Theoretical studies into operating capacity of a thermionic power-generating element with a gas-venting device

This survey (relating to the multi-element thermionic power-generating assemblies with increased electrical power density) addresses the issues of assuring a long-run operating capacity of the fuel-emitting assembly (FEA) of a power-generating element (PGE). It was shown that with the use of dioxide fuel the life of a high- temperature pressure-tight PGE with power density above 7 W/cm2 is less than a thousand hours. To assure its longevity, one needs to set up the removal of gaseous fission products (GFP) through a special gas-venting device (GVD) in the form of a jet tube. Fuel re-condensation process results in a significant transformation of the core thermal field and a possibility of the GVD sealing up and turning FEA into a pressure-tight assembly with limited life. The paper reviews methods for computing the thermal field for a FEA core with a GVD. Theoretical formulas are derived for calculating the possibility of FEA getting sealed up with fuel condensate both outside the jet tube and inside the GVD tube. The need is demonstrated to route the GVD tube to the hot end of the PGE (without switching jumper). Theoretical rationale is provided for developing a jet tube with a high ratio of its length to the orifice diameter, and the feasibility of such a jet tube is demonstrated. Nomograms were obtained for determining the boundaries of the PGE serviceability domain in the generalized variables space as a function of temperature drop along the GVD tube and relative resistance of the jet tube and the space between the ends of adjacent PGEs. Formulas are derived for calculating removal of fuel vapors through GVD together with GFP. Nomogram is obtained for calculating PGE residual life in case of FEA getting sealed off in the course of its operation.