No known amount..... well, we all live in risk, and flying on a plane gives you a higher dose, as does laying on a beach, or lighting off fireworks....etc
But the radiation nuclear workers and others are expected to sustain is lower than that of coal workers and the public from radioactive elements being released from its combustion. So lets not squabble over semantics, a safe amount is far greater than what is allowed, and sure we are always trying to mitigate exposure and worse case scenario, but its unrealistic to expect a worker to receive less than an airline pilot.
Breeder reactors are exactly how Plutonium is made for these missions. Thus the name Breeder.
https://neutronbytes.com/2017/03/05/nasa-re-starts-pu-238-production-at-two-sites/ Its just that the reactors use Neptunium to enrich specifically during a Fission reaction to increase yield, but again, reactor design can create or "burn" newly formed Plutonium depending on design.
Nuclear weapons are useless against countries like Russia, France, China.... etc as its becomes a zero sum game. They are ONLY effective against smaller countries which cannot retaliate against a nuclear deterrent, the only question then becomes, what do they have to do for us or someone else to use a nuke? North Korea, if they launched an actual weapon against the US for example.... or China, or Korea. Or Palestine, lets say they finally got their hands on one and decided to use it against Israel, what then, you have a country where a large percent of the population feel that Jews are the enemy and are OK with their death.
Are Hiroshima and Nagasaki uninhabitable? Nope, higher power nuclear source means shorter half life and less dangerous byproducts. It would be worse for someone to use conventional explosives and elements with longer half lives.
Also, Pepto Bismol emits Alpha particles
https://en.wikipedia.org/wiki/Bismuth-209 https://en.wikipedia.org/wiki/Bismuth, as does Plutonium
https://en.wikipedia.org/wiki/Plutonium-238 , and Plutonium in non-critical mass can be shielded with a few sheets of paper or aluminum foil
https://solarsystem.nasa.gov/rps/docs/APP RPS Pu-238 FS 12-10-12.pdf " Shielding: Pu-238 decays by the alpha emission process,
and these alpha particles can be stopped by material as thin
as a sheet of paper. This greatly enhances radiation protection
safety. Other radioisotopes would require much greater (and
heavier) shielding to protect both the humans working on the
power source and its spacecraft on the ground before launch,
as well as the many radiation-sensitive electronic parts onboard
a spacecraft."
. HMWP are common shields in labs as they absorb the occasional neutron easily with no secondary decay effect.
