U.S. Small Nuclear Power Sources

(Updated 1 March 2014)

Radioisotopes for possible use in Space Power Units

Isotope

Radiation

Half-Life

Po-210

Alpha

138.4 days

Cm-242

Alpha

162.5 days

Ce-144

Beta and Gamma

285 days

Pm-147

Beta

2.6 years

Sr-90

Beta

27.7 years

Pu-238

Alpha

86.4 years

References:
Spacecraft Power Generation by William C. Cooley, NASA (24-26 August 1960)

1960s Costs for a RTG:

For a 100 w(e) Ce144-fueled generator with a one year lifetime and 5% thermionic efficiency, the fuel costs were to be around $70,000. With 15% thermionic conversion efficiency, the fuel costs for the same electrical output fell to $28,000.

A Pu238 fueled generator was reported to cost only $5,000 without the fuel.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

Systems for Auxiliary Nuclear Power (SNAP)

Notes: Even-numbered SNAP systems utilized fission reactors, while odd-numbered ones utilized radioisotopes as an energy source.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

Full Length OCRed Documents on SNAP:

Summary of Snap Nuclear Space Power Systems, E.B. BAUMEISTER Compact Power Systems Department, Atomics International, Canoga Park, Calif. (405~ kb PDF)

SNAP-1A RTG

Fuel Element: Ce144
Electrical Power: 125W
Mass: 200~ lbs

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

TRW SNAP-2 Fission Reactor

Fuel Element Type: Uranium – Zirconium Hydride (U-ZrH), NaK cooled.
Thermal Power:
57.9 kW(t)
Electrical Power:
3 kW(e)
Mass: 250 lbs (reactor); 750 lbs (unshielded system)
Useful Life: 12 months.
Reactor Outlet Temperature: 1,200°F
Mercury Boiling Temperature: 900°F
Radiator Temperature: 600°F

Notes: Development began in 1956. The objective was to develop a compact 3 KW(e) reactor-turboelectric system for space applications using a mercury vapor turboalternator. Program terminated in 1964.

Two complete reactors were built during the program:

SER – SNAP-2 Experimental Reactor (50 kWt): Ran from 5 November 1959 to 19 November 1960. As part of the test program, it ran for 5,200 hours at nearly full power, including a continuous 1,000 hour endurance run at 50 kWt.

S2DR – SNAP-2 Developmental Reactor (65 kWt): Run from 5 November 1959 to December 1962. As part of the test program, it ran for 10,500 hours at nearly full power.

References:
Quarterly Progress Report to the Joint Committee on Atomic Energy, April-June 1958. U.S. Atomic Energy Commission
AEC Annual Report to Congress, 1961.
Summary of Snap Nuclear Space Power Systems, E.B. BAUMEISTER
Advanced Nuclear Systems for Portable Power in Space: A Report; Appendix C by U.S. National Research Council.
SNAP Overview by Glen Schmidt (7 February 2011) (7.39 MB PDF)
An Appraisal of the Advanced Electric Space Power Systems, May 1962 by Lewis Research Center (NASA)
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

SNAP-3 RTG

Fuel Element: Po210

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

SNAP-3A RTG

Fuel Element: Pu238 (1.6 kilocuries’ worth)
Electrical Power: 3 W(e)

References:
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-3B RTG

Electrical Power: 2.7 W(e)
Useful Life: 5 years

Notes: Powered the Transit 4A [SNAP-3B7] and 4B [SNAP-3B8] navigational satellites.

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
CRC Handbook of Thermoelectrics edited by D.M. Rowe

SNAP-4 Fission Reactor

Electrical Power: 1 to 2 MW
Dimensions: 7 foot diameter, 16 foot height
Mass: 40,000 lbs (does not include pressure vessel)
Useful Life: 2 Megawatt-Years

Notes: The objective was to develop a compact reactor-turbolectric system for use in unmanned underwater or remote terrestrial location.

References:
Quarterly Progress Report to the Joint Committee on Atomic Energy, April-June 1958. U.S. Atomic Energy Commission
AEC Annual Report to Congress, 1961.

SNAP-7A RTG

Fuel Element: Sr90 (41 kilocuries’ worth)
Electrical Power: 10W

Notes: Operated for three years powering a U.S. Coast Guard Buoy.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-7B RTG

Fuel Element: Sr90 (225 kilocuries’ worth)
Electrical Power: 60W

Notes: Operated for two years powering a U.S. Coast Guard Lighthouse, then in August 1966 relocated to an offshore oil platform.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-7C RTG

Fuel Element: Sr90

Notes: Was being developed for the US Coast Guard and US Navy for use in navigational/weather stations such as buoys. The -7C version was specifically chosen to power a USN automatic unmanned Antarctic weather station.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Quarterly Progress Report to the Joint Committee on Atomic Energy, April-June 1958. U.S. Atomic Energy Commission
AEC Annual Report to Congress, 1961.

SNAP-7D RTG

Fuel Element: 20~ lbs of Sr90 (225 kilocuries’ worth)
Electrical Power: 60W
Length: 34.5 inches
Diameter: 22 inches
Mass: 4,600 lbs (including shielding)

Notes: Placed into operational service in the Gulf of Mexico in January 1964, some 350~ miles south of New Orleans, powering a U.S. Navy NOMAD (Navy Oceanographic Meteorological Automatic Device) mounted in a small moored boat. The battery powered NOMADs needed recharging every six months, while the SNAP-7D powered version could operate for 10 years. Part of the design specifications for the -7D version was the requirement to withstand ramming by a 20,000 ton merchant ship traveling at 20 knots.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Nuclear Power for Space Research, New Scientist (20 February 1964)
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-7E RTG

Fuel Element: Sr90 (31 kilocuries’ worth)
Electrical Power: 5W (or 7.5W)
Useful Life: 24 months
Shielding: Shielded by 8” of cast iron and depleted uranium.

Notes: Placed into operational service powering a sonar transducer at 15,600 feet in July 1964.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-7F RTG

Electrical Power: 60W (225 kilocuries’ worth)

Notes: Placed onboard an offshore oil platform.

References:
Radioactivity in the Marine Environment by the U.S. National Research Council

Aerojet General SNAP-8 Fission Reactor

Fuel Element Type: Uranium – Zirconium Hydride (U-ZrH), NaK cooled.
Thermal Power:
300 kW(t)
Electrical Power:
30 kW(e)
Mass: 300 lbs (reactor); 1500 lbs (unshielded system)
Useful Life: 12 months.
Reactor Outlet Temperature: 1,300°F
Mercury Boiling Temperature: 1,100°F
Radiator Temperature: 700°F

Notes: Joint NASA/AEC project designed to develop a 30 to 60 KW(e) reactor with a specific weight of 50 lb/kw(e) and a 10,000 hour operational lifetime. Effectively a scale-up of the SNAP-2 system. The official objective was:

The ultimate objective of the SNAP-8 program is to design and develop a 30-kw Electrical Generating System for use in various space missions. The power source for this system will be a nuclear reactor furnished by the AEC. The SNAP-8 system will use a eutectic mixture of sodium and potassium (NaK) as the reactor coolant; the system will operate on a Rankine cycle with mercury as the working fluid for the turbogenerator. The SNAP-8 system will be lightweight and highly reliable. It will be launched from a ground base and will operate unattended at full power for a minimum of 10,000 hours. After the system is placed in orbit, both activation and shutdown may be accomplished by ground command.”

Two complete reactors were built during the SNAP-8 program:

SNAP-8 Experimental Reactor (S8ER), which was ground tested in an inerted containment vessels for 12,000 hours and operated for 1 year at power and temperature. Used non-flight hardware. Was a significant improvement in technology – for the same amount of unshielded reactor mass as a SNAP-10A system, S8ER could deliver over 6 times the energy.

SNAP-8 Developmental Reactor (S8DR), which was ground tested for 7,000 hours at power levels from 600 to 1,000 kW(t) using flight-type reactor components and neutron shielding.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Quarterly Progress Report to the Joint Committee on Atomic Energy, April-June 1958. U.S. Atomic Energy Commission
AEC Annual Report to Congress, 1961.
Summary of Snap Nuclear Space Power Systems, E.B. BAUMEISTER
Report No. 0390-04-6 Development of SNAP-8 Nuclear Power Conversion System Model AGAN 0010 (7 February 1962)
Technological Implications of SNAP Reactor Power System Development for Future Space Nuclear Power Systems Activities by R.V. Anderson (9.1 MB PDF)
SNAP Overview by Glen Schmidt (7 February 2011) (7.39 MB PDF)
An Appraisal of the Advanced Electric Space Power Systems, May 1962 by Lewis Research Center (NASA)
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

SNAP-9A RTG

Fuel Element: Pu238 (16 kilocuries’ worth)
Electrical Power: 25 W(e)
Mass: 12.3 kilograms
Useful Life: 5 years in space after 1 year in storage on earth

Notes: Powered the Transit 5BN-1, Transit 5BN-2, and Transit 5BN-3 satellites completely. Transit 5BN-3 broke up and burned up on re-entry after a launch vehicle upper stage failure, with the Pu238 on-board burning up totally during atmospheric re-entry, which was the design philosophy at the time. Due to this accident, U.S. space power design philosophy was changed to require full containment of the fuel during an inadvertent re-entry to/from Earth Orbit. Became the basis for the SNAP-19 family.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Nuclear Power for Space Research, New Scientist (20 February 1964)
Radioactivity in the Marine Environment by the U.S. National Research Council
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
CRC Handbook of Thermoelectrics edited by D.M. Rowe

SNAP-10A Fission Reactor

Fuel Element Type: Uranium – Zirconium Hydride (U-ZrH), NaK cooled.
Thermal Power:
30 kW(t)
Electrical Power:
500 W(e)
Mass: 250~ lbs (reactor) 550 lbs (unshielded system), 750 lbs (shielded)
Useful Life: 12 months

Notes: Was designed to provide a lightweight, low power electric source for space operations. Utilized the SNAP-2 reactor design.

Because of the safety concerns about launching a reactor into space, a full up Re-Entry Flight Demonstration test was conducted in which a full-scale non-fueled replica of the SNAP-10A reactor was launched into a suborbital flight path to support theoretical modeling in aerodynamic heating and reactor dis-assembly.

Additionally, a series of reactor transient tests called the SNAPTRAN experiments were conducted. SNAPTRAN-3 was designed to characterize what would happen if a SNAP-10A reactor was flooded with water following a launch abort. After the chamber was flooded with water, the reactor was destroyed on command.

Several Reactors and full scale replicas were built during the SNAP-10 program:

S10PSM-1 – Prototype System Mockup 1was used for structural testing.

S10PSM-1A – Prototype System Mockup 1A was used for structural testing.

S10PSM-1B – Prototype System Mockup 2B was used for heat shield/launch vehicle compatibility tests.

S10PSM-2 – Unknown

S10PSM-3 – Prototype System Mockup 3 was used for NaK fill and thermal-vacuum testing.

S10FSM-1 – Flight System Mockup 1 was used for shock/vibration and thermal vacuum testing.

S10FSEM-2 – Flight System Electrical Mockup 2 was used for Electrical Testing and Agena compatibility testing.

S10FSEM-2A – Flight System Electrical Mockup 2A was used for flight status and launch procedure testing.

S10FSEM-3 – Flight System Electrical Mockup 3 was used for launch contractor compatibility testing.

S10FSM-4 – Flight System Mockup 4 was used for Non Nuclear Qualification Testing

S10FS-1 – Flight System 1 was the ground qualification system – it failed the acceptance test.

S10FS-2 – Flight System 2 was deleted from the program and reassigned to non-nuclear mockup testing.

S10FS-3 – Flight System 3 was the ground thermal-vacuum test system – it was tested for 10,005 hours in vacuum from January 1965 to 16 March 1966.

S10FS-4 – Flight System 4 was launched on 3 April 1965 on the SNAPSHOT spacecraft with an Atlas-Agena launch vehicle into a 700 nautical mile orbit with a 3,500 year orbital lifetime. FS-4 initially produced 580 W(e), which drifted down with time. After the 43rd day of in-space operations, telemetry was lost, due to a voltage regulator on the Agena sending an erroneous shutdown command to the reactor’s permanent shutdown system.

S10FS-5 – Flight System 5 completed its acceptance test and was placed into storage as a Flight Qualified Spare in May 1966.

(SNAP-10A Flight System 3 Photograph)

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Quarterly Progress Report to the Joint Committee on Atomic Energy, April-June 1958. U.S. Atomic Energy Commission
AEC Annual Report to Congress, 1961.
Technological Implications of SNAP Reactor Power System Development for Future Space Nuclear Power Systems Activities
by R.V. Anderson (9.1 MB PDF)
SNAP Overview by Glen Schmidt (7 February 2011) (7.39 MB PDF)
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

SNAP-11 RTG

Fuel Element: Cm242

Notes: At one time was considered to be the primary power generator for the Surveyor spacecraft.

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)
Radioisotope Thermal Generators and Thermoelectric Power Conversion by G. Stapfer

SNAP-13 RTG

Fuel Element: Cm242

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

SNAP-19 RTG

Fuel Element: Pu238 (34.5 kilocuries’ worth)
Electrical Power:
60 W(e)

Notes: Developed from the SNAP-9A program. Pioneer 10 and Pioneer 11 each had four SNAP-19s onboard. Viking 1 and Viking 2 each had two SNAP-19s on board.

References:
Radioisotope Thermal Generators and Thermoelectric Power Conversion by G. Stapfer
Radioactivity in the Marine Environment by the U.S. National Research Council
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
CRC Handbook of Thermoelectrics edited by D.M. Rowe

SNAP-19B RTG

Notes: Designed specifically for use on NASA’s NIMBUS meterological satellites. On 18 May 1968, the Nimbus B-1 meterological satellite was launched from Vandenberg AFB with two SNAP-19B2s onboard. During the launch, the vehicle was destroyed by the range safety officer due to erratic behavior. The RTGs were later found intact on the ocean floor and recovered, with the fuel being used on a later mission.

The Nimbus III satellite launched with two SNAP-19B3s on board.

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
CRC Handbook of Thermoelectrics edited by D.M. Rowe

SNAP-21 Series RTGs

Notes: Would have been produced in 10W(e) and 20W(e) versions powering sonars, boosting underwater cable power, navigational aids, and research instruments.

References:
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-23 Series RTGs

Notes: Would have been produced in 25W(e), 60W(e), and 100W(e) versions powering weather and navigational buoys as well as offshore oil platforms.

References:
Radioactivity in the Marine Environment by the U.S. National Research Council

SNAP-27 RTG

Fuel Element: Pu238 (45 kilocuries’ worth)
Electrical Power:
50 W(e)

Notes: Powered the Apollo ALSEPs on Apollo 12, 13, 15, 16 and 17.

References:
Radioisotope Thermal Generators and Thermoelectric Power Conversion by G. Stapfer
Radioactivity in the Marine Environment by the U.S. National Research Council
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council

Pratt & Whitney SNAP-50 Fast Fission Reactor

Fuel Element: 93% Uranium-235
Electrical Power:
300 kw(e)
Useful Life: 1 year

Notes: Legacy of the USAF Aircraft Nuclear Propulsion Program, because space power versions were designed by Pratt & Whitney under contract to USAF/AEC/NASA.

The program was intended to develop a 300 – 1,200 kW(e) power plant with a 10,000 hour lifetime. To achieve this, a 2.2 MW(t) reactor was being designed, along with a potassium working fluid Rankine cycle power converter capable of producing 300 kW(e). To reach the 1,200 kW(e) upper end of the spectrum, the reactor would be enlarged to provide between 8 – 10 MW(t) and four 300 kW(e) Rankine cycle converter modules would be used.

Program was terminated in 1965 before serious development could begin.

Was used as the basis for a study program at the Lawrence Radiation Laboratory named Advanced Space Nuclear Power Program (SPR). SPR itself was canceled in 1973 when NASA canceled most of it’s space-based nuclear power activities.

(SNAP-50 Configuration)
(SNAP-50 Core Configuration)

References:
Advanced Nuclear Systems for Portable Power in Space: A Report; Appendix C by U.S. National Research Council.
Technological Implications of SNAP Reactor Power System Development for Future Space Nuclear Power Systems Activities by R.V. Anderson (9.1 MB PDF)
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

Other Small Nuclear Systems

Advanced Space Nuclear Power Program (SPR)

References:
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

USAF Decomposed Ammonia Radioisotope Thruster (DART)

Fuel Element: Pu238 (4.7 kilocuries’ worth)
Thermal Power:
157 W(t)

Notes: Provided for a ISP of 310 seconds via passing ammonia over a Pu238 radioisotope element to decompose it.

References:
Radioactivity in the Marine Environment by the U.S. National Research Council, 1971
U.S. Patent 3,724,215 (PDF)

Advanced Radioisotope Thermal Generator (ARTG)

Electrical Power: 280 to 420 W(e) at beginning of mission
Mass:
40~ kg
Conversion Efficiency: 9-14%
GPHS Modules: 12

(MMRTG Chart Sheet)

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council

Thermophotovoltaic (TPV)

Electrical Power: 38 to 50 W(e) at beginning of mission
Mass:
7~ kg
Conversion Efficiency: 15-20%
GPHS Modules: 1

(MMRTG Chart Sheet)

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council

Multi-Hundred Watt (MHW) RTG

Electrical Power: 150 W(e) at beginning of mission, at least 125 W(e) after 5 years.

Notes: Voyager 1 and 2 each carried three MHWs onboard. The LES 8 and LES 9 communications satellites each carried two MHWs onboard.

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
CRC Handbook of Thermoelectrics edited by D.M. Rowe
U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future (8.5~ MB PDF)

General Purpose Heat Source (GPHS) RTG

Electrical Power: 285-300 W(e) at beginning of mission [depending on how ‘fresh’ the Pu 238 is]
Dimensions: 42.2 cm diameter, 114 cm length
Mass:
56 kg
Conversion Efficiency: 6.3%
GPHS Modules: 18

Notes: Galileo carried two GPHS onboard. Ulysses carried one GPHS. Cassini carried three GPHS onboard. New Horizons carries one GPHS.

(MMRTG Chart Sheet)

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future (8.5~ MB PDF)

Multi-Mission Radioisotope Thermoelectric Generator (MMRTG)

Electrical Power: 125 W(e) at beginning of mission
Mass: 44.2 kg
Dimensions: 64 cm diameter (fin to fin) and 66 cm long
Conversion Efficiency:
6.3%
GPHS Modules: 8

Notes: Used on Mars Science Laboratory.

(MMRTG Chart Sheet)

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future (8.5~ MB PDF)

Advanced Stirling Radioisotope Generator (ASRG)

Electrical Power: 140 to 150 W(e) at beginning of mission
Mass:
19 to 21 kg
Conversion Efficiency: 28-30%
GPHS Modules: 2

Notes:

(MMRTG Chart Sheet)

References:
Radioisotope Power Systems: An Imperative for Maintaining U.S. Leadership in Space Exploration, National Research Council
U.S. Space Radioisotope Power Systems and Applications: Past, Present and Future (8.5~ MB PDF)

Medium Power Reactor Experiment (MPRE)

References:
Space Nuclear Power
by Joseph A. Angelo, Jr and David Buden.

General Electric 710 Fast Reactor
aka
High Temperature Gas-Cooled Electric Power Reactor

Thermal Power: 700 kW(t)

Notes: Cooled by helium. Canceled in 1968.

References:
Advanced Nuclear Systems for Portable Power in Space: A Report; Appendix C by U.S. National Research Council.
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

Advanced Liquid Metal Cooled Reactor

Notes: Follow on to SNAP-50/SPUR program. Cancelled 1973.

References:
Space Nuclear Power by Joseph A. Angelo, Jr and David Buden.

Advanced Isotopic Source (AIS)

Fuel Element:
Electrical Power:
500W
Mass: 100~ lbs (exclusive of shielding and converter)
Useful Life: 6 months

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

AiResearch SPUR

Electrical Power: 300 kw(e)
Mass: 3,600~ lbs (12 lb/kw(e))

Notes: Was being developed for the USAF, and would use a high temperature liquid metal Rankine cycle.

References:
JPL TM 33-46 A Survey of Energy Conversion Systems (26 June 1961) (6.9~ MB PDF)

WADD SPUR II

Electrical Power: 1,000 kw(e)
Mass: 8,000~ lbs (8 lb/kw(e))

Notes: Was being developed for the USAF.

References:
JPL TM 33-46 A Survey of Energy Conversion Systems (26 June 1961) (6.9~ MB PDF)

SLLG (Soft Lunar Landing Generator)

Fuel Element: Cm242
Electrical Power: 752 w(e) at encapsulation, 655 w(e) at launch, and 475 w(e) at end of life.

Shielding: 1.6 inches of water for safe ground handling, to reduce radiation levels to 60 mrem/hr at 1 meter (compared to 125 mrem/hr without the shield)

References:
Novel Power Sources for Survival Shelters (Contract OCD-OS-62-243) March 1963 (3.9 MB PDF)

SP-100

Fuel Element: 93% Enriched U235
Thermal Power: 1,470 kW(t)
Electrical Power:
100 kW(e)
Height/Length: 55 cm (reactor); 8.5m (total system)
Diameter: 54.2 cm (reactor), 4.3m (total system)
Mass: 1,980 kg (unshielded system) plus 790 kg of radiation shielding
Useful Life: 7 years of full power operation.

Notes: Began development in 1977 under the DoD for a space reactor with a power range of 10-100 kW(e). Defined as the Space Power Advanced Reactor (SPAR). In FY1982, NASA became a co-sponsor of the program, and among other changes to make it compatible with the Space Shuttle, the name was changed to SP-100 in addition to raising the temperatures and energy density.

References:
A New Generation of Reactors For Space Power by J.E. Boudreau and D. Buden
Advanced Nuclear Systems for Portable Power in Space: A Report; Appendix C by U.S. National Research Council.