INTRODUCTION

This is Gulf General Atomic’s final report on the development of a thermionic reactor space power system.

Initial experimental work led to the award of our first AEC thermionic contract AT(04-3)-167, Project Agreement 14, on May 1, 1962, for the research and development of fission heated thermionic cells with an operating life of 10,000 hours or more. This work was primarily concerned with metallurgical studies and the construction of experimental thermionic converters. Two types of converters were fabricated: 1) electrically heated, and 2) fission heated where the fuel was either uranium carbide or uranium oxide.

A similar contract was awarded to the General Electric Company. A competition between GGA and GE ensued to develop a thermionic fuel element (TFE) and to design a thermionic reactor to be tested as an experimental reactor. This competition was climaxed on July 1, 1970 by the award to GGA of a five-year contract, AT(04-3)-840, renewable for three years, to develop an in-core thermionic reactor. At that time GGA had successfully operated out-of-pile converters in excess of 20,000 hours and a two-cell thermionic fuel element in excess of 2,500 hours.

The objectives for the Contract AT(04-3)-840 work were:

· To evolve and maintain space power reference designs. The principal projected uses were to have been a reliable man-rated system with a lifetime goal of five years for possible orbital space systems applications and a reliable lightweight one to two year life system for unmanned electric propulsion missions.

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· Continued development of a thermionic fuel element (TFE) to a lifetime of one to two years for electric propulsion, with a goal of five years for secondary power applications. This included design, material and fabrication technology, and testing.

· Design, construction, installation, and operation of a thermionic reactor experiment. This effort was to have included measurement of the operational characteristics of a thermionic reactor core, or of more than one if necessary.

· Development of thermionic space power reactor technology in sufficient depth to allow user agencies in the late 1970s to commit to mission use in the 1980s.

The contract statement of work read as follows:

· Materials Technology

The objective of this task is to develop specific in-depth materials technology in support of the thermionic reactor development. The task will emphasize developing and characterizing the specific emitters, collectors, fuels, insulators, seals, and joints planned for use in the thermionic reactor Fabrication, development, testing, analysis and post-test metallurgical examination will be involved.

· Converter and Fuel Element Development

The objective of this task is the development of thermionic fuel element (TFE) technology for a lifetime of at least one year and a goal of five years.

This task covers design, development, fabrication and testing of all components required in a thermionic fuel element and includes:

Single Cell Prototype Converters - These devices incorporate the essential components of the diode and will be used to study and evaluate basic processes associated with the converter, such as thermionic electrical performance, fuel venting, envelope integrity, design variations, and fabrication techniques. This configuration will allow detailed instrumentation and diagnostic procedures not possible in multicell tests.

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Partial Length Fuel Elements - These devices include all the components of reference thermionic fuel elements, but are reduced to a length compatible with the irradiation facilities normally used. These partial length elements will be the principal TEE development test vehicle. Every TEE component will be contained in these elements.

Fuel Length, Reference Fuel Elements - This type of device is intended to be a complete reactor fuel element corresponding to the most up-to-date reference design. They will be used to verify results obtained with partial length elements and demonstrate performance and mechanical integrity prior to the reactor experiment test.

Converter and Fuel Element Testing - This involves primarily the inpile (or out-of-pile) testing of converter assemblies fabricated under this task and the associated neutron irradiation requirement. The objectives will be the demonstration of long life, high thermionic output and performance stability. Diagnostic and performance analysis of the nuclear characteristics, thermal behavior, and electrical output will be conducted.

· Reactor Technology

The objective of this task is the understanding of the characteristics of a thermionic reactor. Included in this task will be at least the following: neutronic characteristics of the reactor core and reflector, reactor stability and dynamic behavior, control system design and logic, mechanical design and analysis, thermal/hydraulic analysis, electrical performance analysis, reliability analysis, and nuclear safety analysis.

· Reactor Experiment

The primary objective of the thermionic reactor experiment is to explore and evaluate the dynamic and steady-state operating characteristics of a thermionic reactor core. This task provides for a comprehensive reactor experiment program including the design, fabrication, installation, checkout and test operation of the experiment, including the facility. In addition, research and development associated with the construction line item will be performed under this task.

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· System Technology

The objectives of this task are to study the applicability of thermionic reactors for various mission applications and to provide technology for selected components as necessary for such systems.

The system design analysis effort includes: tailoring the reactor design for specific potential missions; shielding design and analysis; power conditioning and electrical distribution network; heat rejection systems including pumps, heat exchangers, and radiators; specification of launch environmental conditions; power system reliability and safety; system integration considerations; and other factors.

System component development for thermionic reactors is to rely as much as possible on components which have been previously or concurrently developed for other space reactor systems. In cases where such technology does not exist or is not being developed adequately, technology development is to be undertaken in the thermionic program. It is contemplated that work on reactor components not required for the reactor experiment will not be initiated until after the reactor experiment. Hardware development of non-reactor components of the flight reference design power system is not required.

· Program Management

Program management includes: continuing review, planning, and technical direction of the program;

recognition and response to problems which arise; schedule and cost control; reports and related matters.

Milestones set forth in the contract were as follows:

1. Place first full length TFE on in-pile test March 1971
2. 10,000 test hours on in-pile partial length TFE September 1971
3. 2,000 test hours on in-pile full length TFE December 1971
h. 5,000 test hours on in-pile full length TFE December 1972
5. 20,000 test hours on in-pile partial length TFE June 1974

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6. 10,000 test hours on in-pile full length TFE December 1974
7, 30,000 test hours on in-pile partial length TFE December 1976
8. 20,000 test hours on in-pile full length TFE December 1976

9. Select Reactor Experiment Site August 1971
10. Submit PSAR June 1972
11. Construction Authorization January 1973
12. Submit FSAE June 1975

13. Complete Construction & Installation January 1976
(Contractor Beneficial Occupancy Date)
14. Full Power Operation (1.5 Mwt) (without TFEs) January 1977
15. Insert first TFE June 1977
16. Begin operation with full TFE core June 1978

TFE development milestone numbers 1, 2, 3, and 4 were met according to the above milestone schedule. The Reactor Experiment milestones required appropriation of operating and facility money for the Reactor Experiment; the money was not appropriated, and the work was not done.

On January 5, 1973 GGA was notified of program termination by the AEC. At that time a six-cell prototypical thermionic fuel element had passed 8000 hours of operation in the Gulf General Atomic TRIGA test reactor.

This report is organized by tasks as follows:

Task 1 Thermionic Research

Task 2 Materials Technology

Task 3 Thermionic Fuel Element Development

Task 4 Reactor Technology

Task 5 Systems Technology

A bibliography of reports prepared for this program is presented in Appendix C.

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