Nuclear Reactors for Space

Briefing Paper # 82

February 2003

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 several 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 from the earth using a series of small nuclear explosions to propel it. The project commenced in 1958 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

So far, radioisotope thermoelectric generators (RTGs) have been the main power source for US space work over more than 40 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. 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 44 RTGs have powered 24 US space vehicles including Apollo, Pioneer, Viking, Voyager, Galileo and Ulysses space missions as well as many civil and military satellites. The Cassini spacecraft carries three RTGs providing 870 watts of power en route to 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. The Viking and Rover landers on Mars depended on RTG power sources.

The latest RTG is a 290 watt system known as General Purpose Heat Source (GPHS). Each module containing four Pu-238 fuel pellets clad by iridium is 5 cm tall and 10 cm square, weighing 1.44 kg, and 18 modules make up one GPHS. The Multi-Mission RTG (MMRTG) will use 8 GPHS units producing 2 kW which can be used to generate 100 watts of electricity and is a focus of current research.

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 100-120 watts of electric power. The SRG has been extensively tested but has not yet flown.

Russia has also 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. There will be eight on each of the US Mars Rovers launched in 2003.

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

Fission systems

Over 100 kWe, fission systems have a distinct cost advantage over RTGs.

The US SNAP-10A launched in 1965 was a 45 kWt thermal nuclear fission reactor which produced 650 watts using a thermoelectric converter and operated for 43 days but was shut down due to a satellite (not reactor) malfunction. It remains in orbit.

The last US space reactor initiative - a joint NASA-DOE-Defence Dept program known as SP-100, with a 2 MWt fast reactor unit and thermoelectric system delivering 100 kWe, was terminated in the early 1990s after absorbing nearly $0.5 billion.

There was also a Timberwind pebble bed reactor concept under the Defence Dept Multi-Megawatt (MMW) space power program during the late 1980s, in collaboration with DOE. This had power requirements well beyond any civil space program.

Between 1967 and 1988 the former Soviet Union launched 31 low-powered fission reactors in Radar Ocean Reconnaissance Satellites (RORSATs) on Cosmos missions. They utilised thermoelectric converters to produce electricity, as with the RTGs. Romashka reactors were their initial nuclear power source, a fast spectrum graphite reactor with 90%-enriched uranium carbide fuel operating at high temperature. Then the Bouk fast reactor produced 3 kW for up to 4 months. Later reactors, such as on Cosmos-954 which re-entered over Canada in 1978, had U-Mo fuel rods and a layout similar to the US heatpipe reactors described below.

These were followed by the Topaz reactors with thermionic conversion systems, generating about 5 kWe of electricity for on-board uses. This was a US idea developed during the 1960s in Russia. In Topaz-2 each fuel pin (96% enriched UO2) sheathed in an emitter is surrounded by a collector and these form the 37 fuel elements which penetrate the cylindrical ZrH moderator. This in turn is surrounded by a beryllium neutron reflector with 12 rotating control drums in it. NaK coolant surrounds each fuel element.

Topaz-1 was flown in 1987 on Cosmos 1818 & 1867. It was capable of delivering power for 3-5 years for ocean surveillance. Later Topaz were aiming for 40 kWe via an international project undertaken largely in the USA from 1990. Two Topaz-2 reactors (without fuel) were sold to the USA in 1992. Budget restrictions in 1993 forced cancellation of a Nuclear Electric Propulsion Spaceflight Test Program associated with this.

For spacecraft propulsion, once launched, some experience has been gained with nuclear thermal propulsion systems (NTR) which are said to be well developed and proven. Nuclear fission heats a hydrogen propellant which is stored as liquid in cooled tanks. The hot gas (about 2500°C) is expelled through a nozzle to give thrust (which may be augmented by injection of liquid oxygen into the supersonic hydrogen exhaust). This is more efficient than chemical reactions. Bimodal versions will run electrical systems on board a spacecraft, including powerful radars, as well as providing propulsion. Compared with nuclear electric plasma systems, these have more thrust for shorter periods.

However, attention is now turning to nuclear electric systems, where nuclear reactors are a heat source for electric ion drives expelling plasma out of a nozzle to propel spacecraft. Superconducting magnetic cells ionise the hydrogen fuel, heat it to extremely high temperatures (millions °C), accelerate it and expel it at very high velocity. Research for one version, the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) draws on that for magnetically-confined fusion power (tokamak) for electricity generation, but here the plasma is deliberately leaked to give thrust. The system works most efficiently at low thrust, with small plasma flow, but high thrust operation is possible.

Heatpipe Power System (HPS) reactors are compact fast reactors producing up to 100 kWe for about ten years to power a spacecraft or planetary surface vehicle. They have been developed since 1994 at the Los Alamos National Laboratory as a robust and low technical risk system with an emphasis on high reliability and safety. They employ heatpipes to transfer energy from the reactor core to make electricity using Stirling or Brayton cycle converters.

Energy from fission is conducted from the fuel pins to the heatpipes filled with sodium vapour which carry it to the heat exchangers and thence in hot gas to the power conversion systems to make electricity. The gas is 72% helium and 28% xenon.

The reactor itself contains a number of heatpipe modules with the fuel. Each module has its central heatpipe with rhenium-clad fuel sleeves arranged around it. They are the same diameter and contain 97% enriched uranium nitride fuel, all within the cladding of the module. The modules form a compact hexagonal core.

Control is by six stainless steel clad beryllium drums each 11 or 13 cm diameter with boron carbide forming a 120 degree arc on each. The drums fit within the six sections of the beryllium radial neutron reflector surrounding the core, and rotate to effect control, moving the boron carbide in or out.

Shielding is dependent on the mission or application, but lithium hydride in stainless steel cans is the main neutron shielding.

The SAFE-400 space fission reactor (Safe Affordable Fission Engine) is a 400 kWt HPS producing 100 kWe to power a space vehicle using two Brayton power systems - gas turbines driven directly by the hot gas from the reactor. Heat exchanger outlet temperature is 880°C. The reactor has 127 identical heatpipe modules made of molybdenum, or niobium with 1% zirconium. Each has three fuel pins 1 cm diameter, nesting together into a compact hexagonal core 25 cm across. The fuel pins are 70 cm long (fuelled length 56 cm), the total heatpipe length is 145 cm, extending 75 cm above the core, where they are coupled with the heat exchangers. The core with reflector has a 51 cm diameter. The mass of the core is about 512 kg and each heat exchanger is 72 kg.

A smaller version of this kind of reactor is the HOMER-15 - the Heatpipe-Operated Mars Exploration Reactor. It is a15 kW thermal unit similar to the larger SAFE model, and stands 2.4 metres tall including its heat exchanger and 3 kWe Stirling engine (see above). It operates at only 600°C and is therefore able to use stainless steel for fuel pins and heatpipes, which are 1.6 cm diameter. It has 19 sodium heatpipe modules with 102 fuel pins bonded to them, 4 or 6 per pipe, and holding a total of 72 kg of fuel. The heatpipes are 106 cm long and fuel height 36 cm. The core is hexagonal (18 cm across) with six BeO pins in the corners. Total mass of reactor system is 214 kg, and diameter is 41 cm.

Space Reactor Power Systems

US US Russia Russia Russia Russia-US US
dates 1965 1992 1967 1977 1987 1992 2007?
kWt 45.5 2000 40 <100 150 135 400
kWe 0.65 100 0.8 <5 5-10 6 100
converter t'electric t'electric t'electric t'electric t'ionic t'ionic t'electric
fuel U-ZrHx UN UC2 U-Mo UO2 UO2 UN
reactor mass, kg 435 5422 455 <390 320 1061 512
neutron spectrum thermal fast fast fast thermal thermal/
control Be Be Be Be Be Be Be
coolant NaK Li none NaK NaK NaK Na
core temp. °C, max 585 1377 1900 ? 1600 1900? 1020

In the 1980s the French ERATO program considered three 20 kWe turboelectric power systems for space. All used a Brayton cycle converter with a helium-xenon mix as working fluid. The first system was a sodium-cooled UO2-fuelled fast reactor operating at 670°C, the second a high-temperature gas-cooled reactor (thermal or epithermal neutron spectrum) working at 840°C, the third a lithium-cooled UN-fuelled fast reactor working at 1150°C.

Project Prometheus 2003

In 2002 NASA announced its Nuclear Systems Initiative for space projects, and in 2003 this was renamed Project Prometheus and given increased funding. Its purpose is to enable a major step change in the capability of space missions. Nuclear-powered space travel will be much faster than is now possible, and will enable manned missions to Mars.

One part of Prometheus, which is a NASA project with substantial involvement by DOE in the nuclear area, is to develop the Multi-Mission Thermoelectric Generator and the Stirling Radioisotope Generator described in the RTG section above.

A more radical objective of Prometheus is to produce a space fission reactor system such as those described above for both power and propulsion that is safe to launch and which will operate for many years. This will have much greater power than RTGs. Power of 100 kW is envisaged for a nuclear electric propulsion system driven by plasma.

The FY 2004 budget proposal is $279 million, with $3 billion to be spent over five years. This consists of $186 million ($1 billion over 5 years) building on last year's allocation plus $93 million ($2 billion over five years) for a first flight mission to Jupiter - the Jupiter Icy Moon Orbiter, expected to launch in 2009-10 and explore for a decade.

Poston, D.I. 2002, Nuclear design of SAFE-400 space fission reactor, Nuclear News, Dec 2001.
Poston, D.I. 2002, Nuclear design of HOMER-15 Mars surface fission reactor, Nuclear News, Dec 2001.
Vrillon et al, 1990, ERATO article, Nuclear Europe Worldscan 11-12, 1990.
US DOE web site - space applications. 21/5/00, 16/6/00, 22/7/00, 17/1/03, 7/2/03.
NASA 1/2/02,
Delovy Mir 8/12/95.
G.Kulcinski, University of Wisconsin material on web

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