Fission Systems Space Propulsion

Fission systems - space propulsion

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 much more thrust for shorter periods and can be used for launches and landings.

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 already in space. Superconducting magnetic cells ionise hydrogen or xenon, heat it to extremely high temperatures (millions °C), accelerate it and expel it at very high velocity (eg 30 km/sec) to provide thrust.

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 (which can be sustained), with small plasma flow, but high thrust operation is possible. It is very efficient, with 99% conversion of electric to kinetic energy.

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.

SAFE has also been tested with an electric ion drive.

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

	SNAP-10 US	SP-100 US	Romashka Russia	Bouk Russia	Topaz-1 Russia	Topaz-2 Russia-US	SAFE-400 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/ epithermal	fast
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. 

In 2010 the Russian government was to allocate RUR500 million (about US$170 million) of federal funds to design a space nuclear propulsion and generation installation in the megawatt power range. In particular, SC Rosatom is to get RUR 430 million and Roskosmos (Russian Federal Space Agency) RUR 70 million to develop it. The work will be undertaken by N.A. Dollezhal NIKIET (Research & Development Institute for Power Engineering) in Moscow, based on previous developments including those of nuclear rocket engines, but beyond that the design envisaged is not known.   Russia's Energia space corporation is planning to start work in 2011 on standardized space modules with nuclear-powered propulsion systems, initially involving 150 to 500 kilowatt systems. A conceptual design is expected in 2011, with the basic design documentation and engineering design to follow in 2012. The life-service tests are planned for 2018. The first launches, are envisaged for about 2020.

The Director of Roskosmos says that development of megawatt-class nuclear space power systems for manned spacecraft is crucial if Russia wants to maintain a competitive edge in the space race, including the exploration of the moon and Mars. The project will require funding of some RUR 17 billion ($540 million). Energia earlier said that it is ready to design a space-based nuclear power station with a service life of 10-15 years, to be initially placed on the moon or Mars. It is also working on a concept of a nuclear-powered space tug, which could be used for launching satellites.