Nuclear Waste Half-Lives

It's Plutonium 239 with a half life of 2.411×10⁴ years, so still bad.

That's the trouble with radioactive isotopes. A hundred thousand years is long enough that it will be radioactive for longer than we can foresee, many times longer than written history, but it's short enough that it's still very active. Even a gram will have billions of decays per second. And while our body is of course capable of repairing a small amount of DNA damage, evolution never prepared us for a continuous bombardement by the subatomic equivalent of a machine gun.Pu-239

The half life of U-235 is hundreds of millions of years, and U-238 even longer. So it's not decaying fast enough to be dangerous.

The radioactive strength and decay time is not that long, right?

The radioactivity of spent fuel decays exponentially. After a few hundred years, most of it is gone, although there is of course the danger due to the chemical toxicity, just like any other heavy metal deposit. Obsessing over what could happen if somebody digs it up after 10000 years is not anchored in reality.

It's radioactive so the half-life has a serious effect. Its half-life is 87 years so it's not even used up one. I guess it wasn't really very overdimensioned. But it wasn't meant to last this long.

Not at all. Uranium-238 has a half-life of billions of years, and Uranium-235 has a half-life of about 2/3 of a billion years. This means that the natural decay rate, and thus the rate of generation of radioactivity is very low. However, if you pull that out of the ocean and put back things like Radium, I-131, Sr-90, etc. you will vastly increase that rate because such isotopes have a much lower half-life.

If the half life is that long, it's not very dangerous as energy / time is very low.

Well U-238 half life is 4E9 years so there might be some remnant ill feelings for a while.

Waste with a half-life of 500,000 years?