Robert O'Brien

An equation was developed to predict fracture toughness of green powder compacts. The model combines crack tip toughness predicted by Kendall's model with crack tip shielding due to bridging of moisture meniscuses across the crack. The model predicts that crack tip shielding due to moisture should be dominant. Fracture tests on ceria green pellets verified that storing pellets at a high relative humidity (98%RH) for an extended period of time led to fracture strength more than double those stored at lowerRH. However, at lowerRHthere is no significant increase in fracture strength with increasedRHas predicted by the model. The lower strength at lowRHis due to insufficient capillary and surface forces but may also be related to the lack of sufficient adsorbed moisture to form bridging meniscuses. The high green strengths achieved by storing pellets at a highRHsuggest a method of strengthening green parts without adding binder.

The feasibility of the fabrication of tungsten based nuclear fuel cermets via Spark Plasma Sintering (SPS) is investigated in this work. CeO₂ is used to simulate fuel loadings of UO₂ or Mixed-Oxide (MOX) fuels within tungsten-based cermets due to the similar properties of these materials. This study shows that after a short time sintering, greater than 90 % density can be achieved, which is suitable to possess good strength as well as the ability to contain fission products. The mechanical properties and the densities of the samples are also investigated as functions of the applied pressures during the sintering.

The requirements for performance by planetary exploration missions are increasing. Landing at a single location to take data is no longer sufficient. Due to the increasing cost, the missions that provide mobile platforms that can acquire data at displaced locations are becoming more attractive. Landers have also had limited range due to power limitations, limited lifetime of subsystems, and the inability to negotiate rough terrain. The Center for Space Nuclear Research has designed an instrumented platform that can acquire detailed data at hundreds of locations during its lifetime — a Mars Hopper. The Mars Hopper concept utilizes energy from radioisotopic decay in a manner different from any existing radioisotopic power source — as a thermal capacitor. By accumulating the heat from radioisotopic decay for long periods, though, the power of the source can be dramatically increased for short periods. Thus, a radioisotopic thermal rocket is possible. The platform will be able to ‘hop’ from one location to the next every 2—3 days with a separation of 10—20km per hop. Each platform will weigh around 50kg unfuelled which is the condition at deployment. Consequently, several platforms may be deployed on a single launch from Earth. With a lifetime estimated at 10 years, the entire surface of Mars can be mapped in detail by a couple dozen platforms. In addition, hoppers can collect samples and deliver them to the Mars Science Laboratory for more detailed analysis. Furthermore, the basic platform can be deployed to Europa, Titan, and even Venus with alterations — the propulsion system and operations essentially will be the same.