3(d)**, but the fuel elements tend to be larger than their NERVA counterparts and contain more coolant channels. The hexagonal fast concepts do not require tie tubes.

The Russian NTP program considered a variety of fuel element shapes (see Ref. [15]) among which the twisted ribbon design depicted in **Figure 3(e)** was selected as the most promising option. Usually, each twisted ribbon is referred to as a fuel element; it should be noted that each twisted ribbon is significantly smaller than a NERVA fuel assembly. Twisted ribbons are inserted into a fuel bundle that is wrapped by insulating material. The fuel bundle is in turn inserted into a fuel assembly that is then placed into the core.

The Korea Advanced NUclear Thermal Engine Rocket (KANUTER) fuel assembly design is depicted in **Figure 3(f)**. The fuel shown in red in the figure consists of wavers forming square flow channels; interlocking of the fuel wavers forms a square lattice [45]. The fuel is surrounded by insulating carbon wrappers and a metal hydride moderator. The fuel assemblies in the KANUTER core are arranged in a hexagonal pattern.

The recent NASA/BWXT design is depicted in **Figure 3(g)** with the progression from the smallest to largest part from left to right in the figure. Each fuel element is cylindrical with round flow channels and is surrounded by an insulator. The flow channels in each element are arranged in cylindrical clusters in CANDU reactors (i.e., one central hole and six flow channels placed on a circle around the center with 12 flow channels placed on a larger circle surrounding those, etc.). The fuel elements are wrapped with structural support and then placed in holes bored through the monolithic core structure, as observed in the second picture from the right in **Figure 3(g)**. The monolithic core structure is made up of a metal hydride moderator. The fuel elements in the monolith are arranged in a cylindrical cluster, just as the coolant channels are arranged in the fuel element.

The core geometry can be distinguished by the structural support concept for the fuel elements. In the NERVA designs, a tie tube is connected to the six fuel assemblies around it, and a spring keeps the fuel elements in tension to avoid damage to the core structure by flow-induced vibrations and support the core against the axial pressure drop [47]. The tie tubes are connected to a support plate located at the cold end of the core. Additional axial support is provided by pedestals in some reactors (e.g., PEWEE) [17].
