Areas of Research



The 2020 CSNR Summer Fellowship Program, June 8 through August 14, 2020, will be the 15th year of this intense, innovative exploration of applications of nuclear energy in space.  Previous years have explored a variety of research topics such as mobile lunar base design, assessment of dual-mode nuclear thermal rockets, advanced radioisotope power source design and mapping Asteroid Belt resources.  In the last three years we have focused our study topics on near-term challenges in the production and operational behavior of Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) units.

The renewed interest in Nuclear Thermal Propulsion and returning to the Moon, presents several interesting challenges in materials science and nuclear engineering.  Nuclear Thermal Propulsion provides a specific impulse twice that of chemical rockets and thus enables faster trips and more payload on missions to Mars and beyond.

Two challenges we’ll tackle during the 2020 CSNR Summer Program will involve the operational dynamics of an NTP reactor:

  1. In order for an NTP rocket to use the hydrogen propellant most efficiently, rapid startup and shutdown thermal transients are required. In this first challenge we’ll focus on the phenomena of rapid shutdown which haven’t been widely considered.  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 during shutdown at elevated temperature for diffusion prevents the hydrogen from diffusing out of the tube, so it 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.
  2. In order to improve heat transfer (and perhaps provide an escape path for hydrogen at shutdown) some NTP fuel designs have proposed the inclusion of very small tubes (0.01 mm diam, 0.001 mm wall) in the fuel.  In this second challenge we’ll model thermal-mechanical effects of NTP carbide fuel thermal expansion leading to ratcheting creep of the W-Re tubes. This will be performed in W-Re alloys with and without a carbide dispersion.  This work will provide an opportunity to explore the innovative designs and materials being considered for nuclear thermal propulsion.

Providing radioisotope fuel for future NASA missions to Mars and the outer solar system is a continuing issue:

  1. Studies during the last three years have used monte carlo codes to assess and optimize the production of Pu-238 in the Advanced Test Reactor at INL.  Two questions that haven’t been answered are the use of targets with >50% neptunium oxide or use of neptunium metal to increase the throughput of Neptunium-237 for irradiation in the reactor. Strategies for delaying the chemical processing of the targets will be explored in order to decrease the amount of Pu-236 (and its decay isotopes) to a level within acceptance limits. These strategies may be necessary to use additional interior ATR positions with a harder neutron spectrum.   This third challenge for the summer of 2020 will involve more use of time-dependent generation and decay codes like ORIGEN.

 

The research topics are chosen before the beginning of the summer so that Summer Fellows are able to investigate some of the topics’ issues and history before arriving in Idaho Falls.  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.