Space Power Reactors

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This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).

Introduction:

After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions. While Russia has used over 30 fission reactors in space, the USA has flown only one the SNAP-10A (System for Nuclear Auxiliary Power) in 1965.

The SNAP-10A reactor. (Source:NASA)

From 1959-73, there was a US nuclear rocket program—the Nuclear Engine for Rocket Vehicle Applications (NERVA)—focused on nuclear power replacing chemical rockets for the latter stages of launches. NERVA used graphite-core reactors, heating hydrogen and expelling it through a nozzle. Some 20 engines were tested in Nevada and yielded thrust up to more than half that of the space shuttle launchers. Since then, "nuclear rockets" have been about space propulsion, not launches. The successor to NERVA is today's nuclear thermal rocket (NTR). 

Another early idea was the US Project Orion, which would launch a substantial spacecraft from the Earth using a series of small nuclear explosions to propel it. The project commenced in 1958 and was aborted when the Atmospheric Test Ban Treaty of 1963 made it illegal, but radioactive fallout could have been a major problem. The Orion idea is still alive as other means of generating the propulsive pulses are considered.

* Radioisotope power sources have been an important source of energy in space since 1961.

* Fission power sources have been used mainly by Russia, but new and more powerful designs are under development in the USA.

After a gap of several years, there is a revival of interest in the use of nuclear fission power for space missions.

While Russia has used over 30 fission reactors in space, the USA has flown only one - the SNAP-10A (System for Nuclear Auxiliary Power) in 1965.

Early on, from 1959-73 there was a US nuclear rocket program - Nuclear Engine for Rocket Vehicle Applications (NERVA) which was focused on nuclear power replacing chemical rockets for the latter stages of launches. NERVA used graphite-core reactors heating hydrogen and expelling it through a nozzle. Some 20 engines were tested in Nevada and yielded thrust up to more than half that of the space shuttle launchers. Since then, "nuclear rockets" have been about space propulsion, not launches. The successor to NERVA is today's nuclear thermal rocket (NTR).

Another early idea was the US Project Orion, which would launch a substantial spacecraft - about 1000 tonnes - from the earth using a series of small nuclear explosions to propel it. The project was commenced in 1958 by General Atomics and was aborted in 1963 when the Atmospheric Test Ban Treaty made it illegal, but radioactive fallout could have been a major problem. The Orion idea is still alive, as other means of generating the propulsive pulses are considered.

Radioisotope Systems - RTGs

So far, radioisotope thermoelectric generators (RTGs) have been the main power source for US space work over nearly 50 years, since 1961. The high decay heat of Plutonium-238 (0.56 W/g) enables its use as an electricity source in the RTGs of spacecraft, satellites, navigation beacons, etc and its alpha decay process calls for minimal shielding. Heat from the oxide fuel is converted to electricity through static thermoelectric elements (solid-state thermocouples), with no moving parts. RTGs are safe, reliable and maintenance-free and can provide heat or electricity for decades under very harsh conditions, particularly where solar power is not feasible.

So far 45 RTGs have powered 25 US space vehicles including Apollo, Pioneer, Viking, Voyager, Galileo, Ulysses and New Horizons space missions as well as many civil and military satellites. The Cassini spacecraft carries three RTGs providing 870 watts of power as it explores Saturn. Voyager spacecraft which have sent back pictures of distant planets have already operated for over 20 years and are expected to send back signals powered by their RTGs for another 15-25 years. Galileo, launched in 1989, carried a 570 watt RTG. The Viking and Rover landers on Mars in 1975 depended on RTG power sources, as will the 900 kg Mars Science Laboratory Rover due to be launched in 2011 (the two Mars Rovers operating 2004-09 use solar panels and batteries).

The latest RTG is a 290 watt system known as the GPHS RTG. The thermal power for this system is from 18 General Purpose Heat Source (GPHS) units. Each GPHS contains four iridium-clad Pu-238 fuel pellets, stands 5 cm tall, 10 cm square and weighs 1.44 kg. The Multi-Mission RTG (MMRTG) will use 8 GPHS units producing 2 kW thermal which can be used to generate some 110 watts of electric power. It is a focus of current research and will be used in the Mars Science Laboratory, which will be a large mobile laboratory, the rover Curiosity, which is about five times the mass of previous Mars rovers.

The Stirling Radioisotope Generator (SRG) is based on a 55-watt electric converter powered by one GPHS unit. The hot end of the Stirling converter reaches 650°C and heated helium drives a free piston reciprocating in a linear alternator, heat being rejected at the cold end of the engine. The AC is then converted to 55 watts DC.

This Stirling engine produces about four times as much electric power from the plutonium fuel than an RTG. Thus each SRG will utilise two Stirling converter units with about 500 watts of thermal power supplied by two GPHS units and will deliver 100-140 watts of electric power from about 1 kg Pu-238. The SRG and Advanced SRG have been extensively tested but has not yet flown. NASA plans to use two ASRGs for its probe to Saturn's moon Titan (Titan Mare Explorer - TiME) or that to the comet Wirtanen.

Russia has developed RTGs using Po-210, two are still in orbit on 1965 Cosmos navigation satellites. But it concentrated on fission reactors for space power systems.

As well as RTGs, Radioactive Heater Units (RHUs) are used on satellites and spacecraft to keep instruments warm enough to function efficiently. Their output is only about one watt and they mostly use Pu-238 - typically about 2.7g of it. Dimensions are about 3 cm long and 2.5 cm diameter, weighing 40 grams. Some 240 have been used so far by USA and two are in shut-down Russian Lunar Rovers on the moon. Each of the US Mars Rovers which landed in 2004 uses eight of them to keep the batteries functional.

The Idaho National Laboratory's (INL) Centre for Space Nuclear Research (CSNR) in collaboration with NASA is developing an RTG-powered hopper vehicle for Mars exploration. When stationary the vehicle would study the area around it while slowly sucking up carbon dioxide from the atmosphere and freezing it, after compression by a Stirling engine.

Meanwhile a beryllium core would store heat energy required for the explosive vaporisation needed for the next hop. When ready for the next hop, nuclear heat would rapidly vaporise the carbon dioxide, creating a powerful jet to propel the craft up to 1000 metres into the 'air'.

A small hopper could cover 15 km at a time, repeating this every few days over a ten-year period. Hoppers could carry payloads of up to 200 kg and explore areas inaccessible to the Rovers. INL suggests that a few dozen hoppers could map the Martian surface in a few years, and possibly convey rock samples from all over the Martian surface to a craft that would bring them to Earth.

Both RTGs and RHUs are designed to survive major launch and re-entry accidents intact, as is the SRG.

Sources:

1. Poston, D.I. 2002, Nuclear design of SAFE-400 space fission reactor, Nuclear News, Dec 2001.
2. Poston, D.I. 2002, Nuclear design of HOMER-15 Mars surface fission reactor, Nuclear News, Dec 2001.
3. Vrillon et al, 1990, ERATO article, Nuclear Europe Worldscan 11-12, 1990.
4. US DOE web site- space applications.
5. Space.com 21/5/00, 16/6/00, 22/7/00, 17/1/03, 7/2/03.
6. www.nuclearspace.com
7. Delovy Mir 8/12/95.
8. G. Kulcinski, University of Wisconsin material on web.
9. Kleiner K. 2003, Fission Control, New Scientist 12/4/03.

10. OECD 1990, Emergency Preparedness for Nuclear-Powered Satellites.
11.  NASA web site