Areas of Research

The 2022 CSNR Summer Fellowship Program, June 6 through August 12, 2022, will be the 17th 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 2022 CSNR Summer Program as a hybrid internship. 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. Housing will be provided to interns who choose the in-person internship.

The challenges we’ll tackle during the 2022 CSNR Summer Program will involve the operational dynamics of an NTP rocket and the behavior of the fuel in the NTP reactor.

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.

  • A mission to Mars lasting 300 days requires four burns of the NTP system. 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. During operation the hydrogen propellant/coolant diffuses into the W-Re coolant tubes, where temperatures exceed 2700 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.
  • During the first NTP burn cycle the flow tubes will experience radial creep in the flow tubes at the high temperature end where temperatures exceed 2000K driven by the internal pressure. This creep will proceed until any gap between the flow tube and the fuel element is closed and intimate contact will occur. At this point the fuel will reinforce the flow tubes in resisting the hoop and axial stresses and reduce the creep deformation rates in the flow tubes. The intimate contact will eliminate the need for radiation heat transfer between fuel and flow tubes. The flow tubes and the uranium, zirconium carbide fuel elements have substantially different coefficients of thermal expansion, elastic modulus and Poisson’s ratio, so that the cooling of the flow tubes after reactor shutdown will create substantial thermal stresses. The thermal stresses in the flow tube will hit a maximum at the exit end when the temperature has cooled to 600K, where a large delta T of cooling has occurred during shutdown. This is also the temperature where the ductile to brittle transformation of irradiated refractory alloysbegins to occur.

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.