Dmitry V. Paramonov

Development and Comparison of a TOPAZ-II System Model with Experimental Data

An integrated model of the TOPAZ-II space nuclear reactor system is developed and compared with measurements from the TOPAZ-II, V-71 unit tests. For a given reactor thermal power, the model calculates the coolant flow rate, temperature, and pressure throughout the system; load electric power; and overall system efficiency. Model predictions showed good agreement with the experimental data. The calculated coolant temperatures and pressure are within 15 K (< 2%) and 12% of the measurements, respectively. Analysis showed that at the nominal operating thermal power of the system (115 kW), and NaK coolant is highly subcooled. The largest subcooling of 365 K occurs at the exit of the electromagnetic pump, where coolant pressure is highest, and the lowest subcooling of 275 K occurs at the exit of the reactor core, where coolant temperature is highest.

Start‐up simulation of a thermionic space nuclear reactor system

The Thermionic Transient Analysis Model (TITAM) is used in this paper to simulate the start‐up of the TOPAZ‐II space nuclear power system in orbit. The start‐up procedures simulated herein are assumed for the purpose of demonstrating the capabilities of the model and may not represent an accurate account of the actual start‐up procedures of the TOPAZ‐II system. The temperature reactivity feedback effects of the moderator, UO2 fuel, electrodes, coolant, and other components in the core are calculated and their effects on the thermal and criticality conditions of the reactor are investigated. Also, estimates of the time constants of the temperature reactivity feedback for the UO2 fuel and the ZrH moderator during start‐up, as well as of the total temperature reactivity feedback as a function of the reactor steady‐state thermal power, are obtained.

Test results of Ya-21u thermionic space power system

The Soviet-made TOPAZ-II space nuclear power system unit designated Ya-21u underwent a total of 15 tests both in the Union of Soviet Socialist Republic (USSR) (1989--1990) and in the US (August 1993 to March 1995) for a cumulative test/operation time of 7681 h at conditions far exceeding design limits. These tests included steady-state operation at different power levels, fast start-ups and power optimizations, and shock and vibration tests. Test results are presented and analyzed. Results indicate a gradual change in the performance parameters such as the optimum cesium pressure and optimum load voltage. The electric power and conversion efficiency of the unit at an input thermal power of 105 kW decreased from 3.7 kW (electric) and 4% in the test in the USSR to 2.13 kW (electric) and 2.3% in the last test in the US. A discussion and qualitative assessment of potential causes of the performance changes of the Ya-21u unit are given.

Transient analysis and startup simulation of a thermionic space nuclear reactor system

The thermionic transient analysis model is used to simulate the startup of the TOPAZ-2 space nuclear power system in orbit. The simulated startup procedures are assumed for the purpose of demonstrating the capabilities of the model and may not represent an accurate account of the actual startup procedures of the TOPAZ-2 system. The temperature reactivity feedback effects of the moderator, UO[sub 2] fuel, electrodes, coolant, and other components in the core are calculated, and their effects on the thermal and criticality conditions of the reactor are investigated. Also, estimates of the time constants of the temperature reactivity feedback for the UO[sub 2] fuel and the ZrH moderator during startup, as well as of the total temperature reactivity feedback as a function of the reactor steady-state thermal power, are obtained.

An Integrated Model of the TOPAZ-II Electromagnetic Pump

A detailed model of the electromagnetic pump of the TOPAZ-II space nuclear reactor power system is developed and compared with experimental data. The magnetic field strength in the pump depends not only on the current supplied by the pump thermionic fuel elements in the reactor core but also on the temperature of the coolant, the magnetic coil, and the pump structure. All electric and thermal properties of the coolant, wall material of the pump ducts, and electric leads are taken to be temperature dependent. The model predictions are in good agreement with experimental data.

Effect of oxygen on the operation of a planar thermionic converter for isothermal and isoflux heating conditions

Abstract In this paper, the effects of oxygen on the performance of and the emitter material transport in a planar thermionic converter were investigated. The oxygen and cesium partial pressures in the interelectrode gap were varied parametrically, and their effects on the operational characteristics of the converter were quantified for both isothermal and isoflux emitter heating conditions. With isothermal heating, a significant increase in the electric power density in the presence of oxygen (by a factor of 3–10, depending on the emitter temperature and the initial composition of the collector surface) was predicted. The load voltages corresponding to the maximum output power were ∼0.3–0.5 V lower than those corresponding to the optimum efficiency. With an isoflux emitter heating of 20 W/cm 2 , the introduction of oxygen into the interelectrode gap of a converter with an initially pure tungsten collector increased the electric power density by only ∼19%; however, no improvement occurred for an initially oxidized tungsten collector.

Effect of oxygen on performance and mass transport in a single-cell thermionic fuel element

The introduction of tracer amounts of oxygen into the interelectrode gap of a thermionic converter has been shown to improve converter performance. Excess oxygen, however, increases the loss rate of emitter material, reducing the converter performance and shortening its lifetime, owing to the increase in the effective emissivity of the electrodes, the change in the collector work function, and the deposition of emitter material oxides on spacers and insulators. In this paper, a model was developed which calculated the emitter material loss rate, composition of the emitter material deposits on the collector surface and investigated the effect of performance of a single-cell thermionic fuel element (TFE) in the presence of oxygen and cesium oxides in the interelectrode gap. The amount of oxygen and the cesium pressure in the interelectrode gap were varied parametrically and the TFE volt-ampere characteristics, and axial distributions of current density and emitter material loss rate along the TFE were calculated.

An analysis of Ya‐21U thermionic fuel elements test results

The Soviet made Ya‐21U unit of TOPAZ‐II space nuclear power system underwent numerous tests for almost 8000 hours, both in the USSR and in US. A change of the Ya‐21U unit performance was measured during these tests. In an attempt to identify the most probable causes of this change in performance, test results of the working section Thermionic Fuel Elements (TFEs) were examined and their volt‐ampere characteristics analyzed with the help of the Thermionic Transient Analysis Model (TITAM). Results showed significant increase in the effective emissivity and the cesiated work functions of the electrodes which caused both the emitter temperature and the output electric power to decrease. Impurities and air incursion in the interelectrode gap had changed surface properties of the electrodes, causing the average effective emissivity of the Ya‐21U unit TFEs to be ∼0.03–0.035 higher than for as fabricated TFEs. The cesiated work functions of the electrodes were higher than that for clean electrode materials, causi...

Analysis of Ya-21u thermionic fuel elements

The Ya-21u unit of the Soviet-made TOPAZ-II power system has recently been tested at the Thermionic Evaluation Facility in Albuquerque, New Mexico. A change in the unit performance was measured during these tests. In an attempt to identify the causes of this change performance, data were examined and used to estimate surface properties of electrodes of thermionic fuel elements (TFEs) of the power system. The effective emissivity was estimated at {approximately}0.03 to 0.035 higher than for as-fabricated TFE and cesiated work functions of the electrodes, which were higher than for as-fabricated TFEs. These changes in the effective emissivity and cesiated work functions, caused by gaseous impurities and air incursion in the TFEs interelectrode gap, lowered both the emitter temperature and the output load voltage thus contributing to the measured decrease in output power.

Analysis of emitter material transport in thermionic converter

Output power and efficiency of a thermionic converter depend on temperatures, cesiated work functions, and emissivities of electrodes as well as the interelectrode gap size. Operation lifetime of a thermionic converter is directly related to the values as well as the stability of these parameters, which can be seriously altered by the transport of emitter material to the collector during operation. Loss rate of tungsten, a preferred emitter material, by sublimation at typical operating temperatures is small (about 3×107 atom/cm2 sec at 2000 K). The loss rate, however, can be several orders of magnitude higher in the presence of gaseous contaminants. Accelerated transport of emitter material to collector surface changes the effective emissivity and work functions of the electrodes, resulting in performance degradation. A phenomenological model was developed to simulate emitter material transport to the collector in the presence of oxygen, water vapor, and carbon oxide contaminants. The model accounts for i...