Tungsten oxidizes in air beginning around 600°C and as the temperature increases, the tungsten oxide layer scales off, exposing underlying metal to further oxidation. (see, for example, http://labfus.ciemat.es/AR/2011/C_004/AM_4x.pdf)
Tungsten is great for high temperature use in vacuum, neutral (the inert gases) or reducing environments (hydrogen, for example). You can use it nearly up to its melting point in those conditions if you aren't too dependent on structural integrity.
In oxidizing environments (air, oxygen, water, halogens, silicates, etc.) it fails quite rapidly. Molten rock is replete with chemical species that react with tungsten at elevated temperatures.
At 2000°C, the tungsten blanket covering the Co60 heat source would be corroded away, I'll guess, within a week of launch on its journey to the center of the earth.
Although it would be incredibly costly, they might have better luck with iridium or rhenium.
Nevertheless, a fun mission to think about.
Even more so if you wonder if it couldn't be a way to dispose of waste. Also, if corrosion was an issue, perhaps they could coat it with something?
Happened to see it while archiving the inventor's papers. :)
[1] https://en.wikipedia.org/wiki/Artemis_Fowl:_The_Opal_Decepti...
Okay, so you have something that's so self-heating that it'll easily melt rock. In fact, it's hot enough that it self-liquifies quickly at STP. Cobalt reacts weakly with oxygen, but you'll still have to be careful with it in air, so you'll have to seal it in something; at 2000K, there are only a few materials with which you can hold and seal it, tungsten being one. Also, it's radioactive, and the tungsten sphere you put it in isn't nearly sufficient to stop the gammas.
So, you get your tungsten sphere all ready to go, let the cobalt liquify itself, pour it into the sphere, and then lower/drop it into a borehole. Better not be any water down there, or it might come back up.
Once you've got it doing it's melting thing and it's really deep at the bottom of a borehole, it probably can't hurt anyone.
I can't imagine a funding agency being ballsy-enough to fund it, and _really_ can't imagine a nuclear regulatory agency being interested in letting you build a source that could get itself so thermally hot.
TL;DR -- lots could go wrong.
It's still a spooky thing to assemble, but it does appear that if the shielding is preserved, that it could be done without ridiculous quantities of assembly/launch shielding.
(Apparently the cobalt would be inserted into the tungsten sphere as a not-yet-liquid sphere. That would make the multiple deliveries and robotic assembly more difficult, so maybe a liquid-assembly like you suggest could be arranged.)
How the hell do you weld tungsten anyway? Maybe make the sphere in two halves, then spin them up in opposite directions and push them together, to friction weld them?
Why would it make any noise?
Or would the magma just freeze as it travels through the relatively cool hole?
"Heat generated from the decay of radioactive cobalt-60 allows the probe to melt its way into the Earth. The probe is estimated to melt down to a depth of 20 km in ~1 year. As the probe descents deeper, the rate of descent will gradually slow until the probe reaches a depth of 100 km after ~30 years. By melting its way into the Earth, the probe will leave behind a wake of molten material. Subsequent re-crystallisation of the molten material will generate intense acoustic signals."
Worked fine with all the depleted uranium dropped on Iraq.
The article doesn't mention - why will the probe stop descending?
Not so simple. You need a hot source and a cold sink to transform energy. Where's your cold sink? This thing is intended to melt what's around it, and the outside of the probe is not that different in temperature from the inside.
Only if it's thermal energy. Given the small amount of power needed, they could easily use a betavoltaic generator.
And the tungsten is touching the cobalt, with no air gap (i.e. to opportunity to harvest power from the electrons returning to the cobalt to neutralize charge).
How would you harvest the electrons?
