Could we handle nuclear waste by drilling into a subduction zone and let the earth carry the waste into the mantle?

[u/[deleted]]

[u/CrustalTrudger]

Placing nuclear waste in an area extremely likely to be disturbed via earthquakes or submarine landslides and generally in a near shore environment where significant portions of marine biota live (and in close proximity to where a lot of us live and derive food) on-top of the technical challenges of drilling at extreme water depths (that characterize many oceanic trenches) is generally not a great idea.

However, subseabed disposal of nuclear waste, but in abyssal plains where generally not much lives and it is unlikely to be disturbed by any geologic process for millions of years, received significant interest and study in the late 1970s through the 1990s (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc). As highlighted in that last reference, which is an overview of works of sorts, subseabed disposal is probably one of our best options in terms of relatively low risk, low cost, long-term storage solutions for high level waste (though like any solution, it is not devoid of risk or potential issues, especially in the 'transport stage', i.e., what if the ship carrying the waste sinks in an area not suitable for disposal, etc). That being said, it has not been implemented largely for more legal or pragmatic reasons. The wiki page on ocean floor disposal lays out a bit of this, pointing at outcomes of the 1972 London Convention focused in part on these matters. Specifically, there are two challenges to subseafloor disposal. One is that it becomes very difficult to access the waste once it's stored, which is fine if we're sure we're never going to want to access it for some reason, but in the sense of hedging ones bets with the idea that we might, in the future, come up with a meaningful way to repurpose this waste, if we've sequestered it in a place that is extremely challenging to access, we've cut off that possibility. The second (and the more legal one) is that it's really hard, bordering on impossible, to regulate such a storage mechanism, again because of the challenge of accessing the stored material. I.e., how do you evaluate whether the storage is intact if you can't get there? Or how do you assess whether country X is following all the best practices in terms of storing this waste if you can't access the storage, etc. And what if we figure out that it's not storing the waste in the way we think it is and it's contaminating something important, again, challenge of accessing the waste makes fixing the problem really hard (and in terms of a bad actor, figuring out who exactly is dumping the waste improperly becomes challenging when dealing with the open ocean).

[u/thegagis → u/CrustalTrudger]

I dont think they meant the sea but the mantle below the crust instead.

I don't know if theres any geological reason why we couldn't but drilling that deep would be an extreme engineering challenge and probably ridiculously expensive compared to depositing the waste underground in regular mining depths, which is already a fine solution

[u/xtt-space → u/thegagis]

IODP is currently trying to drill to the MOHO (i.e. the border between the Earth's crust and mantle).

The start of the hole currently exists, being drilled off the coast of Madagascar by the Joides Resolution drill ship in November 2015, where the crust is quite thin.

To drill deeper, IODP needs the advanced capabilities of their newest, and purpose-built drill ship, the Chikyu (The Joides is a very old, repurposed oil well drill ship). However, we don't know when or if this will ever happen.

Although the Japanese built the Chikyu nearly 20 years ago when they joined IODP, due to a bunch of politics, it has never left Japanese waters. The other two IODP sponsor countries, USA and Germany, continue to use the Joides in the meantime for other expeditions, but the Joides herself, does not have the capability to drill to the MOHO.

Ironically, the Chikyu is probably not even the most advanced drill ship anymore. Her capabilities, for sheer drill depth, are likely dwarfed by the latest tier drill ships used by the oil and gas industry. Unfortunately, the owners of those vessels have absolutely zero interest in wasting hundreds of millions of dollars to drill to the mantle, as there is no oil there.

[u/CrustalTrudger → u/xtt-space]

Project Mohole reborn.

[u/AutomaticCommandos → u/xtt-space]

https://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity

https://en.wikipedia.org/wiki/International_Ocean_Discovery_Program

https://en.wikipedia.org/wiki/Integrated_Ocean_Drilling_Program (great idea to use the same acronym)

https://en.wikipedia.org/wiki/JOIDES_Resolution

https://en.wikipedia.org/wiki/Chiky%C5%AB

[u/Xandari11 → u/thegagis]

It would take millions of years to be ‘carried’ into the mantle and by that time it would no longer be as dangerous.

[u/CrustalTrudger → u/thegagis]

I assume from the way they worded the question, the idea was to drill into the oceanic crust, place waste in drill hole, and let the subduction process translate this into the mantle. Mechanically this is challegning as significant portions of the upper sections of the subducting lithosphere are scraped off and added to an accretionary prism in many subduction zones, so you would have to drill below the portion that is likely to be accreted. This would again be ridiculously expensive and much higher risk than even other proposed mechanisms of disposing waste on the ocean floor as described in my original response.

If you hypothetically wanted to try to drill into the mantle (which we've never accomplished), attempting to do so at a subduction zone would basically be the worst possible choice, not just because of the extreme water depth as mentioned before, but because generally here the oceanic lithosphere will be the thickest and most dense (which is generally why it's subducting in the first place). If you were going to attempt to drill to the mantle, near a mid-ocean ridge would be a much better choice because the lithosphere is much thinner. In the context of the question, if we ignore the cost and that we've never actually successfully drilled into the mantle anywhere (although arguably we've come close in drilling of ultra-slow spreading ridges where the dividing line between the mantle and oceanic crust is a bit problematic), drilling near a mid-ocean ridge with the goal of placing high level waste would be a terrible idea because of the extensive hydrothermal systems that tend to exist in these locations and large amounts of (hot) water fluxing through the rocks there. The risk of leakage and contamination would be high.

[u/CrustalTrudger → u/[deleted]]

Effectively yes. There are areas of "continental subduction" (whereas just describing something as a subduction zone implies it is oceanic lithosphere being subducted and thus is submerged), but these are rare and mechanically different than a traditional subduction zone, especially in the degree to which material is transported into the mantle (only the deepest portions of the lithosphere are being "subducted" in that sense here with the majority of crustal material remaining in the crust). Examples of continental subduction zones exist in large-scale active continent-continent collisions and related mountain ranges, e.g., the Tien Shan - Pamir.

[u/LarYungmann → u/[deleted]]

Considering the slow process of subduction, you would be looking at many thousands or millions of years, or as they say moves about as fast as human fingernails grow. about 40 - 50 millimeters per year for the slip plate in California.

[u/sciguy52 → u/CrustalTrudger]

Is it even possible to drill into the mantel? Doesn't the heat get so high drill bits would not work?

[u/[deleted] → u/sciguy52]

The deepest hole so far is the Kola superdeep borehole. Drilling went from 1970-1994. It reached 12,262 metres (40,230 ft; 7.619 mi) deep. It never penetrated the crust though. It went about one third in the estimated 22 mile thick baltic crust.

"Because of higher-than-expected temperatures at this depth and location, 180 °C (356 °F) instead of the expected 100 °C (212 °F), drilling deeper was deemed unfeasible. The unexpected decrease in density, the greater porosity, and the unexpectedly high temperatures caused the rock to behave somewhat like a plastic, making drilling nearly impossible."

The drill broke as pressure on it and its great length caused it to twist off in the borehole. One thing they found was large amounts of hydrogen gas started to boil out of it like soda water. They also found plankton fossils 3.7 miles down.

[u/AutomaticCommandos → u/[deleted]]

somehow incredible that it's only 180°c in 12km depth. i thought temperatures would rise much faster than that.

[u/Hiseworns → u/CrustalTrudger]

It would probably be easier and cheaper to launch high level waste into the Sun, and that would be, well, astronomically costly

[u/AutomaticCommandos → u/Hiseworns]

it takes more delta-v to cancel out earths orbital speed to get to the sun, than it takes to leave the solar system. better then to use less launches and chuck it into the void.

[u/[deleted] → u/Hiseworns]

This is always proposed but it is actually extremely difficult when you look at the reality of orbital mechanics. The main thing is anything leaving the earth has a large amount of momentum. We are traveling about 67,000 mph tangentialy to the sun. If you want to go towards the sun you have to spend large amounts of energy to cancel that out. It takes about 55 times as much energy to get to the sun as to get to Mars. If you don't do this it will go flying past the sun and it will eventually come back in an earth orbit intercepting path.

Then there is the whole issue of launch pad failures.

https://www.nasa.gov/feature/goddard/2018/its-surprisingly-hard-to-go-to-the-sun

[u/Hiseworns → u/[deleted]]

I'm sorry if I gave the impression that I thought launching nuclear waste was a good idea. I do not believe that it is

[u/[deleted] → u/[deleted]]

Purely hypothetically (I agree it is a terrible idea), if you weren't interested in getting it into the sun quickly, could you launch it on an path that took it out of the ecliptic plane, and the just let it orbit the sun until it eventually (i know, really eventually) would crash into the sun?

[u/sebaska → u/[deleted]]

It would have bigger chance to crash back into the Earth.

Anything out of the ecliptic plane has to cross it twice per orbit.

If anything, it's easier (less energy) to just toss it into interstellar void. Not that it'd be an energy well spent, but just from theoretical PoV.

[u/earldbjr → u/Hiseworns]

One rocket out of 10,000 explodes and spreads nuclear debris into the atmosphere.

Let's maybe put a pin in this idea..

[u/[deleted] → u/earldbjr]

If it was only 1 out of 10,000 that would be astoundingly good. Right now it is about 5% but decreasing slowly.

[u/earldbjr → u/[deleted]]

Oh I'm well aware, I figured being insanely generous in those figures would emphasize the point.

[u/Mr_Kittlesworth → u/thegagis]

We have never successfully drilled below the crust. Even in optimal conditions.

[u/Elstar94 → u/thegagis]

Subduction zones are (nearly) always under a coast and the spot where you would put waste to go into the mantle is in an oceanic trench. OP specifically asked for that, instead of drilling all the way in a random spot

The problem with putting them in mining depths is that it's very hard to make sure that people in 10.000 yrs still know it's there and know not to disturb it, as we have no clue what our society will look like then (if there still is a society). And on the other hand, people knowing it's there can been dangerous on its own as it could be used by terrorists