A Comparison of Materials Issues for Cermet and Graphite-Based NTP Fuels
This paper compares material issues for cermet and graphite fuel elements. In particular, two issues in NTP fuel element performance are considered here: ductile to brittle transition in relation to crack propagation, and orificing individual coolant channels in fuel elements. Their relevance to fuel element performance is supported by considering material properties, experimental data, and results from multidisciplinary fluid/thermal/structural simulations. Ductile to brittle transition results in a fuel element region prone to brittle fracture under stress, while outside this region, stresses lead to deformation and resilience under stress. Poor coolant distribution between fuel element channels can increase stresses in certain channels. NERVA fuel element experimental results are consistent with this interpretation. An understanding of these mechanisms will help interpret fuel element testing results.
Preliminary conceptual design of a new moderated reactor utilizing an LEU fuel for space nuclear thermal propulsion
Abstract Nuclear Thermal Rocket (NTR) propulsion is a viable and meritorious option for human exploration into deep-space because of its high thrust, improved specific impulse, well established technology, bimodal capability, and enhanced mission safety and reliability. The NTR technology has already been investigated and tested by the United States of America and Russia and the former Soviet Union. The representative Nuclear Engine for Rocket Vehicle Applications (NERVA) type reactors traditionally used Highly Enriched Uranium (HEU) fuels, shaped in hexagonal fuel element geometries because of the importance of making a high power reactor with a minimum size. Although the HEU-NTR designs are the best choice in terms of rocket performance and technical maturity, they inevitably provoke nuclear proliferation obstacles not only for all research and development activities by civilians and non-nuclear weapon states but also for potential commercialization. To overcome the security issues due to HEU, the non-proliferative, small-size NTR engine with low thrust levels of 41 kN–53 kN (9.2 klb f  ∼ 11.9 klb f ), Korea Advanced NUclear Thermal Engine Rocket utilizing a Low-Enriched Uranium fuel (KANUTER-LEU), is being designed for future generations. Its design goals are to make use of an LEU fuel for its fairly compact core, but to minimize the rocket performance sacrifice relative to the traditional HEU-NTRs. To achieve these goals, a new space propulsion reactor is conceptually designed with the key concepts of a high uranium density fuel with resistance against high heating and H 2 corrosion, a thermal neutron spectrum core, and a compact and integrated fuel element core design with protective cooling capability. In addition, a preliminary design study of neutronics and thermal-hydraulics was performed to explore the design space of the new LEU-NTR reactor concept. The result indicates that the innovative reactor concept has great potential, both to implement the use of an LEU fuel and to create comparable rocket performance, compared to the existing HEU-NTR designs.
The plastic deformation of uranium oxides above 800° C
Abstract Sintered uranium oxide of composition UO2.06 or UO2.06 and density exceeding 95 % theoretical can be deformed plastically at about 800° C, but UO2.oo becomes plastic only above ≈ 1 600° C. The bendcreep characteristics of UO(2+x) are similar to those of metals inasmuch as they obey the expression g3 = A sinh ( σ σ0 ) exp ( Q RT ) where g3 is the strain-rate, σ the stress and T the absolute temperature. A, σo and Q are material constants. The activation energy of flow, Q, varies with the oxygen/uranium ratio, being about 65 k cal for UO2.16, 72 k cal for UO2.06 and >/ 95 k cal for UO2.00. This latter value for UO2.00 is comparable with the activation energy of 95–120 k cal experimentally determined for the diffusion of uranium in UO2.0.