Whenever I see an article about storing nuclear waste for such longer periods of time, I can't help but think we will find a use for nuclear waste and end up digging it all back up way before 100,000 years.
It reminds me of that concept where if you want to travel to another star, the best time to leave is not 'as soon as possible'. It may actually be better to wait several decades or centuries to develop new propulsion technologies (possibly based on entirely new branches of science), launch later, and arrive earlier.
Blast, I can't remember what that's called.
And if we don't develop these machines, our AI augmented successors will.
Ironically, it's just really short-sighted to plan for something that far in the future.
It's not something making smaller and faster transistors can solve.
The mantle is ... large.
The total volume of the oceans is about 1.35 billion cubic kilometers.
The total volume of the mantle is 909 billion cubic kilometers, roughly 1,000 times greater.
Moreover, the radioactive minerals which came out of the Earth's crust either originated within, or are otherwise found in, the Earth's mantle and core.
We're also talking about timecycles of hundreds of thousands of years, and, frankly, a mixing dynamic which is probably not well known.
I'm actually not much a fan of nuclear power (it's potentially useful, but highly problematic, and much more limited in capacity than is generally understood), but deep-mantle-injection would actually be, on the grand scheme of things, statistical noise so far as any radioactive risk is concerned.
We're also not aware of any biological activity occurring within the mantle. The problems with ocean pollution -- plastics, metals, fertilisers, etc. -- were that these are:
1. Generally unevenly distributed, with concentrations in specific areas.
2. Highly interactive with life forms -- biological concentration of heavy metals, forming algael blooms, etc.
3. For lighter detritus (especially plastics), confined to what's effectively a film at the surface of the oceans, rather than mixed throughout the full volume of the ocean.
Even for substances which do mix with seawater, such as CO2, the rate of mixing through the entire benthic column is a concern.
Where radioactive waste has entered seawater (numerous reactor cores, mostly from the nine nuclear-powered submarines which have sunk, waste disposal, and liquid discharge e.g., from Daichi-Fukushima), dilution with seawater tends to make this a very low-level threat at any distance from the immediate site.
(Despite this, I strongly discourage the practice.)
My point: in the list of risks to worry about, this isn't one I'd spend much time on. I've already spent more than it's worth.
Or even if you count the awesome toys that Boston Dynamics puts out; they're still just that, toys. Incredible toys, but nowhere near taking over from humans.
We're nowhere near the AI that futurists of the past thought we'd have by now. Same goes for cancer cures (CRISPR).
Drawing targets around what you shot at regardless of where you're aiming has a name: Texas Sharpshooting.
It's an exceptionally poor rebuttal or response to the observation that forecasts have failed. Moreso if in doing so the reasons for the deviation isn't specifically analysed.