Thorium
Thorium is the most abundant radioactive element.
It occurs as a byproduct of rare earth mining in great volume. A good fraction of the mined material there is Thorium. These are mountains of radioactive thorium not just a few thousand barrels. This thorium has not ever seen a reactor core. Thus it is not as radioactive as high level nuclear waste (except in a few natural nuclear reactor sites - yes they exist). But still it this thorium is radioactive.
Putting the "waste" thorium back into the ground where it came from creates a worse situation than before. The fact that the amount of the radioactive material is a little less and the level of radioactive material is a little higher is not so much of a problem. The real problem is that today (2016) the mining waste usually is in a fine grained powdery form (TODO: check facts in detail) which is (unlike the original monazite rock) suszeptible to ground water washout. If the mining remnants are dumped into an attle heap (open air mining dump) winds may carry away thorium laden dust.
Contents
Safety depends strongly on the "packaging" on all size scales
Historically Thorium oxide glass thorium dioxide has been used for lenses in camera optics (e.g. olympus lenses turning yellow due to radiation damage) There the thorium it is pretty safe since it can't disperse. Vitrification [1] of those massive amounts of mining remnant thorium laden material is currently not done presumably because of expensive equipment and high energy requirement. With advanced atomically precise technology energy should become much more cheap and maybe vitrification can be done non-thermally. Non thermal vitrification may involve atomically precise disassembly (TODO: Is chemical preperatory dissolving possibe?) which is nontrivial and not expectable early on.
See: mobility prevention guideline
In short nature provided safety by "packaging" the thorium in big rocks of the mineral monazite. If we do similar we can be as safe as nature.
Related: diamondoidivity and machine phase
Thorium as today's nuclear waste
Thorium is a main component of today's radioactive waste. It still carries the biggest part of valuable energy to its grave.
Thorium as tomorrow's resource
Thorium is a fertile nuclear fuel material. That means if exposed to neutrons of the right kind of energy it can be bred (transmuted) to fissile material. See: APM and nuclear technology
Occurrence
- New results from measurements on natural neutrino fluxes seem to indicate that most of the thorium in Earth is concentrated in the crust and has not sunk down towards the earth's center (as one might suspect due to thorium being a heavy element) (wiki-TODO: add reference) (Does this have to do with its chemical properties? Electropositivity? Interaction with oxygen?). Metallic meteorites (when stemming from a smashed and segregated proto-planetary seed) seem to confirm that with their composition. This may be bad news for mining of metallic asteroids for nuclear fuel.
