Energy Conversion Efficiency-Mass Optimization and Transient Analysis of Megawatt-level Space Nuclear Power Systems

ABSTRACT

The space nuclear power system (SNPS), which integrates a gas-cooled reactor with a closed Brayton cycle, is a leading candidate for megawatt-level aerospace power applications due to its high energy conversion efficiency, substantial power output, and independence from solar illumination. However, the design of such systems faces substantial challenges, including narrow safety margins, complex optimization requirements, strong component coupling, and rapid dynamic responses. These issues are primarily driven by strict payload constraints during space launch, the inherent trade-offs between efficiency and mass, and the unique operational environment of outer space. Despite its potential, the development of megawatt-class SNPS remains in its early stages, with limited understanding of the factors influencing system performance, safety evaluation, and operational control. This study addresses these critical challenges of high-power SNPS by focusing on the dual-objective optimization of efficiency and mass, along with the transient behavior and safety performance under various operational scenarios. First, a high-fidelity and rapid mass estimation model was developed, covering key components such as the reactor, shielding, and energy conversion systems. A system-level design and optimization tool, Megrez, was created, incorporating the mass estimation model, thermodynamic model and a non-dominated sorting genetic algorithm to achieve efficiency-mass co-optimization. Second, the RELAP5 code, originally developed for light-water reactors, was modified to accommodate the specific characteristics of SNPS, including reactor thermal-hydraulics, turbomachinery, and counterflow heat exchangers. Finally, using Megrez, the modified RELAP5, and the OpenMC neutron transport code, a 1 MWe SNPS was designed and optimized, including detailed neutronic and thermal-hydraulic design of a space-based gas-cooled reactor. Transient scenarios such as pipe rupture, reactivity insertion, and shaft failure were simulated, yielding critical insights into reactor power behavior and peak fuel temperature evolution. The results provide essential support for safety analysis, control strategy development, and robust system design of SNPS technologies.

Scalar Dispersion in Outdoor and Indoor Flow Applications

ABSTRACT

Hazardous air pollutants released in public spaces are a threat to national security and can have long-lasting repercussions on the public health and economy. Managing the consequences of these incidents often requires time-sensitive decisions that need to be supported by science. As such, it is important to have the capability to model the scalar dispersion of hazardous air pollutants accurately and quickly. In this presentation, I will discuss the key findings of experimental campaigns which focus on two relevant applications: the scalar dispersion of a ground-level point-source in (1) turbulent boundary layer flows and (2) a supply ventilated empty room model. Using an improved understanding on the scalar transport mechanism in these applications, the application of turbulent diffusivity models to point-source dispersion problems will be discussed.