operational phases (startup, full thrust and shutdown) is accomplished by matching the TPA-supplied LH2 flow to the reactor power level. Multiple control drums, located in the reflector region surrounding the reactor core, regulate the neutron population and reactor power level over the NTR’s operational lifetime. The internal neutron and gamma radiation shield, located within the engine’s pressure vessel, contains its own interior coolant channels. It is placed between the reactor core and key engine components to prevent excessive radiation heating and material damage.

[Figure: Figure 3.—Schematic of “expander cycle” NTR engine with dual LH2 turbopumps.]

The fuel elements (FE) tested in the Rover/NERVA program (Ref. 8) consisted of a “graphite matrix” material that contained the U-235 fuel in the form of either coated particles of uranium carbide (UC2) or as a dispersion of uranium and zirconium carbide (UC-ZrC) referred to as “composite” fuel. Each FE (see Fig. 4) had a hexagonal cross section (~0.75 in. across the flats) and 19 axial coolant channels that were coated with niobium carbide (NbC) initially, then with zirconium carbide (ZrC) using a chemical vapor deposition (CVD) process. This protective coating, applied to the exterior FE surfaces as well, helped reduce hydrogen erosion of the graphite. Individual elements were 1.32 m (52 in.) in length and produced ~1 MWt. This basic FE shape and length was introduced in the KIWI-B4E reactor and became the standard used in the 75 klbf Phoebus-1B, 250 klbf Phoebus-2A, 25 klbf Pewee and the 55 klbf NERVA NRX series
of engines (Ref. 8). The Rover program’s Pewee engine (Ref. 8) was designed and built to evaluate higher temperature, longer life fuel elements and improved coatings. It set several performance records including the highest average fuel element exit gas temperature of ~2550 K, and the highest peak fuel temperature ~2750 K. Other performance records included average and peak power densities in the reactor core of
~2340 MWt/m3 and ~5200 MWt/m3, respectively. A new CVD coating of ZrC was also introduced in
Pewee that showed performance superior to the NbC coating used in previous reactor tests. In addition to FEs, the engine reactor cores also included tie tube (TT) elements of the same hexagonal shape that provided structural support for the FEs. A coaxial coolant tube of Inconel inside each TT supplied a source of heated hydrogen for turbine drive power and a sleeve of zirconium hydride (ZrH) moderator material could also be incorporated in the TTs to help raise neutron reactivity (shown in Fig. 4). In the larger size engines tested during the Rover/NERVA programs, a “sparse” FE—TT arrangement was used with each FE having two adjacent TTs and four adjacent FEs comprising its six surrounding elements (Ref. 11). In this sparse pattern, the FE to TT ratio is ~3 to 1. In the Small Nuclear Rocket Engine (SNRE) design developed by Los Alamos National Laboratory near the end of Rover/NERVA (Ref. 12), shorter (0.89 m/35 in.) FEs were used so additional TTs were included in the reactor to increase core reactivity. With the “SNRE” FE—TT pattern each FE has three adjacent TTs and three adjacent FEs surrounding it (shown in Fig. 4) and the FE to TT ratio is ~2 to 1. An important feature common to both the sparse and SNRE FE—TT patterns is that each tie tube provides mechanical support for six adjacent fuel elements.

[Figure: Figure 4.—Coated particle and composite SNRE fuel element and tie tube arrangement.]