Huimin Xue

Transient and load-following characteristics of a fully integrated single-cell thermionic fuel element

Abstract : Practical application of thermionic space nuclear power systems necessitates the investigation of such systems responses to changes in operation and design parameters as well as load-following characteristics. The principle building block in these systems is the thermionic fuel element (TFE). A Thermionic Transient Analysis Model (TITAM) is developed to simulates transient and steady state operations of a fully integrated, single cell TFE. TITAM is used to investigate the responses of the TFE to a step input in reactivity, a change in Cs pressure and/or in the size of the interelectrode gap, a change in the coolant temperature, and a change in load demand. The effects of these parameters on load electric power, emitter temperature, overall conversion efficiency, and load-following characteristics of the TFE are determined. Results show that although nuclear reactors having negative temperature reactivity coefficients are always load-following, TFEs are only partially load-following. However, for a given load electric power need there are several combinations of fission powers and load resistances. Results also show that for TFEs having a large interelectrode gap, it is desirable to conserve Cs by lowering its vapor pressure at the beginning-of-life, since increasing the Cs pressure insignificantly affects the load electric power. However, should fuel swelling, alter operating the reactor for an extended period of time, reduces the width of the interelectrode gap, both the conversion efficiency and the load electric power will decrease. In this case, the load electric power could be restored by increasing the fission power, and only partially by changing the coolant temperature and/or increasing the Cs vapor pressure.

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.

Two-dimensional steady-state and transient analyses of single-cell thermionic fuel elements

A two-dimensional transient model is developed to simulate steady-state and transient operations of single-cell thermionic fuel elements (TFEs). Model predictions are in good agreement with published data to within 4.5 and 5.5% for fission and electrically heated TFEs of the TOPAZ-II type, respectively. In addition, the results of a transient analysis simulating the startup of an electrically heated TFE, following a step function increase in thermal power, are in presented and discussed.

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.

‘‘TITAM’’ thermionic integrated transient analysis model: Load‐following of a single‐cell TFE

TITAM, a dynamic model that simulates transient and steady‐state operation of a fully integrated single‐cell thermionic fuel element (TFE), has been developed. Operation modes investigated include transient response to a step input in reactivity, effects of changing the Cs pressure and/or the width of the interelectrode gap on the load electric power, overall conversion efficiency, and the load‐following characteristics of a TFE. Results show that although the nuclear reactor is always load following, a TFE is only partially load‐following. Results also show that in a thermionic (TI) nuclear power system, which employs single‐cell TFEs having a large interelectrode gap, it is desirable to conserve Cs by lowering its vapor pressure at the beginning‐of‐life, since increasing the Cs pressure insignificantly affects the load electric power. However, should fuel swelling, after operating for an extended period of time, reduce the width of the interelectrode gap, both the conversion efficiency and the load elec...

Simulation of Fission Heated Thermionic Fuel Elements Using Uniform Electrical Heating

An important advantage of the single‐cell, Thermionic Fuel Elements (TFEs) is that they can be tested in a fully assembled nuclear reactor core using electrical heating instead of fission heating. To simulate the effects of the non‐uniform fission power profile, short electrical heaters are inserted inside the emitter tubes to uniformly heat the middle section of the active emitter length. This paper investigates the accuracy of simulating the operation of fission heated, TOPAZ‐II type TFEs using uniform electrical heating. The effects of changing the heater length on the emitter temperature and emission current axial profiles, as well as the load electric power, are calculated as functions of the thermal power to the TFE. Results show that electrical heating of the middle 0.35 m section, of the 0.375 m long emitter, more accurately simulate the operation of fission heated, TOPAZ‐II type TFEs. Shorter heaters overestimate the load electric power and the emitters temperature and emission current density.

A Topaz‐II bimodal design assessment study and system analysis

A preliminary conceptual design study is performed to investigate the feasibility of near‐term, simple, minimum design changes to the Russian TOPAZ‐II space power system to provide bimodal operation, electric power and direct thermal propulsion for future missions. To simulate the fully integrated, TOPAZ‐II Bimodal system the capabilities of the Thermionic Transient Analysis Model (TITAM) are extended to take into account the proposed design modification of the Thermionic Fuel Elements (TFEs), waste heat recovery, pre‐injection electric heating of the hydrogen propellant. This system model is used to characterize and identify promising design options and assess the performance parameters of the TOPAZ‐II Bimodal system. Results of the study indicate that the TOPAZ‐II system can be easily modified to support bimodal missions at a thrust level of 2.5 to 7.5 N and a corresponding specific impulse of 830 s to 600 s, respectively. Testing of TOPAZ‐II Bimodal system can be accomplished at the Air Force Thermionic System Evaluation Test (TSET) facility in Albuquerque, New Mexico. The proposed design modification will build on the vast expertise and technology base available at the different Institutes, particularly at the Central Design Bureau of Machine Building (CDBMB) in St. Petersburg.

An investigation of natural circulation decay heat removal from an SP‐100 reactor system for a lunar outpost

A transient thermal‐hydraulic model of the decay heat removal from a 550 kWe SP‐100 power system for a lunar outpost has been developed and used to assess the coolability of the system by natural circulation after reactor shutdown. Results show that natural circulation of lithium coolant is sufficient to ensure coolability of the reactor core after shutdown. Further improvement of the decay heat removal capability of the system could be achieved by increasing the dimensions of the decay heat exchanger duct. A radiator area of 10‐15 m2 would be sufficient to maintain the reactor core safely coolable by natural circulation after shutdown. Increasing the area of the decay heat rejection radiator or the diameter of the heat pipes in the guard vessel wall insignificantly affects the decay heat removal capability of the system.

Two‐dimensional steady‐state analysis of an electrically heated thermionic fuel element

A two‐dimensional transient model of a single cell, long Thermionic Fuel Element (TFE) is developed and its predictions are compared with published calculations and experimental data on steady‐state operation of electrically heated, TOPAZ‐II type TFEs. The operation parameters of the TFE, such as axial distributions of the emitter temperature, emission current density, and the electrode voltage are calculated and discussed. Results show that despite the excellent agreement between the model predictions of the axial distribution of the emitter temperature, its predictions of the maximum emission current density was lower by about 17%. This difference is attributed primarily to the J‐V characteristics in the model, which could be different than those of the TOPAZ‐II TFE, hence additional data on the latter is needed. When compared with experimental data, the model predictions of the electric power output are in excellent agreement with the data at thermal power input of 3.5 kW or higher, but within 10% of t...