Re: Nuclear heater for Expeditions

Eugene Leitl (
Thu, 8 Jan 1998 19:44:52 +0300 (MSK)

On Wed, 7 Jan 1998, Dan Clemmensen wrote:

> True. but the original question was (more or less) "can we build 'mister
> fusion'?" I responded "Yes, given nanotech."

'Mr. Fusion' as in 'Back to the Future'? Not with hot fusion (while the
cold one being still mythical). Reactor volume way too small (even JET is
too small). Perfect diamondoids (which are as susceptible to rad ageing
as everything else) are no Unobtainium.

> > 2) If nanotechnology _can_ work, it can do very little to increase safety
> > of nuclear processes. At best, you can expect a near 100% rate of
> > waste isotope reclaiming. Otoh rad damage seems to be incompatible with
> > 'living' nano devices, and will certainly drastically reduce lifetimes
> > of nonliving ones.

> I disagree. There are two major perceived dangers of nuclear energy:
> waste isotopes and massive energy release. If nanotech can permit us to

Yes. But massive energy release and waste isotope release are combined in
a catastrophic failure. To prevent catastrophic failure you must operate a
a sensor-permeated system way out the critical regime, scrapping, and
rebuilding it when it grows brittle enough. Continous autorepair is
impossible due to high fluxes (thermal, rad) in a small volume. At these
hellish conditions your nanoagents will drop as flies, faster than you
could remake them. (As a side note, design of radresistant nanosystems is
an absolute prerequisite for the really fast interstellar probes, both
because of the relativistic interstellar medium rad load and due to the
nuclear propulsion and/or energy source emissions. (Then there is laser
propulsion, and Forrester's nanoprobe catapults)).

As I already said, you can quantitatively reclaim nuclear ashes. Question
is, can you safely exclude catastrophic failure of the reactor device? Not

> build tiny reactors, the massive energy release goes away: your "mister
> fusion" is more like your gas tank than like a supertanker or an oil
> refinery in this regard. If, as you say, nanotech can permit complete

The smaller the reactor, the more critical the operation regime. The
enrichened uranium high-flux research reactor currently being erected in
Munich, Garching has a core the size of a wastebasket. Its energy density,
and large surface of the cooling vents at a record neutron flux do not
seem to make for a passively safe system.

> recovery of the dangerous isotopes generated by neutron activation, then
> all that remains is deactivation of those same isotopes. This is also
> perfectly feasible, because these isotopes can be exposed to still more
> fast neutrons by recycling them back into the fusion reactor. Most of

This is a feature I also do not like about planned fusion tritium
breeders. Though power density of a hypothetical production tokamak will
be relatively low, the wall radiation load will be quite demanding. We
have already miles and miles of tritium-contaminated liquid lithium pipes,
now we must also intertwine them with hot waste ducts. I do not see how
this will limit the total waste curie output.

> atoms will eventually transform to stable isotopes under additional
> exposure to fast neutrons. The tiny percentage that do not can be
> separated (again by nanotech) and deactivated in a tiny particle

Not by nanotech. It just makes conventional isotope separation cheaper,
and increases the yield. It is difficult to see how dirt cheap but precise
macroscopic structures could make such a big difference.

> accellerator. These processes are on balence highly exothermic, so the

Why tiny? I do not see how nanotech can decrease the accelerator size
significantly. Novel acceleration principles do not rely on nanotech for

> entire process of deactivation should yield a substantial net gain in
> useable thermal energy. All of this hold true even if your reactor must
> use D-D or D-T, both of which produce a lot of primary fast neutrons and
> therefore a relatively large amount of isotopic transformation.
> Remember that nanotech is likely to permit the use of diamondiod
> structural components instead of steel, so the reactor will be
> activating carbon instead of iron: that is, most of the resulting

Do diamond vacuum chamber walls age gracefully under high
thermal/radiation load? I think not.

> isotopes will be fairly benign. Now, if nanotech permits us to use the
> H-H reaction instead of D-D, we won't have much of a problem to solve,

I don't see how a chemical-energy-level technology could significantly
assist nuclear reactions, besides increasing cheapness of construction,
enhanced supervision and waste recycling yield.

> because there are no fast neutrons generated in the primary reaction.