[pjc50]

Each decay event is when radiation is emitted.

So: very short half-life is good, because the element turns into something else very quickly and ceases to be a problem. This is the nanoseconds-to-days range.

Very long half-life: not actually all that radioactive. e.g. U238 itself with a half-life in the billions of years.

Medium half-life: emits a dangerously high level of radiation in the process of decaying. This is the real problem stuff as "medium" can mean "centuries".

[roenxi]

If it can generate enough energy to be dangerous then it probably has an economic use if enough of it can be gathered in one place Like the sun - as I recall per-m3 it isn't all that energetic but there is enough sun that it provided the energy for ~99% of all life on earth. Lots of not-quite-enough energy is enough energy.

That is part of why this "no human should set foot for 100,000 years" is silly. We only have recorded history going back a few thousand years, and all of civilisation was invented in that time. If humans are exist in 100,000 years we'll be using that century-long half life material for something important.

[pjc50]

> If it can generate enough energy to be dangerous then it probably has an economic use if enough of it can be gathered in one place

This is the basis of the radiothermal generator (RTG); but generally, the spent fuel is deemed spent in the first place because it's no longer emitting enough heat/neutrons to be worth keeping in the reactor. It's already got to the point of "it's no longer worth the hassle of handling this and dealing with all those neutrons/gamma radiation in exchange for a mediocre amount of warmth".

[AnthonyMouse]

> spent fuel is deemed spent in the first place because it's no longer emitting enough heat/neutrons to be worth keeping in the reactor.

It's in a fission reactor and those isotopes aren't fissile, and aren't a huge proportion of the "spent fuel" to begin with. To be useful it has to be separated.

Short-lived highly radioactive substances are commercially valuable as radiation sources. Medium-lived radioactive substances are useful in RTGs (not fission reactors). Long-lived radioactive substances are often fissile and therefore useful as reactor fuel.

But none of them are useful when they're all mixed together, because what they're each useful for is a different thing. So separate them.

[ben_w]

> If it can generate enough energy to be dangerous then it probably has an economic use if enough of it can be gathered in one place Like the sun - as I recall per-m3 it isn't all that energetic but there is enough sun that it provided the energy for ~99% of all life on earth.

The sun is also a third of a million times the mass of the entire planet, or about 1.4 billion times the mass of all our oceans.

And the power output being in the form of ionising radiation is really bad: the power density of the core of the sun is 276.5 W/m^3, but in a form which will, if you leant against it for a minute and given reasonable guesses as to your body mass and shape, give you a remaining conscious lifetime of vomiting, diarrhoea, seizures, bleeding everywhere inside and out, relieved only by being followed with a coma after about an hour then death within a day or two.

(That's ignoring the fact that it's also hot and dense and would immediately explode, it's just the effect of the radiation coming from it).

> If humans are exist in 100,000 years we'll be using that century-long half life material for something important.

There are three possible futures: business as usual, collapse, transcendence/singularity.

With business as usual, there's a fairly good chance that everything from our era will be forgotten and dismissed as myth and legend.

With collapse, all of society might of forgotten how the abstract concepts of "money" and "writing" work, reinvented them, gotten up to our level, and then collapsed again 50 times over.

With the singularity: the planet itself and every star visible to the naked eye (and many which aren't) may have been physically disassembled in that time frame.

I think we should be the kind of civilisation that plans for how to minimise the damage of bad outcomes, even if only to make sure we don't mess up the "singularity" option.

[darkclouds]

And I'm to assume that decay events are like a fundamental law of physics in that they will never change, so they cant be speeded up, or slowed down, or even reversed?

[SideburnsOfDoom]

Sure you can speed it up: get something else to decay neutrons, protons or alpha particles, on to it.

An atomic bomb is when you convince a lot of Uranium-235 or Plutonium to decay all at once in an uncontrolled way.

A nuclear reactor is what happens when you convince material to decay at a controllable rate.

It's way more complex than that but you can look up the rest, e.g. "Nuclear chain reaction".

Where I think you're going with this is

https://physics.stackexchange.com/questions/594598/destroyin...

tl;dr: it's more trouble than it's worth, since you need radioactive materials as the neutrons sources, and stray neutrons tend to bump into other matter and cause yet more radioactive waste.

[darkclouds]

> Sure you can speed it up: get something else to decay neutrons, protons or alpha particles, on to it.

So if you could control and direct these NPAP's to behave like a Newton Cradle, you could accelerate them away faster?

Something for CERN to try maybe?

[SideburnsOfDoom]

> Something for CERN to try maybe?

I get the impression that your understanding of the current state of particle physics is approximately 80 years behind the state of the art. You're catching up with Leo Szilard's ideas in the 1930s

[pjc50]

Well .. yes? (I wouldn't count neutron activation as "decay")

[SideburnsOfDoom]

Decay, like anything else can be (from our subjective point of view) be slowed down by accelerating it away from us at speeds approaching that of light.

As far as the parent comment's implied question, "and is that useful for radioactive waste disposal?" the answer is a strong "no, there are far better uses for the energy required, within and outside of radioactive waste disposal", including using this energy instead of energy from the nuclear reactor that makes waste.