The post states that tungsten "...has a low corrosion rate at elevated temperatures." This is not accurate.
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.
> A self-sinking probe is basically a dumb probe measuring less than 100 cm in diameter - a lump of nuclear waste encapsulated in a tungsten sphere and sunk into the ground.
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?
Nuclear waste is waste because it doesn't produce enough heat anymore to be useful in production of electricity. So I doubt that spent nuclear fuel would be able to produce 1000C+ temperatures for 30 years to bury itself deep enough.
Nuclear waste is waste because it doesn't contain enough fissile nuclides (i.e. nuclides, which could produce heat in fission). It has almost nothing to do with radioactivity or decay power. Radioactivity just makes it harder to manage.
But you have it right: The decay power density of nuclear waste is quite soon not enough for such a probe.
IANAG, but it probably doesn't need to get 100km under the ground. It only needs to go to the depth at which the nuclear waste stops being a problem.
With a good amount kilometers of rock between the surface and the waste, the danger is already gone. Furthermore, melting the waste and have it move thorugh the mantle will lower its concentration and will over time have the heavy materials fall into the core.
My guess is that the point at which the nuclear waste becomes less problematic than current methods will probably occur quite quickly, just a few kilometers if that. The point at which the tungsten corodes and the waste can be swept away doesn't seem to be that deep either.
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.