NASA may try to demonstrate a space-rated nuclear reactor on the Earth's Moon first, instead of on a mission to explore the icy moons of Jupiter. Administrator Sean O'Keefe tells the NASA Advisory Council it remains an open question how the Project Prometheus space nuclear power initiative will demonstrate its hardware. "Where you go and exactly how you accomplish that task first is of no moment," he says. A lunar-surface reactor would meet President Bush's directive to use the Moon as a testbed for eventual human surface operations on Mars, where many believe nuclear power would be very useful. Nuclear propulsion to enable something like the proposed Jupiter Icy Moons Orbiter (JIMO) is "inevitable," O'Keefe says (see p. 56). But he suggests that the Energy Dept.'s naval reactors office, which will develop the space reactor, has its own ideas of how best to proceed that may not match those of the scientific community, and a lunar demonstration might be a quicker way to prove out the basic technology. JIMO faces major technical challenges and a program review next spring.
I think this actually makes sense. Putting the first reactor on the moon allows a full up test of the reactor, control and power generation systems on a remote, yet stable environment under conditions similar to where the unit will operate (vacuum.) Further, a cammera toting rover could be used to monitor the unit visually from a distance (or remote controlled cammeras closer to the reactor) to visually correlate problems such as coolant leaks, etc. Further, a rover could be used to make far field radiation measurements--crucial data for shielding design of subsequent spacecraft.
This makes good sense. Also, eventually, we're going to need a nuclear reactor on the moon one day anyways if we are ever to establish humans on the moon semipermanently. We might as well go ahead and test the technology.
are they effectivly beefed up switchable RTG's or do they use moving parts such as sterling engines or brayton cycle turbines (problematic i would gave thought on a spacecraft but OK for a lunar/mars etc instalation where it is on the ground and in a gravity field)
It shouldn't really matter: Brayton cycle turbines and Sterling engines should work quite well in a vacuum. The choice of working fluid and operating pressure, along with desired power output would be the deciding factors in estimating system size and mass. The system must use a thermal radiator for exhausting heat because of the vaccuum (in the vacuum of space there are no flowing streams of cooling water, or air to heat in a convection cooling tower.) This could be quite easily accomplished using a cruciform array of panels on a short tower somewhere near the power conversion units. The cruciform pattern is so that during daylight on the moon, atleast two of the panels would be in constant shade, thus increasing their thermal capacity. [This obviates the need to use a sun tracker to automatically position the radiator panels to be always in shade--thus eliminating complicated pressure joints which would probably leak!]
For a small conventional reactor I should think that a Stirling Cycle unit could do quite well up to about 200kWe. Above that, I would imagine that a Brayton cycle gas turbine with helium as a working fluid would be more efficient and mass less than its reciprocating cousin. If the turbine/compressor unit used magnetic bearings (already being used on small gas turbine generators on Earth!) then I don't see why this unit could not function continuously for years.
RTG's are dead last in efficiency--with less than 1% thermal conversion into electricity. However for probes like Voyager, Pioneer, Galileo, and Cassini, RTG's are a preffered choice because of their extreme longevity. Being a solid state device (no moving parts) RTG's can last for decades!
I dont have a problem with reciprocating/rotating machinary on a moonbase but I can see there would be issues on board a spaceship. special attention would have to be paid to vibration and conservation of angular momentum (turbine spins one way, spaceship spins the other) the magnetic bearings would, i immagine, be extreemly desirable since I understand that lubrication can be an issue in zero g and we are looking at machinarry that will have to run unattended for decades.
Ironically many of these problems would go away if we were considering a reactor on an Orion, not least because "scotty" could do regular oil changes ang go and fix it if the bearings wore out!
As I recall, there are at least three other alternative energy conversion schemes suitable for medium-to-high-power application : the recently demonstrated acoustic thermo-electric generator; thermionic energy conversion (usually using cesium as working fluid); and of course VCR-MHD.
As Googlenaut says, the heat rejection radiators would be necessary in all cases. However, once again, the permanently-shadowed polar regions would be advantageous siting locations from the point of view of employing efficient heat sinks. Perhaps the heat rejection could even be combined with a lunar ice extraction plant: heating dirt containing a small percentage of water & ammonia ice, and condensing them in heat-traced storage tanks.
Bingo, Jaro! Waste heat could easily be used to drive extraction of volatiles. No problem there. A network of flexible tubing containing warm working fluid laid directly on the ground with a thin layer of plastic sheeting to catch evaporates.
As far as turbines and generators go on a space craft, Dusty you bring up an excellent point. It would be my opinion that such units may have to operate in pairs, with one unit counter rotating from the other. Momentum wheels could be used to absorb and accumulate small differences in angular momentum. A fairly straightforward momentum 'dump' could then be performed periodically (slowing down and stopping the momentum wheels) whenever a relatively large delta-v burn was necessary. Magnetic and gas bearings should mitigate most of the lubrication issues, although Du Pont Krytox is just about the industry's very best cryogenic rated, oxygen resistant lubricating grease for high speed bearings. This grease is used on the space shuttle main engine turbopump bearings. It might work pretty well in a Brayton cycle turbine, provided that the bearings are cooled (as they should be.)
