TASK 5. SYSTEMS TECHNOLOGY

Many studies of thermionic power systems have been made over the last 10 years. Early studies were largely parametric in nature, and were aimed at establishing design constraints on thermionic converters that could serve as fuel elements in nuclear reactors. These studies (References 1, 2, 3, and 4) were concerned initially with reactor parameters, but were later broadened to include the other portions (shielding, heat rejection, and power conditioning) of the system. The emitter and overall fuel element dimensions of the E- and F-series TFEs were chosen on the basis of these studies.

Applications studies were performed to determine requirements for thermionic reactors for nuclear electric propulsion (References 5 and 6), These studies showed that a power level of 100 kWe was optimum for unmanned missions, but that the optimum was very broad, so that power levels up to 300 kWe were suitable.

Point design studies were made of thermionic power plants for several applications. One of the first studies (Reference 7) dealt with a 300 kWe unit-cell reactor, in which the fuel elements were single thermionic cells. This study included transient and control analyses and reliability analyses, which were later extended into further studies (References 8 and 9). Other point design studies treated 40 kWe systems for application to manned space stations (Reference 10) and systems ranging from 5 to 134 kWe for unmanned applications (References 11 and 12) including nuclear electric propulsion. Studies were also made comparing alternate point designs in the same power range (References 13 and 14). The comparisons included reactors with fast driver fuel elements (UN or UO₂) or moderating driver fuel elements (U-ZrH) as well as those in which all of the fuel is in the TFEs. The most recent design study dealt with thermionic reactors for undersea applications (References 15 and 16).

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5 .1 SYSTEM DESIGN

The objective of this subtask is to design thermionic reactor power systems for manned and unmanned auxiliary power applications in space, for unmanned space electric propulsion applications, and for undersea application.

The approach used is to study reactor and system parameters within application guidelines and constraints and within engineering and technology constraints of system components. A baseline design is selected to serve as an initial model for more detailed analysis of system components and their interaction. Improvements made to the baseline design result in a reference design upon which detailed engineering, operational and integration analyses are based.

Previous work includes a design study of 40 kWe power systems for a manned space laboratory. Two systems were designed: one for rigid boom-mounting on the space laboratory and the other for deployment at the end of a 3.2 km (2-mile) tether. Baseline designs were also prepared for 5 and 10 kWe systems for unmanned auxiliary power and for a 120 kWe power system for electric propulsion of unmanned spacecraft.

The major effort of the current fiscal year is the reference design of a reactor power system at 120 kWe for nuclear electric propulsion. The side thrust NEP spacecraft and the thermionic reactor power system are illustrated in Figure 5-1. This system design is defined by the specifications recently published in Reference 17. Nuclear and structural analyses of the reactor are discussed in Sections 4.1 and 4.2.

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In this reporting period, a design study was concluded for the application of thermionic reactors as undersea power plants. Point designs were completed for a 30 kWe compact power plant and a 5 kWe, natural-convection-cooled reactor plant f or sea bottom application. The designs are described in a topical report (Reference 16) and summarized briefly in the following section.

5.1.1 Undersea Power Systems (D. Allen, C. Carney, W. Leech)

Baseline designs are complete for two undersea power plants. The first power plant embodies a compact, 30 kWe thermionic reactor. This plant is called NEMO (Nuclear Electricity for Marine operations). The reactor employs the TRIGA* type U-ZrH rods as driver fuel. TRIGA reactors are the most widely used research reactors, with more than forty in operation around the world. NEMO has 33 thermionic fuel elements (TFEs) and 73 U-ZrH rods. Core cooling is by forced convection, and heat exchange to sea water is via a tube-in-shell heat exchanger. The result is a compact power plant. The NEMO power plant is shown in Figure 5-2. The core and pressure vessel are located centrally. They are surrounded by the lead gamma shielding and suspended from the roof of the secondary containment in a volume of borated water which serves as neutron shielding. Reactor auxiliaries are also submerged in the shield water.

The overall schematic flow diagram for NEMO is shown in Figure 5-3. The redundant use of pumps in both primary and secondary circuits can be seen. Double containment of primary coolant is accomplished by using double-walled tubes in the heat exchanger. The isolation valves on the seawater circuit can be closed simultaneously with a reactor trip as an additional containment feature. Reactor afterheat can be conducted and convected to the seawater through the shield water without overheating the core.

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*TRIGA is a registered trademark of the Gulf Oil Corporation.

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The reactor control is accomplished by poison-backed drums on the periphery of the core. Shutdown rods driven into the central region of the core are also provided. The 18 drums are operated by 6 motors. The three rods are driven by spring force and held out by hydraulic pressure. Figure 5-4 is a control schematic for NEMO.

The second undersea system is a 5 kWe power plant similar in design to the widely used TRIGA family of research reactors. The reactor core is predominantly U-ZrH fuel rods cooled by water in natural convection. This power plant incorporates 9 TFEs and conditions the electrical power output to user requirements. Flow in both reactor primary and secondary coolant loops is by natural convection. Heat transfer to the sea is also by natural convection. This power plant is expected to find application to sea bottom installations and is termed the Thermionic Reactor for Installed Oceanic Service (TRIOS). The design was conceived with particular emphasis placed on reliable operation with features to facilitate infrequent maintenance. The power plant elevation is shown in Fig. 5-5.

Significant features of each system are compared in Table 5-1.

Shielding analyses were conducted for the NEMO and TRIOS systems. The geometrical models used are shown in Figures 5-6 and 5-7. Neutron fluxes and doses are calculated using a one dimensional transport code. Gamma doses were calculated using a point-kernel model with exponential attenuation and appropriate build-up factors. Both primary and secondary gamma rays were considered.

The results of the studies are given in Table 5-2, which gives the dose at two different points outside the shield for each system.

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5.2 SYSTEM ANALYSTS

The objective of this subtask is to develop and improve analysis methods, and to perform analysis on the thermionic reactor system. Areas of analysis include performance, safety, dynamics and control, reliability, vibration, radiation shielding, and operations.

The approach used for this subtask is to obtain versions of existing calculational programs developed by national laboratories and other contractors and to integrate them into the analysis of the thermionic reactor system. New programs are written where necessary.

Previous to this reporting period, several computer programs had been written or modified to perform analyses in the above areas. Some of the programs were in need of revision to reflect changes in reactor and system designs.

5.2.1 System Optimization Programs

Updating of the TIRP (Thermionic Reactor Parameters) program continued. The hydraulics portion of the program was enlarged to calculate the total length of piping required outside the core for several arrangements of core, power conditioner, and radiator. The shielding portion of the code was updated. A subroutine (HTPIPE) was added to calculate the size, number, and mass of the heat pipes needed in the radiator. Calculational techniques for the heat pipes are based on work in Reference 18). An updated description of the program and instructions for preparation of input were prepared.

Work on a new computer program to calculate I-V characteristics was terminated. Calculations uncovered a basic theoretical error in the model of Hansen and Warner which was being used in the program.

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5.2.2 Reliability Analysis

The reliability analysis of the NEP reference design was continued. A component functional listing was compiled and a preliminary assessment of the criticality of each of these functions was made. A partial failure mechanisms list was also compiled.

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5.3. TECHNOLOGY EVALUATION AND DEVELOPMENT

The objectives of this subtask axe to determine where technology which has been or is being developed can be applied to the thermionic reactor power system and to determine requirements for new technology. In future years, the development of new technology and adaptation of technology for the thermionic reactor power system will be an objective.

The approach will be to review literature and existing programs which may be applicable and to prepare a technology evaluation report and preliminary technology development plan which are revised and updated annually.

The preparation of the first technology evaluation report was begun in the latter part of FY 71. The completion of this report has been delayed due to a reduction in personnel and a reordering of priorities.

During the last four months, two sections of the technology evaluation report were written. The section on radiation shielding included lithium hydride neutron shields and gamma shields of tungsten, uranium, lead and alloys of tungsten and uranium. The section on radiators included heat pipes, heat pipe panels, finned tubes, emissive coatings, and brazed joints for bonding heat pipes to coolant ducts. These sections of the report and the previously written section on reactor components were consolidated in the close-out files.

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5.4 APPLICATIONS ANALYSIS

The objectives of this subtask are the definition of power requirements and the evaluation of performance capabilities of thermionic nuclear electric systems for space applications.

Information exchange and liaison with potential users in government and aerospace organizations will be maintained. Modifications in the design of thermionic reactor systems will be implemented as necessary to meet integration requirements.

Previous effort in this area has established basic guidelines on power system selection for first generation nuclear electric propulsion missions and assessed the effect of reactor lifetime constraints on mission capability (Reference 19).

Mission analysis studies indicate that a nominal 100 kWe reactor power system with a Titan/Centaur or a Shuttle/Centaur is appropriate for most solar system missions with payloads under 2000 kg. Required thrust times for typical missions are in the range of 10,000 to 20,000 hours, excluding power use during coast period or at the target.

Other recent activities in this task have been related to identifying potential non-propulsion applications for thermionics. The following table lists those which had suggested further investigation:

Application Power Requirement

DOD Satellite 5-10 kWe

Undersea Propulsion 30-50 kWe

Undersea Stationary Power 5-100 kWe

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1. Perry, L. W., and Homeyer, W. G., "Synthesis of Thermionic", Power Conversion to Nuclear Reactors for Space Power Applications, Presented at the International Conference on Thermionic Electrical Power Generation, September 1965.

2. Homeyer, W. G., et.al., "Thermionic Reactor Power-Plants-Interrelation of Performance, Reliability and Technology, USAEC Report GA-7868, Gulf General Atomic, May 22, 1967.

3. Homeyer, W. G., "Performance and Technology Parameters of Thermionic Reactors", USAEC Report GA-7302, Gulf General Atomic, June 6, 1967.

4. Homeyer, W. G., "Thermionic Reactors for Electric Propulsion-Parametric Studies", Presented at the Second International Conference on Thermionic Electrical Power Generation, May 1968.

5. Ingber, J. F., and Higdon, D. T., "Reactor Requirements for First Generation Thermionic Nuclear Electric Propulsion", USAEC Report GA-9046, Gulf General Atomic, November 1, 1968.

6. Ingber, J. F., and Higdon, D. T., "Reactor Requirements for First Generation Thermionic Nuclear Electric Propulsion", USAEC Report GA-9046(Rev.), Gulf General Atomic, January 16, 1969.

7. Allen, D. T., "A Design Study of the Unit-Cell Thermionic Reactor for Space Application. Final Report", USAEC Report GA-8917, Gulf General Atomic, January 13, 1969.

8. Heath, C. A., "Possible Instabilities in a Thermionic Reactor with Negative Reactivity Coefficients", Presented at the Thermionic Conversion Specialist Conference, October 1968.

9. Heath, C. A., "The Effect of Coolant Loop Configuration Upon the Reliability of Thermionic Reactor Systems, USAEC Report GA-8991, Gulf General Atomic, March 31, 1969.

10. Gietzen, A. J., et.al., "A 40 kWe Thermionic Power System for a Manned Space Laboratory", USAEC Report GA-A10535, Gulf General Atomic, July 1, 1971.

11. Gietzen, A.J., and Homeyer, W. G., "100 kWe Thermionic Power Systems Design", Presented at the Intersociety Energy Conversion Engineering Conference, September 21, 1970.

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12. Gietzen, A. J. and. Homeyer, "Thermionic Reactor Power Systems, Presented at the 3^rd International Conference on Thermionic Electrical Power Generation, May 2, 1972.

13. Cohen, S. C., et. al., "Comparison of Thermionic Reactor Concepts for 50 to 100 kWe", USAEC Report GA-9509, Gulf General Atomic, October 1969.

14. Homeyer, W. G., "The Performance of Thermionic Reactors for 50 to 100 kWe", Presented at the Thermionic Conversion Specialist Conference, November 1969.

15. Fisher, C. R., "Thermionic Reactor Power Systems for Undersea Application, USAEC Report GA-A12302, Draft, September 10, 1972.

16. Allen, D. T., et al. "Undersea Thermionic Reactors," USAEC Report Gulf-GA-A12419, Gulf General Atomic, February 1973.

17. Ingber, J. F., et al., "Nuclear Thermionic Power System for Electric Propulsion," USAEC Report GA-C12377, Gulf General Atomic, January 1973.

18. "Development of a Thermionic Reactor Space Power System," Summary Report for Period July 1, 1972 through October 31, 1972, Draft: USAEC Report GA-A12397, Gulf General Atomic.

19. Ingber, J., and Higdon, D., "Reactor Requirements for First Generation Thermionic Nuclear Electric Propulsion", Presented to AIAA 7th Electric Conference, Williamsburg, Va., March 3-5, 1969.

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