[Someone]

> They’re digging one mile (1.6 km) holes dropping low enriched nuclear fuel to the bottom, and filling them with water.

> When the fuel is used up, they can leave it where it is since it’s below the water table.

To do both, they’ll have to guarantee that that column of water stays isolated from groundwater for a long time after the fuel is used up.

Reading https://www.deepfission.com/faq, they answer that question with

“Importantly, the mile-deep column of water in the borehole is expected to provide the pressure conditions required for safe reactor operation. Water within the borehole is also intended to contribute to the reactor’s thermal management system”. So, what do they do if their drill hole starts leaking, and they lose pressure?

and

“Our boreholes are expected to be lined with multiple layers of casing, including steel casing and concrete intended to maintain structural integrity and isolate surrounding geological formations”

I don’t think those are good answers. They say what they want to do, but almost nothing about how they’ll do that, and try to avoid making hard statements on the what by using “is expected” and “is intended”.

[quotemstr]

What a waste of perfectly good reprocessing input. "Spent" convention nuclear fuel retains 95% of its energy. Discarding "spent" fuel is a shamefully profligate energy practice we can get away with because we're using not nearly enough nuclear power, making and virgin fissiles are dirt cheap.

[pseudosaid]

they just recently nailed converting spent fuel into tritium

[Asooka]

> To do both, they’ll have to guarantee that that column of water stays isolated from groundwater for a long time after the fuel is used up.

I wonder if just letting the water gradually dissolve the uranium might not be fine, actually. If it is done far from wells and rivers used for drinking water, then the small amount of radioactive minerals that slowly seep out might not pose a danger. I can't find any studies to back it up, but I imagine there are places on Earth at which enriched uranium buried 1.6km underground poses no threat. I am no expert, so I would love to hear what others think.

[leonidasrup]

Of the all the radioactive elements in a nuclear reactor, uranium is one of the least dangerous (is very weakly radioactive, but it's toxic, it's a heavy metal). It's all the radioactive fission products (many have short half-life and in process of decay produces lot of ionizing radiation) and trans-uranic elements (because they have very long half-life and produce a bit small bits of ionizing radiation for very long time).

One possible of measure of danger is median lethal dose LD50:

Uranium LD50 in mice 114 mg/kg (about the same as Cocaine LD50 96 mg/kg)

Plutonium LD50 in dog 320 μg/kg

Caesium-137 LD50 245 μg/kg

Polonium-210 LD50 10 ng/kg (estimated)

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

There are places underground with high concentration of uranium, they are called uranium ore and sometimes they are mined for uranium.

"The deposit is located at depth of 450 m (1,480 ft), surrounded by and isolated within a layer of water-impermeable illite-chlorite clay, within the Athabasca Sandstone formation. Its age is estimated to be 1.3 billion years. Due to natural containment and lack of any traces of radioactive elements on the surface, the deposit is used as an example of an effective natural deep geological repository."

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

[JumpCrisscross]

On the upside, the half life of those byproducts is measured in days to decades (15 years for Pu-241, 88 for Pu-238, 30 for Cs-137 and 139 days for Po-210).

[leonidasrup]

https://radioactivity.eu.com/articles/radioactive_waste/fuel...

A spend nuclear fuel rod, freshly taken out of nuclear reactor, will kill you in minutes, if stand near it without any shielding. Therefor manipulation with spend nuclear fuel is done remotely under water. Few meters of water are excellent radiation shield.

With each year radioactive products decay into stable elements.

After about 400 years in storage, the most radioactive elements decay away and the dominant part radiotoxicity of spend nuclear fuel is now Plutonium. From the point of toxicity, you could now handle the spend nuclear fuel as other industrial toxic waste - you have to breathe in, or ingest the spend nuclear fuel to be dangerous.

On the other hand, from the geopolitical point of view, spend nuclear fuel is dangerous from many thousands of years (in contrast to other industrial toxic waste), because for many thousands of years Plutonium can be used make a nuclear weapons. This is the real reason why spend nuclear fuel gets so much attention.

[cucumber3732842]

>I don’t think those are good answers.

Steel and concrete are what we use above ground so.....

>They say what they want to do, but almost nothing about how they’ll do that, and try to avoid making hard statements on the what by using “is expected” and “is intended”.

They say nothing about how because those are trivial problems in the well (and oil) drilling industry.

[AnimalMuppet]

We use steel and concrete. We don't usually expect them to last 10,000 years, though.

[bronson]

> those are trivial problems in the well (and oil) drilling industry.

Ah yes, an industry well known for its adherence to safety standards and never having surprising blowouts.