b'Closed-form Solution for theModeling the fundamental physics related to nuclear thermal propulsion makes Movement of Hydrogen inspace travel faster and safer.Nuclear Thermal PropulsionF uture human exploration of Mars and the outer solar system will require the use of nuclear energy to reduce travel time and the exposure of the crew to Reactor Fuels duringenergetic protons (i.e., cosmic rays and solar flares) in space. The propulsion TransientOperation concept that has been developed the most is nuclear thermal propulsion, a very high temperature reactor in which hydrogen is heated from 20 K to about 2700 K before exiting through a nozzle at about 10 km/s, producing a specific impulse of ~900 s, nearly double that of conventional rockets. Since 2018, the U.S. National Aeronautics and Space Administration has been sponsoring renewed research, some at INL, to develop nuclear fuels for such very high temperature, high specific impulse reactors.TOTAL APPROVED AMOUNT:The main project used data and correlations from the nuclear engine for rocket $275,000 over 1 year vehicle application program to develop a model for the solubility of hydrogen in the PROJECT NUMBER:tungsten-rhenium coolant tubes as a function of temperature and time, then to use 21P1063-001 that model to estimate the pressure of hydrogen in bubbles along grain boundaries in the coolant flow tubes as a function of time and of location within the reactor PRINCIPAL INVESTIGATOR:during the shutdown transient. The goal of the model is to provide criteria for the Stephen Johnson ramp-down of hydrogen coolant flow needed to prevent decohesion of grains in CO-INVESTIGATORS: the coolant tubes. If the hydrogen coolant flow remains too high following control Brad Kirkwood, INL drum rotation, then the reactor will cool rapidly, trapping hydrogen along the grain Stephen Herring,boundaries in the tubes and failing the tubes. If the hydrogen coolant flow decreases Center for Space Nuclear Research abruptly following control drum rotation, then there will be insufficient transport of the reactor decay heat and the fuel, already operating near its melting point, may COLLABORATOR: overheat and deform. Thus, it is critical to know the boundaries for safe shutdown of Center for Space Nuclear Research the nuclear thermal propulsion reactor.A second, smaller project was to develop a model for the highbay room at the Materials and Fuels Complex where the fueled Multi-Mission Radioisotope Thermoelectric Generator is loaded into its shipping cask. The purpose of this model is to allow the experimental measurement of the neutron and gamma fluxes and spectra produced by the Multi-Mission Radioisotope Thermoelectric Generator within the concrete-walled highbay and the extrapolation of those measurements to the materials and configuration of the spacecraft and surroundings during its mission. This capability is particularly important where there are sensitive instruments, such as optical spectrometers or pulsed neutron sources on a drone in a planetary atmosphere such as is being proposed for the Dragonfly mission to Titan, the largest moon of Saturn.52'