Feasibility of Low-Enriched Uranium Fueled Nuclear Thermal Propulsion in the Low-Thrust Region Below 16klbf

Abstract

This paper establishes the feasibility of Low-Enriched Uranium fueled Nuclear Thermal Propulsion (LEU-NTP) reactors in the low-thrust region below 16klbf (71kN). A reference, 7.5klbf, High-Enriched Uranium (HEU) design is converted to 19.75% enriched LEU and shown to be capable of reaching criticality and meeting other performance requirements with a minimal mass increase. At this smaller scale, historical LEU-NTP conversion techniques that focus on maximizing neutron moderation or minimizing leakage within the active core are insufficient. Thus, the 7.5klbf preliminary design requires several unique modifications, including a reduction in the number of control drums and adjustments to selected materials. To verify the design’s feasibility, several key neutronic and thermal-hydraulic performance parameters including burnup, Xenon worth, and submersion criticality are characterized with Serpent 2.0 and the Space Propulsion Optimization Code. The methods applied in this work reveal an opportunity for further LEU-NTP optimization that may directly translate to increased scalability and efficiency for future designs.

Introduction

Nuclear thermal propulsion (NTP) is unchallenged as the technologically superior transportation method for near-term, crewed missions to Mars. Its fundamental performance capabilities are double that of current, state-of-the-art technologies, making it an optimal solution for the high mass, long distance architectures necessary for human spaceflight beyond cislunar space. NTP is the only currently viable option that delivers six astronauts to Mars within three months, reduces the total required number of launches, enables mission profiles with either short-term or extended stays on the Martian surface, and offers abort scenarios at any point during transit (Sager, 1992, Durante and Bruno, 2010, Drake, 2013, Joyner, 2017).

Fortunately, nuclear thermal propulsion already possesses significant historical data from the original research conducted during the Rover/Nuclear Engine for Rocket Vehicle Application (NERVA) program from 1955 to 1972. Most of today’s design work involves improving upon the program’s final design iteration, the 16.4klbf Small Nuclear Rocket Engine (SNRE) (Fig. 1).

Several attempts to launch this technology into space took place in the decades following the NERVA program, yet none progressed beyond laboratory experiments. In January 1994, a preliminary proposal was submitted to the Department of Energy presenting a space nuclear system utilizing low enriched uranium (LEU) instead of high-enriched (HEU). Such a system might generate the public support, reduce the security costs, and ease the regulatory burdens enough to finally achieve a sustained source of funding. The proposal was rejected on the basis that the additional mass necessary to achieve criticality with anything less than HEU would be prohibitive (USDOE ASSISTANT SECRETARY FOR NUCLEAR ENERGY, 1994).

Almost twenty years later the first LEU-NTP reactor design was published, a low-enriched version of the SNRE, with initial findings suggesting the change in reactor mass would be negligible (Venneri and Kim, 2016, Venneri and Kim, 2016). A surge of independent reviews verified this work, and by 2015 NASA had redirected its NTP program to experimentally proving LEU-NTP’s feasibility. Since then, almost all NTP publications either assume the use of LEU for mission analysis or seek to further understand the full range of LEU-NTP’s potential and limitations.

These studies have repeatedly shown that LEU-NTP systems are equivalent to their HEU counterparts in both single-mission performance and total mass at both the standard NTP thrust levels for crewed Mars missions, 16-25klbf, and the higher thrusts considered for upper stage orbit transfer maneuvers, greater than35kblf (Patel et al., 2016a, Joyner, 2016, JOYNER, 2018, Eades et al., 2015). This equivalence is primarily because, at thrusts greater than 16klbf, reactor size is driven by cooling requirements rather than criticality (Patel et al., 2016a). Thus, both LEU and HEU-NTP reactors at these larger thrusts are volumetrically constrained by thermal-hydraulics rather than the nuclear physics.

At smaller sizes, however, LEU reactors are more constrained by their neutronics than HEU (Kim et al., 2013, Licht, et al., 2016). Thus, for engines smaller than 16klbf, which would be most useful for missions such as small-scale qualification testing, robotic interplanetary missions, or proposed as a faster means to achieve first-flight, the LEU-NTP reactor configuration was once again assumed to require an unacceptable mass increase.

The actual transition point between nuclear physics and thermal-hydraulics as the primary driver for NTP reactor size is understood to occur at some thrust level below 16klbf. Rather than immediately attempting to derive this value, this work extends the baseline for the minimum feasible LEU-NTP engine by successfully converting the smallest accepted HEU-NTP design, a SNRE-based, 7.5klbf reactor model, to its LEU equivalent (Schnitzler et al., 2011).

The following work begins with a description of the standard and small-scale HEU-NTP SNRE-based reactor configurations and their major similarities and differences. Following this background, the Methodology section details the three-phase approach of recreating Schnitzler’s 7.5klbf HEU-NTP model, converting it to LEU, and then performing various thermal-hydraulic and neutronic analyses to verify the design’s feasibility. The numerical outputs from this process are then presented in the Results and further summarized in the Conclusion section. Additional parametric considerations that were not implemented in the final 7.5klbf LEU-NTP design are discussed in the Appendix A.