Molten Salt Fast Reactor proposal for Mars Vasimr Mission and Other Power Alternatives

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Recently the Space Review had an article by Jeff Foust which indicated that Zubrin believes it is quite optimistic to get an alpha of 20 kg/kW for a power source for a VASIMR plasma rocket. The VASIMR mission architectures with the 39-day travel times had assumed an overall mission mass of approximately 600 tons. The VASIMR-based Mars mission concepts, he said, assume an alpha of 1 kg/kW.

Here we will review ways to approach or exceed 1 kg/kw power sources, which would enable VASIMR rocket to get to Mars in 39 days.

Molten Salt Fast Reactor would take about 50 kg of plutonium and get to about 3 kg/kw.

Uranium nitride reactors are funded and being commercially developed for 2013-2018 and could get to 2-3 kg/kw.

Vapor Core Reactors have a bunch of academic study and are expected to achieve 0.3-1 kg/kw.

Stretched lens solar arrays could go from 3kg/kw to 1kg/kw.

A proposed strontium 90 beta decay thermophotovoltaic system could achieve 10kg/kw and better photovoltaics and other improvements might enable about 5kg/kw

There was a 2003 MIT study for using a small Molten Salt Fast Reactor to power a VASIMR plasma rocket to Mars.

VASIMR is assuming the development of Vapor Core reactors that could achieve 0.3 to 1 kg per kilowatt. This would be three to ten times better than a proposed molten salt fast reactor.

There are small 25 MWe fast reactors being developed by Hyperion Power Generation that would be could be in the 2kg/KWe range and should be ready around 2013-2018. The reactor would weigh 15 tons (another five tons for a cask.) It could also use a proposed thermophotovoltaic system to convert the heat to electricity (in an MIT study which this article will cover). Hyperion Power Generation is not building molten salt reactors, but uranium nitride reactors that are lead-bismuth cooled. There are older studies of uranium nitride reactors for space applications.

MIT was projecting less than 3 kg/KWe for a molten salt fast reactor with 4 MWe.

It would use 50 kilograms of Plutonium. The NASA Cassini probe to Saturn used 28.8 kilograms of plutonium.

Power 11 MWth
Dimensions 20X20X20cm
Total mass 185 kg - (50 kg Pu)
Reflector thickness 6 cm (Zr3Si2)
Coolant - molten salt (NaF-ZrF4) 
       - High Boiling Temp
Fuel - Reactor Grade Pu carbide,
           honeycomb plates 
keff BOL = 1.1
Core lifetime 540 FPD 

A thermophotovoltaic system with 100 watts/kg for the whole power source and 263 Watts/kg for the heat to electricity conversion system.

This is a proposed system for powering a spacecraft in the 5kW – 500 kW power range. This system uses a high-temperature heat source, powered by a beta-decaying isotope (Strontium 90). A heat-to-thrust efficiency of 18 – 27% becomes possible with the use of FEEP/colloid propulsion. For exploratory missions, the final rocket speed estimation is made for a payload mass of 300 kg and the electricity generator weighing 1.3 tons, which provides 125kW electric output. Assuming a 6 ton initial rocket mass, a final rocket speed of over 130 km/s can be achieved in less than 6 years. With future improvements in the temperature stability and spectral efficiency of tandem-filters, higher photovoltaic operating temperature would become possible. The use of custom-designed photovoltaics, which has been outlined in this paper, will increase the power per mass ratio and the feasible rocket performance even further.

Stretched Lens Array (SLA)/SquareRigger enables giant space solar arrays in the 100 kW to 1 MW class, with spectacular performance metrics (300 to 500 W/kg specific power, 80 to 120 kW/m3 stowed power, and operational voltages above 1,000 V) in the near-term (2010) to mid-term (2015). In the longer term (2020-2025), with constantly improving solar cell efficiencies and incorporation of new nanotechnology materials into the lens and radiator elements, SLA’s technology roadmap leads to 1,000 W/kg solar arrays.

More Vapor Core Reactor Info

Development of Liquid-Vapor Core Reactors with MHD Generator for Space Power and Propulsion Applications, DOE Project: DE-FG07-98ID 13635
Principal Investigator: Samim Anghaie
Department of Nuclear and Radiological Engineering, University of Florida August 13, 2002

Optimum Utilization of Fission Power with Gas Core Reactors (GCR) by Robert Norring (74 pages)

The GCR is up to 15 times more efficient than a typical LWR (light Water Reactor) in terms of fuel utilization (percent of fuel fissioned). The GCR produces as little as one-tenth the actinides of a conventional LWR, while limiting the overall waste by two orders of magnitude, and is inherently resistant to weapons proliferation. The GCR shows promise as being a viable means of power production while limiting nuclear waste.

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