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.
Multidisciplinary Simulation of Graphite-Composite and Cermet Fuel Elements for NTP Point of Departure Designs
This paper compares the expected performance of two Nuclear Thermal Propulsion fuel types. High fidelity, fluid/thermal/structural + neutronic simulations help predict the performance of graphite-composite and cermet fuel types from point of departure engine designs from the Nuclear Thermal Propulsion project. Materials and nuclear reactivity issues are reviewed for each fuel type. Thermal/structural simulations predict thermal stresses in the fuel and thermal expansion mis-match stresses in the coatings. Fluid/thermal/structural/neutronic simulations provide predictions for full fuel elements. Although NTP engines will utilize many existing chemical engine components and technologies, nuclear fuel elements are a less developed engine component and introduce design uncertainty. Consequently, these fuel element simulations provide important insights into NTP engine performance.
Fabrication and properties of dense beryllium oxide
Abstract Nuclear grade beryllium oxide powders from commercial suppliers may be pressed at temperatures of 1500° to 1900° C and under pressures from 2000 to 6000 psi in graphite dies into a variety of shapes having densities in the range 98 to 100 % of the theoretical value. Addition of 1 % MgO facilitates the process and improves the rate and uniformity of densification. The compaction of BeO as a function of pressure, temperature and time may be described by a flow relation involving a viscosity and a critical shear stress for grain boundary movement. Thermal expansion is isotropic to about 1700° C and is well described to 2000° C by a quadratic function of the temperature. The specific heat of BeO at 1500° C is 0.52 cal/g-°C, slightly above the Dulong and Petit value. From 500° to about 1800° C the thermal conductivity decreases as 1 T , from a high value at room temperature of about 0.50 cal/cm-°C for dense material. A decrease in density of 5 % diminished the thermal conductivity by approximately 13 %. Information given earlier on the creep, elastic and anelastic properties of dense material has been extended, and relations have been employed showing that significant fractions of thermal stresses encountered in practice may be relaxed if given time. Anelasticity due to grain boundaries also appears to confirm the view on flow during densiflcation in hot pressing. A preliminary estimate of rupture strength for completely dense material with grain size of approximately 35 μ is 36 000 psi at room temperature. Lowering the density 1 % reduces the strength about 4100 psi. The status of knowledge of other properties is reviewed briefly.