Impact of Real Gas Properties on Multiphysics Modeling of an NTP Fuel Element

Advanced nuclear reactors are under investigation as a possible solution to a large array of modern challenges. These areas of research span from microreactors for remote cities or military bases to small modular reactors to support the sizable energy requirement of training artificial intelligence algorithms. Additional applications of current interest are space reactors to provide large amounts of electric power on the surface of celestial bodies and to propel spacecraft for crewed and science missions. The National Aeronautics and Space Administration (NASA), in particular, presently researches three primary nuclear reactor technologies: nuclear thermal propulsion (NTP), nuclear electric propulsion (NEP), and fission surface power (FSP). NASA is interested in NTP as a near-term solution for crewed interplanetary space travel to reach Mars by 2039. This research is led through the Space Nuclear Propulsion (SNP) project at NASA’s Marshall Space Flight Center (MSFC). This technology uses an extremely high temperature nuclear reactor (~3000 K) as a heat exchanger to increase the temperature of the propellant to peak values. Hydrogen is the most desirable propellant for NTP since it enables the highest specific impulse (i.e., “fuel efficiency”) of the spacecraft. Specific impulse is directly proportional to the effective molecular weight of the propellant as well as its temperature in the nozzle chamber. Yielding these extreme fluid temperatures presents numerous challenging multiphysics problems for the reactor to handle during nominal and transient operation.

Risk reduction for the engine, spacecraft, and crew are the paramount concern for NASA’s technology maturation efforts. Traditional technology maturation involves an intensive development strategy that includes testing each component until failure prior to incorporation in the final product. However, technology advancement for space reactor technologies requires high-fidelity multiphysics and system-level models given the harsh environment for experimental testing, significant cost burden, and regulatory hurdles for nuclear safety. Reduced order and higher fidelity models are both utilized depending on the application. This work aims to investigate how a finite element model for a reference NTP fuel element performs when the fluid property database assumption is varied. Many research fields use ideal gas fluid properties given their availability, ease of use, and validity in the operability region of interest. NTP, in contrast, experiences large temperature and pressure variations throughout the reactor and engine cycle, necessitating a better data repository that includes real gas and dissociation effects. To simulate these effects on overall thermal hydraulic system performance, a reference reactor configuration and a robust modeling framework must be established. Subsequent sections of this introduction discuss the requirements for modeling this test case to achieve the desired results.