It's glowing with magical power

They meant radiation-emitting

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.

Emitting plutonium?

Yep. That would be pretty high energy.

Solid sphere of intense radiation emitting, plutonium.

It's just messing that comma indicating that plutonium is the article being described.

Man, those physical markers would do the exact opposite of keep me out. If i found a 'city' of black blocks I'd explore the hell out of it