Re: Radioactive decay and long-term preservation

Robert J. Bradbury (
Fri, 22 Oct 1999 13:31:15 -0700 (PDT)

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:

  1. 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.
  2. 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.