5 kWe Reactor

The 5 kWe Thermoelectric Reactor: Simpler, Cleaner, and More Reliable

Artists Depiction
5 kWe Thermoelectric Reactor, artist’s concept cutaway drawing. Image DOE

The 5 kWe Thermoelectric Reactor (5 kWe reactor) was a reasonably simple adaptation of the SNAP-8 design, intended to be used with a shadow shield. Unsurprisingly, a lot of the design changes mirrored some of the work done on the SNAP-10B Interim design, which was undergoing work at about the same time. Meant to supply 5 kWe of power for 5 years using lead telluride thermoelectric convertors (derived from the SNAP-10A convertors), this system was meant to provide power for everything from small crewed space stations to large communications satellites. In many ways, this was a very different departure from the SNAP-8 reactor, but at the same time the changes that were proposed were based on evolutionary changes during the S8ER and S8DR experimental runs, as well as advances in the SNAP 2/10 core which was undergoing parallel post-SNAPSHOT design evolution (the SNAP-10A design had been frozen for the SNAPSHOT program at this point, so these changes were either for the followon SNAP-10A Advanced or SNAP-10B reactors). The change from mercury Rankine to thermoelectric power conversion, though, paralleled a change in the SNAP-2/10A origram, where greater efficiency was seen as unnecessary due to the constantly-lower power requirements of the systems.

5 kWe Schematic

The first thing (in the reactor itself, at least) that was different about the design was that the axial reflector was tapered, rather than cylindrical. This was done to keep the exterior profile of the reactor cleaner. While aerodynamic considerations aren’t a big deal (although they do still play a minute part in low Earth orbit) for astronuclear power plants, everything that’s exposed to the unshielded reactor becomes a radiation source itself, due to radiation scattering and material activation under neutron bombardment. If you could get your reactor to be a continuation of the taper of your shadow shield, rather than sticking out from that cone shape, you can make the shadow shield smaller for a given reactor. Since the shield is often many times heavier than the power system itself, especially for crewed applications, the single biggest place a designer can save mass is in the shadow shield.

This tapered profile meant two things: first, there would be a gradient in the amount of neutron moderation between the top and the bottom of the reactor, and second, the control system would have to be reworked. It’s unclear exactly how far the neutronics analysis for the new reflector configuration had proceeded, sadly, but the control systems were adaptations of the design changes that were proposed for the SNAP-10B reactor: instead of having the wide, partial cylinder control drums of the original design, large sections (235 degrees in total) of the reflector would be slid up or down around the core containment vessel to control the amount of reactivity available. This is somewhat similar to the SNAP-10B and BES-5 concepts in its execution, but the mechanism is quite different from a neutronics perspective: rather than capturing the unwanted neutrons using a neutron poison like boron or europium, they’re essentially vented into space.

A few other big changes from the SNAP-8 reference design when it comes to the core itself. The first is in the fuel: instead of having a single long fuel rod in the clad, the U-XrH fuel was split into five different “slugs,” which were held together by the clad. This would create a far more complex thermal distribution situation in the fuel, but would also allow for better thermal stress management within the hydride itself. The number of fuel elements was reduced to 85, and they came in three configurations: one set of 27 had radial fins to control the flow that spiralled around the fuel element in a right-hand direction, another set of 27 had fins in the left-hand direction, and the final 31 were unfinned. This was done to better manage the flow of the NaK coolant through the core, and avoid some of the hydrodynamic problems that were experienced on the S8DR.

Blueprint Layout of System
5kWe Thermoelectric power system. Image DOE
Summary Table
Summary Table 2

5 kWe Thermoelectric Reactor

Overall Program

PERFORMANCE MODELING OF THE 5-kwe REACTOR THERMOELECTRIC SYSTEM 1973 https://www.osti.gov/servlets/purl/4480222

Fuel Element Development and Analysis

5-KWE REACTOR T/E UNMANNED SYSTEM TEST NaK Heat Transfe r Study for a 19-Rod Bundle (P/D = 1.05) Employing Helical-Fin Spacers 1972
https://www.osti.gov/servlets/purl/4201152

Nuclear Design and Analysis

REACTOR ASSEMBLY 5-kwe REACTOR THERMOELECTRIC SYSTEM 1973 https://www.osti.gov/servlets/purl/4439973

Therma l Transient and Deflection Analysis of Internal Reflector – 5 Kwe Reactor 1972 https://www.osti.gov/servlets/purl/4201235

CONTROL WORTH OF SLIDING REFLECTORS FOR ZIRCONIUM HYDRIDE REACTORS 1973 https://www.osti.gov/servlets/purl/4460252

Primary Loop and Thermal Design

Partial Temperatur e Coefficients for the 5 Kwe Reactor 1973
https://www.osti.gov/servlets/purl/4174777

5 Kwe System Therma l Losse s and Piping Insulation Performanc e 1972
https://www.osti.gov/servlets/purl/4186297

5 Kwe Reactor Powe r Distributions 1972 https://www.osti.gov/servlets/purl/4201233

Power Conversion System Design and Development

REACTOR ASSEMBLY 5-kwe REACTOR THERMOELECTRIC SYSTEM 1973
https://www.osti.gov/servlets/purl/4439973

5-kwe REACTOR THERMOELECTRIC SYSTEM SUMMARY 1973
https://www.osti.gov/servlets/purl/4447088

COMPACT THERMOELECTRIC CONVERTER SYSTEMS TECHNOLOGY FINAL REPORT 1973 https://www.osti.gov/servlets/purl/4459097

Component Design, Development, and Analysis

5 Kwe Reacto r T E Reflecto r Secto r Driv e Actuato r Design Summar y 1972 https://www.osti.gov/servlets/purl/4195860

Desig n Summar y – Reflecto r Driv e System 1972
https://www.osti.gov/servlets/purl/4185989

Dual Throa t NaK Pum p Performanc e Evaluatio n (Conceptua l Desig n of Prototype ) 1972 https://www.osti.gov/servlets/purl/4196225

MATERIALS INTERACTIONS BETWEEN THE THERMOELECTRIC CONVERTER AND THE 5-kwe REACTOR SYSTEM 1973
https://www.osti.gov/servlets/purl/4466746

Shield Nuclear Design for the 5-kwe TE System 1972
https://www.osti.gov/servlets/purl/4174779

Progress Reports

Safety

Mass Transfe r and Oxygen Analysis in the SNAP 5 KWe Ground Test Reactor 1973 https://www.osti.gov/servlets/purl/4196226

Decommissioning and Safety

OPERATIONAL HAZARDS EVALUATION FOR 5-kwe REACTOR THERMOELECTRIC SYSTEM GROUND TEST 1973
https://www.osti.gov/servlets/purl/4450467