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.
Design research for the porous hexagonal prism shaped fuel element adopted in nuclear thermal propulsion reactor based on neutronics and thermal-hydraulics coupled method
Abstract The Porous hexagonal prism shaped fuel element has the advantages of simple structure, high reliability and good practicability. It has been widely adopted by many nuclear thermal propulsion (NTP) reactor designs. A design method is present in this paper for determining key geometrical parameters of the porous hexagonal prism shaped fuel element. Four sets of fuel elements geometrical parameters are involved, including cases of 127, 169, 217 and 271 cooling passages. And they adopt different flow hole void fractions to have the same distance between adjacent cooling passages, this is done in order to ensure the reliability of fuel element. Three-dimensional neutronics/thermal-hydraulics coupled analyses of them are conducted to study their thermal hydraulic performances. The distributions of power density, coolant mass flow, temperature, coolant velocity and total pressure are obtained. The results indicate that the fuel element with larger numbers of cooling passages and smaller flow hole void fraction tends to have more uniform flow distribution, and the heat transfer efficiency of fuel element is improved at the same time. But those performance improvements by increase in numbers of cooling passages become much smaller when the number of cooling passages exceeds a certain value. Meanwhile, each additional of cooling passages would lead to almost doubled increase in the flow resistance of fuel element. Finally, through contrastive analyses, one set of geometric parameters is considered as the optimal choice which could ensures excellent heat transfer performance and relatively low flow resistance. This study could help set appropriate geometrical parameters for porous hexagonal prism shaped fuel element, thus being beneficial to improve the thermal-hydraulic performance of NTP reactor.
Numerical Simulation of Thermal Sensitivity for Explosives
In order to compare the thermal sensitivity of several typical explosives, this article solved numerically one-dimensional transient heat conduction equation including thermal decomposition for explosives with finite difference method. The explicit difference equations under different boundary conditions were established, and the corresponding stability criterion was derived. The ignition times for typical explosives were calculated, which was used to evaluate thermal sensitivity of explosives.