The Frozen Star - Can U-238 reactors be used as a energy source for 10,000 years?

Question

I was watching this Minecraft Animation video based on a scientific concept about what would happen if we froze the Sun - What happens if we Freeze The Sun- Portrayed by Minecraft.

In this video, some guy freezes the Sun. This scenario causes the Earth to freeze over, causing drastic changes to life. Finally, the protagonist unfreezes the Sun by blasting it with a Ray-Gun powered by the Earth's Core.

Obviously, this scenario has a lot of ridiculous loopholes. Since the Sun is so massive and there is a gargantuan amount of pressure at the Solar core, even if we brought down the Sun to a sufficiently low temperature, such that thermonuclear fusion stopped and we froze the Hydrogen-Helium mix of the Sun (very close to absolute zero), the compression of such a large mass due to cooling would cause it to heat up rapidly and reignite within maybe just a few years or even months, ruining the Everybody-Freezes-to-Death scenario. The Sun is way too massive, and as soon as it froze, it would again reignite and carry on as though nothing happened. Also, even if we managed to keep the Sun frozen, the amount of energy needed to thaw it back into a star would be too much, making the Geothermal Ray-Gun in that scenario utterly impossible.

So, inspired by this video, I worked on a story with an alternate scenario:

Alternate Scenario

Let's say we have this really low-mass star, let's call it Eridanus. It's characteristics are:

• Mass - 0.0898 M☉

• Radius - 0.1192 R☉

• Bolometric luminosity - 0.000553 L☉

• Age - 6.8 BY

• Temperature - 2600 K

Eridanus is an ultracool, low-mass red dwarf, that has just one planet orbiting it. Let's call it Taurus.

Here are few details about Taurus I could work out:

• Mass - 2.4 Earth masses

• Radius - 9,780.83 KM

• Gravity - 10 m/s²

• Axial tilt - 19.8 degrees

• Mean Temperature - 12.8°C

• Day length - 23h 39min

• Semi-Major axis - 3,200,000 KM

Let's handwave away tidal locking for now, and assume that the planet is rotating perfectly normal, despite it orbiting a red dwarf (which have a notorious reputation for tidally locking planets within their habitable zone)

Let's assume that some evil Worldbuilding wizard decided to freeze Eridanus to wreak havoc on the orbiting planet's life-forms.

SNAP!

Eridanus has been frozen. It is no longer a star, but now a ball of solid hydrogen-helium with traces of heavier elements. All nuclear fusion has been stopped due to the fact that the star has been frozen down to nearly absolute zero.

Since light travels at a finite speed, it would take based on my calculations about 10.6740673534 seconds for the residents of this planet to notice that something has gone horribly wrong with their parent star.

The residents of this planet, called Villagers (named because this story was inspired by the Minecraft Animation about freezing the Sun mentioned above), soon detect that their skies are now suddenly dark, as if somebody "turned off" Eridanus. It doesn't take a long time for the Villager astronomers to realize that their parent star "Eridanus" has been frozen to absolute zero by some unknown force. This means that plants will die out very soon and animals will either resort to cannibalism/hunting other animals or starve to death. This also means that oxygen levels will plummet very quickly and sooner or later they will suffocate.

The Villager astronomers calculated how much time it would take for Eridanus to heat up and restart fusion reactions, and to their horror, they found out that since Eridanus is a small red-dwarf star, it would take nearly 100-10,000 years for Eridanus to thaw out and again initiate fusion reactions.

The Villagers are not stupid, they know that they are incapable of producing enough energy to again "artificially" thaw out Eridanus, no matter even if they used all of their energy resources (hydrocarbons, nuclear fuel, solar, hydro etc.) So, instead of foolishly wasting their time on searching for a solution, the Villagers use this plan to make sure they survive long term- "U-238 reactors"

Although Uranium-238 isn't fissile, it sure does produce heat. In fact, according to this article, it produces nearly 0.1 watts/tonne as it decays. Since Uranium-238 is really abundant on Taurus, it is really easy for the Villagers

By using WolframAlpha, I can compute the amount of Uranium-238 required to generate 1 MW of heat, about 10 million tonnes. The volume of such an enormous mass of Uranium-238 is about 523,560 m³, or about half the volume of the Empire State Building. Such a large mass of uranium-238 can reach scorching temperatures due to its own radioactive decay.

First, the large quantity of U-238 is stored in an underground chamber, either natural, or hollowed out. Although Uranium-238 releases little energy due to radioactive decay, since there is nearly 10 MILLION tonnes of uranium clumped together, the intense heat produced causes the Uranium-238 to melt into a red-hot liquid. The Villagers position a boiler over the molten uranium. Water is then pumped through the boiler, which turns into steam and is used to spin a turbine, generating electricity.

The Villagers use this electricity to heat up their colonies, lighting for plants to grow and livestock rearing, and also most importantly, keep the atmosphere from freezing and falling to the surface as snow.

Will the U-238 reactors function for 10,000 years, after which Eridanus thaws out, and becomes a star once more?

Notes-

The technology of the Villagers is nearly the same as that of Humans in 2023, with a few advancements. Average use of electricity per Villager is about 12 KWh. They are not exactly a spacefaring species, but they do have a few active space stations, comparable in size to the Mir space station. The Villagers also use predominantly satellites for broadcasting and radio technology (Wi-fi, Portable Radio, phone calls etc.) They don't use photovoltaic cells for terrestrial power, due to the dimness of their parent star Eridanus, except in satellites. Formerly, wind and hydel was the predominant energy source for the villagers, but following the Frozen Star, they now use U-238 heaters for powering their homes.

The U-238 heaters do not need a constant resupply, as U-238 has a very long half-life of nearly 5 billion years.

My question is asking about, would they function for 10,000 years, without getting disrupted due to other factors such as leaking out of my reactor, or simply detonating its reactor due to the heat produced.

I am not asking How long the fuel will last? but How long will the REACTOR last?. I am not concerned with the fuel, as Uranium-238 is very abundant on Taurus, but I am concerned whether my "reactors" (which are nothing more than underground chambers storing molten U-238) will last, without blowing up/melting/leaking or any kind of damage.

Answer 1

Non-fissile decay of radioactives (including uranium and thorium in natural isotope ratios) is believed to be the primary source of internal heat for rocky planets like Earth (or, presumably, Taurus, since they have lots of U-238), accounting for about half the total 44 TW of internal heat flux from the Earth.

In other words, the Taurans are already sitting on this reactor and it's already producing heat (what we'd call geothermal); they just need to harness it via either Peltier junctions or by pumping water down into the ground and using heat exchangers to run steam turbines. The expected lifetime of such a "reactor" is around ten billion years (though by the time there's complex, industrialized life on the surface, roundly half of that has elapsed).

Answer 2

From Wikipedia about Uranium

Around 99.284% of natural uranium's mass is uranium-238, which has a half-life of 4.468 billion years.

The above means that your decay heat will practically be stable on the time span of 10 thousand years.

However, there is no man-made artifact which has lasted usage for even a fraction of those 10 thousand years.

Archeological findings are found because they are buried underground and that protects them from wear and tear. Turbines, operating at high temperature and high stress, will not have the same luck. They will need frequent replacement, together with all the parts related to the power plants.

But before that, one might question if a civilization will last for 10 thousand years to maintain those power plants operative. Roman aqueducts and roads have sometimes being used until recent times, but those do not require the same level of maintenance and technology of a power plant. Once the knowledge is gone, you will have a hard time keeping things running.

Answer 3

Uranium 238 is Fertile

People still do not consider that modern nuclear technologies are very primitive. Current fission reactors extract only a tiny percentage of the energy available from the nuclear fuel and then throw it away as a nuclear waste.

Uranium 238 is a fertile material which can be turned into fissile by bombarding it with neutrons. It can be done in a fast reactor coupling it with other fissile materials. Or, it can be done with a particle accelerator. If the accelerator is done with modern technologies the energy required to generate the neutrons will be less than the energy released by the bombarded uranium. Another option would be a neutron gun made coupling a material like beryllium and a low grade nuclear waste.

So, the answer is yes U-238 can be used in a way or the other.