The Uranium and plutonium isotopes used in weapons generally aren't themselves super dangerous in terms of radioactivity. The most dangerous radioactive elements produced in a nuclear explosion or disaster such as Chernobyl are the short half life fission products, such as isotopes of iodine, strontium, caesium, etc.

> What's the amount of radiation that thing gave off simply from decay, in a non-critical state?

Not much. U-235 has a 700M year half-life. Pu-239's is 24,000 years.

You're more at risk from ingesting/inhaling bits of it.

>You're more at risk from ingesting/inhaling bits of it.

This is the vaaaast majority of the risk profile for radioactive anything.

The half life doesn't really tell you the whole story, Pu-239 mostly decays into alpha particles which is what makes it "safe", it's still not good for you, and very bad if ingested, but externally alpha radiation doesn't really penetrate deep enough into the body to be a big problem.

To put it another way - from an "immediate risk" point of view, the largest danger from critical-capable amount of fissile material is the prompt radiation emitted during a criticality: the pulse of neutrons, gamma and x-rays thrown off during that period the unit is sustaining a chain reaction.

For relatively brief power excursions like this one (which Slotin himself stopped by removing the reflector), the prompt burst was well over a lethal level but the radiation dose-rate from fission product decay even a tiny time later would not be immediately hazardous.

For those looking to learn more, Los Alamos has a great writeup of every publicly-known incident (in the US, USSR, and elsewhere) that details just how many times similar incidents happened: https://www.orau.org/ptp/Library/accidents/la-13638.pdf