Albert C. Marshall

A reformulation of thermionic theory for vacuum diodes

Abstract Methods for predicting thermionic diode current–voltage characteristics have been in wide use for decades; however, serious limitations have been identified in the conventional formulations of thermionic theory. A reformulation of thermionic theory for vacuum diodes has been developed to improve predictive accuracy, to broaden the scope of application, and to establish a more consistent approach. Electron reflection effects and temperature dependent work functions are shown to be important considerations for thermionic diodes; consequently, the revised formulation focuses on proper treatment of these considerations. Revised equations are derived for predicting current densities, space charge, and electron cooling. In addition, equations are developed that provide energy–angle dependent current density spectra used to compute average transmission coefficients. An approach for computing space charge reflection parameters is also provided. Methods are given that permit application of the revised approach to inhomogeneous (patchy) electrode surfaces. A preliminary comparison shows that the predicted current–voltage characteristics using the revised formulations are in good agreement with test results for a prototype microminiature thermionic converter; whereas, more conventional formulations do not provide accurate predictions. Several new parameters are recommended to take advantage of the more precise methodology provided by the reformulation.

Topaz II preliminary safety assessment

The Strategic Defense Initiative Organization (SDIO) decided to investigate the possibility of launching a Russian Topaz II space nuclear power system. A preliminary safety assessment was conducted to determine whether or not a space mission could be conducted safely and within budget constraints. As part of this assessment, a safety policy and safety functional requirements were developed to guide both the safety assessment and future Topaz II activities. A review of the Russian flight safety program was conducted and documented. Our preliminary safety assessment included a top level event tree, neutronic analysis of normal and accident configurations, an evaluation of temperature coefficients of reactivity, a reentry and disposal analysis, and analysis of postulated launch abort impact accidents, and an analysis of postulated propellant fire and explosion accidents. Based on the assessment, it appears that it will be possible to safely launch the Topaz II system in the U.S. with some possible system modifications. The principal system modifications will probably include design changes to preclude water flooded criticality and to assure intact reentry.

Small fission power systems for NEP

Two nuclear electric propulsion (NEP) power system configurations are presented, each with an output power of 50 kWe and a system mass of about 2500 kg. Both consist of a reactor coupled to a recuperated Brayton power conversion system with a fixed conical radiator driven by loop heat pipes. In one system the reactor is gas-cooled with the gas directly driving the Brayton power conversion system. In the other the reactor is heatpipe-cooled with a heat exchanger between the reactor and the Brayton system. Two variations are described briefly with powers of 100 k We and 150 kWe. The mass scales approximately with the square root of the power.

Low work function material development for the microminiature thermionic converter.

Thermionic energy conversion in a miniature format shows potential as a viable, high efficiency, micro to macro-scale power source. A microminiature thermionic converter (MTC) with inter-electrode spacings on the order of microns has been prototyped and evaluated at Sandia. The remaining enabling technology is the development of low work function materials and processes that can be integrated into these converters to increase power production at modest temperatures (800 - 1300 K). The electrode materials are not well understood and the electrode thermionic properties are highly sensitive to manufacturing processes. Advanced theoretical, modeling, and fabrication capabilities are required to achieve optimum performance for MTC diodes. This report describes the modeling and fabrication efforts performed to develop micro dispenser cathodes for use in the MTC.

Nuclear safety policy working group recommendations for SEI nuclear propulsion safety

Nuclear propulsion has been identified as an essential technology for the implementation of the Space Exploration Initiative (SEI). An interagency Nuclear Safety Policy Working Group (NSPWG) was chartered to recommend nuclear safety policy, requirements, and guidelines for the SEI nuclear propulsion program to facilitate the implementation of mission planning and conceptual design studies. The NSPWG developed a top level policy to provide the guiding principles for the development and implementation of the nuclear propulsion safety program and the development of Safety Functional Requirements. In addition, the NSPWG reviewed safety issues for nuclear propulsion and recommended top level safety requirements and guidelines to address these issues. Safety topics include reactor start‐up, inadvertent criticality, radiological release and exposure, disposal, entry, safeguards, risk/reliability, operational safety, ground testing, and other considerations. All recommendations are in conformance with existing re...

RSMASS‐D: Reactor and shield mass minimization models

Three relatively simple mathematical models have been developed to estimate minimum reactor and radiation shield masses for liquid metal cooled reactors (LMR’s), in‐core thermionic reactors (TI’s) and out‐of‐core thermionic reactors (OTR’s). The approach was based on much of the methodology developed for the RSMASS model (Marshall 1986). The models use a combination of simple equations derived from reactor physics and other fundamental considerations along with tabulations of data from more detailed neutron and gamma transport theory computations. All three models vary basic design parameters within an allowed range to achieve a parameter choice which yields a minimum mass for the power level and operational time of interest. The impact of critical mass, fuel damage and thermal limitations are accounted for in the computations. Thermionic requirements are also accounted for in the thermionic reactor models. All major reactor component masses are estimated as well as instrumentation and control (I&C), boom...

Space reactor safety, 1985–1995 lessons learned

Space reactor safety activities and decisions have evolved over the last decade. Important safety decisions have been made in the SP‐100, Space Exploration Initiative, NEPSTP, SNTP, and Bimodal Space Reactor programs. In addition, international guidance on space reactor safety has been instituted. Space reactor safety decisions and practices have developed in the areas of inadvertent criticality, reentry, radiological release, orbital operation, programmatics, and policy. In general, the lessons learned point out the importance of carefully reviewing previous safety practices for appropriateness to space nuclear programs in general and to the specific mission under consideration.

Startup Simulation of the TOPAZ‐II Reactor System for Accident Conditions

The Thermionic Transient Analysis Model (TITAM) is used in this paper to simulate the startup of the TOPAZ‐II space nuclear power system for a postulated accident condition. The temperature feedback effects of the moderator, UO2 fuel, electrodes, coolant, and other components in the core are calculated using the most recent temperature coefficient data. The reactor behavior predicted by TITAM for a normal startup is in agreement with Russian data for TOPAZ‐II. In the accident investigated, the control drums, starting in the full in position, rotate out at maximum speed of 1.4°/s to the full out position and remain out. The combination of the prompt negative coefficient of reactivity and the very low cold excess reactivity resulted in a predicted gradual rise in temperature rather than a prompt critical excursion.

Integrated Safety Program for the Nuclear Electric Propulsion Space Test Program

The Nuclear Electric Propulsion Space Test Program (NEPSTP) is sponsored by the Ballistic Missile Defense Office (BMDO) to demonstrate and evaluate the Russian‐built TOPAZ II nuclear reactor as a power source for an electric propulsion system in space. From its inception, safety has been a central feature of the NEPSTP program. This paper addresses the work done to define the safety organizational relationships, responsibilities, management, engineering requirements, and documentation to assure an integrated safety program that coordinates the various safety activities in Mission Safety, Range Safety and Nuclear Safety. Because the United States has not launched a nuclear reactor since 1965, much of the focus of the safety program has been directed toward the unique safety considerations of using a nuclear reactor in space. Our preliminary findings indicate that the safe use of the TOPAZ II for the NEPSTP space mission is feasible.

TOPAZ II Temperature Coefficient Analyses

A two‐dimensional model of the Topaz II reactor core suitable for neutronic analyses of temperature coefficients of reactivity is presented. The model is based on a 30° r‐theta segment of the core. Results of TWODANT calculations are used to estimate temperature coefficients associated with fuel, electrodes, moderator, reflector, and tube plates over the range of temperatures anticipated during startup and operation. Results are presented to assess the reactivity effects associated with Doppler broadening, spectral effects and thermal expansion. Comparisons are made between the TWODANT results and empirical Russian curves used for simulating Topaz II system transients. TWODANT results indicate that the prompt temperature coefficients associated with temperature changes in fuel and emitters are negative. This is primarily because of Doppler broadening of the absorption resonances of uranium and molybdenum. The delayed effect of tube plate heating is also negative because fuel is moved radially outward in t...