Thermal-hydraulics and thermionic analysis of TOPAZ-2 thermionic fuel element using HXTOPAZ

Abstract

Extensive experimental data from TOPAZ-2 thermionic fuel element (TFE) provides an excellent opportunity for benchmarking of HXTOPAZ. Previous modeling of the TOPAZ-2 TFE with TFEHX does not includes the heat loss through the ends of the TFE and the effects of the performance of the scandium oxide spacers. Heat removal through the electrical leads changes the axial power distribution and, therefore, the emitter and collector temperatures hereby affecting the thermionic process. Heat conduction through spacers also decreases the emitter temperature in the proximity of the spaces. However, results from TFEHX showed a good agreement for a certain range of the thermal input power. To take into account these phenomena, a new computer code called HXTOPAZ is under development to include the effects of the electrical leads and the scandium oxide spacers. The new model uses both axial and radial power distribution to calculate the temperature field across the TFE. The Russian space reactor TOPAZ-2' provides a good opportunity for evaluating the effects of various important parameters in the design of the reactor and thermionic fuel elements (TFEs). The use of the computer code such as HXTOPAZ allows an easy and quick evaluation of the TFE performance, giving an estimate of the results expected during actual tests. The coupled interactions between the tehrrnionic emission process and the conductive, radiative, and convective heat transfer processes involved with removing heat from the TFE requires detailed modeling. First, the heat generation within the nuclear fuel is generally not uniform along the TFE due to neutron leakage. It typically can be represented by a chopped cosine distribution function. The radial power distribution is also non-uniform due to the self-shielding effect. Second, since the coolant temperature increase as it flows the length of the TFE, the temperature of the cladding surface will be higher at the exit of the TFE coolant channel than at its inlet. Also, heat is transferred across the emittercollector gap via three mechanisms: conduction through the cesium vapor and through the spacers, thermal radiation exchange between the emitter and collector surfaces, electron transport across the emitter-collector gap. Other effects which need to be considered include the dependence of the thermionic emission on the cesium vapor pressure within the interlectrode gap, the energy losses due to electrical resistances of the emitters and collector materials, and the conduction of heat out the electrical leads of the TFE. The additional complication of evaluating the transient performance of these systems during startup, shutdown, and during accidents also requires attention. Previous studies of the TOPAZ-2 TFE with TFEHXL3 assumes no heat losses through the electrical leads, and does not include the scandium oxide spacers in the interelectrode gap. The Sc,O, spacers are distributed along the fuel length of the TFE assuring the separation of the emitter and collector. This allows easy removal of the UO, for replacement with electrical heaters, and prevents shorting out the circuit.