Areas of Research

The 2021 CSNR Summer Fellowship Program, June 7 through August 13, 2021, will be the 16th year of this intense, innovative exploration of applications of nuclear energy in space.  Because of the continued course of the Covid-19 pandemic, we will be conducting the 2021 CSNR Summer Program virtually, as we did during the summer of 2020. We are also increasing the stipends being offered this year for the 10-week session to $6500 for undergraduate juniors and seniors, $8000 for MS candidates and $9000 for PhD candidates. Because the summer program is being conducted virtually, we will not be able to provide housing, as we had until 2019.

Two challenges we’ll tackle during the 2021 CSNR Summer Program will involve the operational dynamics of an NTP rocket and the development of a tool to quantify the radiation field surrounding a drone above the surface of Titan, Saturn’s largest moon.

Future human exploration of Mars and the outer solar system will require the use of nuclear energy to reduce travel time and thus the exposure of the crew to energetic protons (i.e. cosmic rays and solar flares) in space. Such a reduction in travel time cannot be achieved using conventional rockets because of the lower exhaust velocities of the combustion gasses. There are several concepts for attaining higher exhaust velocities (i.e. higher specific impulse) through the use of nuclear energy. All of these concepts are in an early stage of development. Some nuclear space propulsion concepts involve acceleration of ionized particles or plasmas by electrostatic fields. While such concepts achieve high specific impulse, the thrust produced is so small that a long duration burn is required to attain the change in velocity needed. The propulsion concept that been most developed is Nuclear Thermal Propulsion (NTP), 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 about 900 s, nearly double that of conventional rockets. Such reactors were tested in the 1960s and early ‘70s in Nevada with varying degrees of success. Since 2018 NASA has been sponsoring renewed research, some at the INL and CSNR, to develop nuclear fuels for such very high temperature, high specific impulse reactors.

  1. During the summer of 2020, a preceding project developed neutronics models of the NTP reactor for both Serpent and MCNP for a mission to Mars lasting 300 days during which four burns of the NTP system would be required. Since these burns last only 20 or 30 minutes, the fission product inventory of the reactor is far from equilibrium and dependent on the previous operating history. The Serpent model was used to determine the build-up of fission products and the decay heat as a function of time and location. MCNP was used to check the Serpent results and Origen was used to model the fission- and activation-product decay.

    During operation the hydrogen propellant/coolant diffuses into the W-Re coolant tubes, where temperatures exceed 2500 K. At these temperatures the solubility of W-Re for hydrogen is high and a significant amount of hydrogen is absorbed. When the reactor is shut down at the end of its required burn, the coolant channels cool rapidly because of residual hydrogen flow in the channels and through blackbody radiation to space. As the flow channel tubing cools rapidly, the solubility and the diffusion coefficient of hydrogen in the W-Re tube decrease by several orders of magnitude. The reduced diffusion rates coupled with the short time for diffusion during shutdown at elevated temperature prevents the hydrogen from diffusing out of the tube, so the tube is left with a concentration of hydrogen in its lattice, significantly in excess of its reduced solubility. This creates high pressures within voids, microcracks and grain boundaries of the tubing. Experiments on nuclear fuels and other materials have shown that overly-rapid cooling leads to hydrogen bubbles causing grain boundary decohesion. The goal of this task will be to model this decohesion mechanism to determine the maximum cooling rate in candidate tubing materials to prevent cooling tube failure. Rapid startup and shutdown transients in an NTP reactor impose non-equilibrium thermal conditions during which dissolved quantities of the hydrogen coolant may exceed solubility limits, particular along grain boundaries.

    The proposed 2021 project will build on the 2020 models for the temporal and spatial fission power and decay heat distributions in conjunction with a model developed from experiments during the NERVA program. Using Mathlab, Python and/or Nu, we propose to use the NERVA data and correlations to develop a model for the solubility of hydrogen as a function of temperature and time, then to use that model to estimate the pressure of hydrogen in bubbles along grain boundaries in the tungsten or molybdenum coolant flow tubes as a function of time and of location within the reactor during the shutdown transient. The goal of the model is to provide criteria for the ramp-down of hydrogen coolant flow needed to prevent decohesion of grains in the coolant tubes. If the hydrogen coolant flow remains too high following control drum rotation, then the reactor will cool rapidly, trapping hydrogen along the grain boundaries in the tubes and failing the tubes. If the hydrogen coolant flow decreases 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 overheat and deform. Thus it is critical to know the boundaries for safe shutdown of the NTP reactor.
  2. A second, smaller project is to develop a model for the high-bay room where the fueled MMRTG 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 MMRTG 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. The Dragonfly drone would fly in Titan’s predominantly nitrogen atmosphere and look for signs of present or previous life in the organic chemicals on the surface. Using a MCNP surface source technique developed by an earlier CSNR Summer Fellow, Emory Colvin, the MMRTG is modeled as surface source that would then be surrounded by a model of the spacecraft, which in turn would be inserted into models of the atmospheric and/or surface environment. Using these models together the effects of the surrounding walls can be removed from the experimental gamma and neutron flux and energy spectrum measurements. In this way the effects of a given MMRTG on the instrumentation and viewing field of a particular spacecraft can be accurately calculated.

The Summer Fellows are divided into teams, based on the different fields of expertise needed by each project. The Summer Program gives the CSNR Summer Fellows a chance to explore fields beyond those within their majors, so that nuclear, chemical and aerospace engineers have an opportunity to learn a cross-disciplinary approach to some challenging, real-world problems.

During the summer, the Fellows will be asked to make short presentations of the results of their research and a final oral/written presentation to the laboratory management.