Wizards

Plutonium 239 is primarily produced by bombarding Uranium 238 with neutrons.

Uranium 238 + fast neutron bombardment -> Uranium 239 -> Neptunium 239 -> Plutonium 239.

Each step involves a negative beta decay, ie, a neutron in the nucleus becomes a proton, and a high energy electron and antineutrino are thrown out.

That is to say, U 238 + neutron bombardment actually decays into Pu 239, or perhaps, emits Plutonium.

https://www.nuclear-power.com/nuclear-power-plant/nuclear-fuel/plutonium/plutonium-239/

I am not well versed enough in nuclear physics to accurately estimate the actual radioactivity of a one foot diameter sphere of neutron bombarded U 238... but it would be 'significant'... here goes an almost certainly innacurate attempt:

U 239 has a half life of about 23 minutes, Np 239 has a half life of about 2.3 days.

DuckDuckGo's small mistral model ai is giving me 2.58 * 10^21 becquerels for the radioactivity of a solid, one foot diameter sphere of Neptunium 239, it won't even do U 239 because its half life is too short.

But I think this assumes literally 100% of the U 238 being successfully neutron bombarded instantly... which is almost certainly wildly unrealistic...

... but either way, mistral is also saying that 2.58 * 10^21 bq of radioactivity, 3 feet away from a human, with no shielding, would be instantly lethal.

Anyway, a one foot diameter sphere of the stuff, undergoing these reactions, would weigh about 300 kilograms, or about 660 pounds.