From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Fri Oct 22 1999 - 14:31:15 MDT
On Fri, 22 Oct 1999, David Lubkin wrote:
Lots of questions! Gee David, you are almost as curious as me.... :-)
> Once full-blown nanotech is here, is it feasible to go through an object
> atom-by-atom and replace all radioisotopes (not just the C14) with
> non-radioactive isotopes?
It depends on whether the nanobot can physically get to the molecules
containing the radioactive isotopes. Since nanobots themselves contain
billions of atoms the can't go "everywhere". They can sit in a cell
and slowly sort the circulating molecules, removing those containing
radioactive isotopes. Presumably as time goes by and the proteins
and lipids turn over, the radioactive atoms in those would decrease.
>
> How would the waste isotopes be safely removed from the object?
>
The simplest way for the nanobot to "weigh" the molecule and compare
it to the molecular weights from a table of perfect nonradioactive
molecules. Those molecules that are out-of-bounds aren't returned
to the cytoplasm. After the nanobot consumed a "full load" of
radioactive molecules/atoms it would migrate out of the body.
Presumably you would have to go in every month or year and execute
a "hot" nanobot recall protocol that would allow the physician to
collect the nanowaste. He then turns it over to an isotope
processing facility that breaks it down into atoms and sorts
them completely into pure isotopes. It then takes the worst
offenders and subjects them to nuclear bombardment to turn them
into stable isotopes, re-sorts them and releases the stable
isotopes to the environment. Or it could use the radioactive
isotopes as a breeder for 148Gd that gets returned to you in
your nuclear power nanobots.
See Nanomedicine Sect. 4.4.3 and 6.3.7 for the details.
> Could this be done in vivo to living objects?
Yep, within limits of available power and capacity for heat
dissipation and the turnover time between radioactive atoms
sequestered in slow-turnover molecules (bone, DNA, etc.)
and the cytoplasm.
You are also limited in the on-board carrying capacity of the
nanobots for nuclear waste. After all, they are nanotech
and they don't like high radioactivity any more than your
body does.
> Can a form of utility fog be used to prevent new radioisotopes in the
> object by blocking (a) intake of radioisotopes and (b) creation of
> radioisotopes by external influences (cosmic rays, etc)?
I think utility fog would not be the best approach.
You probably can have your nano-refrigerator-microwave-replicator
feed you only non-radioactive food (since it can go through the
same process with its feedstock that your internal nanobots go
through). Your house nano-filtration systems can probably
purify the air & water of 14C, radon and other isotopes.
You can certainly build structures strong enough to support
roofs & walls filled with iron or lead in sufficient quantities
to reduce your cosmic ray exposure by an order of magnitude or more.
So while you can cut down on the intake of the radioactive isotopes,
unless you decide never to go outside, drink or eat any "natural"
substances you are still going to have some exposure to radioactive
isotopes. Gives new meaning to the term "environmantal hazards". :-)
> Is it an inherent consequence of quantum mechanics that you
> can't prevent formation of radioisotopes within an object?
Probably. So long as an object receives some level of background
radiation, you will get the formation of radioisotopes. The
only way I can think of is to put a relatively small object
within a shield container built of nonradioactive material
that cuts the incoming radiation flux so low that the
probability of transforming a non-radioactive atom in the
object into a radioactive one would take millions of years.
This approach obviously has limits in terms of the amount
of material you can put in the shield and the size of the
object being protected.
>
> Beyond Spike's examples, what uses would there be for completely
> non-radioactive objects? Perhaps in scientific research?
>
Isotopically pure materials do have interesting properties.
I've got one conference note that isotopically pure 12C has
thermal conductivity 45% greater than natural diamond (but I would
want to check the reference to make sure). If Spikes points
regarding the decay of 14C in coal (and presmably diamond)
are valid, the difference must be due to the ~1% abundance of 13C.
This would probably make sense since slight distortions in the
crystal lattice should distort heat flow, perhaps electron
conductivity as well. I seem to recall that there is a company
providing isotopically pure Si for use in semiconductors.
> What uses would there be for objects whose radioactivity is precisely
> positioned?
Well, I'm pretty sure that radioisotopes in proteins may alter their
NMR properties.
Small concentrations of radioactive materials do make great power
sources (for spacecraft as we now sometimes build them, or in
humans as Nanomedicine points out). If we get the costs of
producing the isotopes low enough it seems to make sense
to build your air-car using nuclear power. Some of our rocket
engineers might want to comment on the tradeoffs due to shielding
requirements. It might not buy you much if anything.
>
> What would be the biological and geological repercussions if the entire
> planet, from core to biosphere, were made radioactivity-free? What
> would the impact on evolution have been if this had been done by a
> Power a few billion years ago?
Well, you could make it radioactivity free but unless you put it
in a big shield, the cosmic rays will slowly radioactiv-ize it
again. To get rid of the uranium and other radioisotopes
in the core you would really have to dismantle the planet and
reassemble it. According to my handy-dandy chart on planetary
disassembly, that will take ~22 days for Earth if you have the
full power of the sun at your disposal. You might get away with
doing it somewhat faster because you don't actually have to lift
all of the atoms out of the gravity well, but merely separate them
and weigh them and ship the radioactive ones to the sun. However
letting the heat out of the core and releasing the pressure from
gravitational binding will not be simple things to do without
disrupting the activities on the surface by quite a bit. Better to
relocate everyone to Mars & Venus while you do it.
Ultimately though to get the absolute lowest radiation levels,
you want to turn the planets inside out with the non-useful
material (iron?) serving as your overhead shield. Living on
the inside is the way to go given the natural radioactivity
of the universe. Two structures seem feasible:
a) A core sphere support of diamond radiating tapering diamond
pillars up to an overhead roof constructed of meters of iron
interlaced with buckytube cables. You could either make the
diamond core large enough to provide some gravity or rotate
the sphere in which case you will have variable artificial
gravity from the equatorial to polar regions on the inside
of the outer roof.
b) A core cylinder supporting pillars or cables up to a similar
cylinder roof (this is an O'Neill colony type architecture).
The cylinder rotates at a rate to give the inside of the
roof a reasonable level of gravity. Whether you need
pillars or cables depends on the central cylinder mass,
internal atmospheric mass, the mass of the roof and the
rotation rate.
There are probably some interesting design tradeoffs in these
structures to give you the best use of materials and the
lowest radioactivity levels.
Thinking about building these things requires a reprogramming
of your mindset to allow for the element ratios in the solar
system (using the values from Physics & Chemistry of the Solar
System, J.S. Lewis, Table II.4), I get:
O/C: 2.4x; C/N: 3.2x, C/Ne: 3.0x, C/Fe: 11.2x; C/Al: 118x
Given that the iron isn't good for much else you would put it
in the roof, but since you want to use all of your C for structural
materials (diamond & buckytubes), it probably makes sense to fill
the roof with H2O, O2, N2 and Ne. If the inside is a high power
production region (so you have lots of heat to get rid of), you build
it as a enveloped gas pressure container (with diamond+steel walls).
If power production low enough, or the sphere is really large and the
orbit is far enough from the sun, you can probably get away with
a supporting surface and an ocean of LN2/LO2 on which you had
floating icebergs in which you had encapsulated the Ne. But
given the mass available that isn't good for much else, I'm
pretty sure you can reduce the background radiation levels to
very low values. Just gotta hope that your roof doesn't
spring a leak.
Good questions.
Robert
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