Modeling Topaz-II system performance

The US acquisition of the Topaz-11 in-core thermionic space reactor test system from Russia provides a good opportunity to perform a comparison of the Russian reported data and the results from computer codes such as MCNP (Ref. 3) and TFEHX (Ref. 4). The comparison study includes both neutronic and thermionic performance analyses. The Topaz II thermionic reactor is modeled with MCNP using actual Russian dimensions and parameters. The computation of the neutronic performance considers several important aspects such as the fuel enrichment and location of the thermionic fuel elements (TFES) in the reactor core. The neutronic analysis included the calculation of both radial and axial power distribution, which are then used in the TFEHX code for electrical performance. The reactor modeled consists of 37 single-cell TFEs distributed in a 13-cm-radius zirconium hydride block surrounded by 8 cm of beryllium metal reflector. The TFEs use 90% enriched 235U and molybdenum coated with a thin layer of 184W for emitter surface. Electrons emitted are captured by a collector surface with a gap filled with cesium vapor between the collector and emitter surfaces. The collector surface is electrically insulated with alumina. Liquid NaK provides the cooling system for the TFEs. The axial thermal power distribution is obtained by dividing the TFE into 40 axial nodes. Comparison of the true axial power distribution with that produced by electrical heaters was also performed