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.
MICROSTRUCTURAL EVOLUTION OF HIGH DENSITY W-CERMETS EXPOSED TO FLOWING HYDROGEN AT TEMPERATURES EXCEEDING 2000 K
Nuclear thermal propulsion (NTP) shows promising potential for crewed space exploration by enabling high specific impulse and thrust. The development of NTP systems presents unique fuel material challenges due to requirements for high operating temperatures, exceeding 2500 K, and chemical compatibility with a hydrogen (H2) propellant (coolant) during operation. NASA has been investigating ceramic-metal (cermet) fuels due to their high temperature capability and H2 compatibility of the refractory metal matrix. For this study, subscale tungsten (W) cermet specimens, with 60 vol% zirconia surrogate (ZrO2), were consolidated via spark plasma sintering (SPS). Sintered samples were tested at 2000°C for 60 minutes and 2500 ̊C for 5 minutes in flowing H2. After testing, as produced and tested specimens were cross sectioned for microstructural examination using optical microscopy, scanning electron microscopy, and microhardness in order to understand the stability of the bulk cermet microstructure under the different conditions. While the specimens retained structural integrity throughout testing with minimal mass loss, the microstructural investigation revealed H2 attack and migration of ZrO2 particles. Overall, the W matrix showed minimal grain growth and embrittlement as a result of testing.
Experimental study on bond behavior between high-strength grout and deformed steel bars
Abstract Engineers use high-strength grout to connect precast concrete members for its low shrinkage and good fluidity. This paper designed 87 central pull-out specimens using the control variable method to investigate the influence of factors on the rebar-grout bond property such as compressive strength, steel grade, bar diameter, transverse stirrups, and bond length. The test result showed that improving grout compressive strength (85.8 MPa–122.7 MPa) cannot improve the bond strength, which was different from the previous conclusions of concrete pull-out tests. Simplified formulas were presented for calculating the characteristic values of bond strength, residual strength, peak slip, and relative slip of load-holding platform based on parameter analysis. Then, a constitutive model was proposed to predict the full-range bond stress-slip curve of high-strength grout and steel bar specimens from bearing to failure. Finally, FEA models were established to evaluate the applicability and accuracy of the constitutive model.