Why Not Dispose of Waste in Ocean Trenches?

It seems to be a perennial suggestion: let's put our most hazardous wastes into the deepest sea trenches. There, they will be drawn down into the Earth's mantle well away from children and other living things. Usually, people are referring to high-level nuclear waste, which can be dangerous for thousands of years. This is why the design for the proposed waste facility at Yucca Mountain, in Nevada, is so incredibly stringent.

The concept is relatively sound. Just put your barrels of waste in a trench - we'll dig a hole first, just to be tidy about it - and down they inexorably go, never to bring harm to humanity again.

At 1600 degrees Fahrenheit, the upper mantle isn't hot enough to alter the uranium and make it nonradioactive. In fact, it's not even hot enough to melt the zirconium coating that surrounds the uranium. But the purpose isn't to destroy the uranium, it's to use plate tectonics to take the uranium hundreds of kilometers into the Earth's depths where it can naturally decay. 

It's an interesting idea, but is it plausible? 

Ocean Trenches and Subduction

Deep-sea trenches are areas where one plate dives beneath another (the process of subduction) to be swallowed up by the Earth's hot mantle. The descending plates extend down hundreds of kilometers where they are not the least bit of a threat.

It isn't completely clear whether the plates disappear by being thoroughly mixed with mantle rocks. They may persist there and become recycled through the plate-tectonic mill, but that wouldn't happen for many millions of years. 

A geologist might point out that subduction is not really secure. At relatively shallow levels, subducting plates become chemically altered, releasing a slurry of serpentine minerals that eventually erupt in large mud volcanoes on the seafloor. Imagine those spewing plutonium into the sea! Fortunately, by that time, the plutonium would have long since decayed away.

Why It Won't Work

Even the fastest subduction is very slow - geologically slow. The fastest-subducting location in the world today is the Peru-Chile Trench, running along the west side of South America. There, the Nazca plate is plunging beneath the South America plate at around 7-8 centimeters (or approximately 3 inches) per year. It goes down at about a 30-degree angle. So if we put a barrel of nuclear waste in the Peru-Chile Trench (never mind that it's in Chilean national waters), in a hundred years it will move 8 meters - as far away as your next-door neighbor. Not exactly an efficient means of transport. 

High-level uranium decays to its normal, pre-mined radioactive state within 1,000-10,000 years. In 10,000 years, those waste barrels would have moved, at maximum, just .8 kilometers (half a mile). They would also lie only a few hundred meters deep - remember that every other subduction zone is slower than this.

After all of that time, they could still be easily dug up by whatever future civilization cares to retrieve them. After all, have we left the Pyramids alone? Even if future generations left the waste alone, the seawater and seafloor life would not, and the odds are good that the barrels would corrode and be breached.

Ignoring geology, let's consider the logistics of containing, transporting and disposing of thousands of barrels each year. Multiply the amount of waste (which will surely grow) by the odds of shipwreck, human accidents, piracy and people cutting corners. Then estimate the costs of doing everything right, every time.

A few decades ago, when the space program was new, people often speculated that we could launch nuclear waste into space, maybe into the sun. After a few rocket explosions, nobody says that any more: the cosmic incineration model is infeasible. The tectonic burial model, unfortunately, isn't any better.

Edited by Brooks Mitchell