Made, not found
Plutonium does not occur in nature in any meaningful quantity. Whatever the Earth formed with decayed away long ago; the traces present now are produced by neutron capture in uranium ore, at concentrations of parts per trillion.
It was made in Berkeley in 1940 by Glenn Seaborg’s group, bombarding uranium with deuterons. They identified element 94 and, following Neptune, named it for Pluto.
Within two years it was clear that plutonium 239 was fissile, that it could be produced in bulk in a reactor, and that this was an easier path to a weapon than enriching uranium.
⚠️ The Manhattan Project therefore ran two programmes at once. Oak Ridge separated uranium 235 by physical means; Hanford bred plutonium in reactors and separated it chemically — which is far easier, because plutonium and uranium are different elements.
The reason for implosion
Little Boy was a gun: two pieces of uranium fired together down a barrel. Simple enough that it was used untested.
Plutonium cannot be assembled that way, and the reason is plutonium 240.
Any plutonium made in a reactor contains some 240, formed when a 239 nucleus captures another neutron instead of fissioning. Plutonium 240 fissions spontaneously and emits neutrons on its own.
In a gun assembly, taking milliseconds, those stray neutrons start the chain reaction while the pieces are still approaching. The material heats, expands and blows itself apart before most of it has reacted — a fizzle, with a fraction of the intended yield.
⚠️ Implosion solves it by being fast. Shaped explosive lenses compress a sub-critical sphere to super-critical density in microseconds, faster than a stray neutron can spoil it. Getting the shock wave symmetrical was one of the hardest engineering problems of the project, and the reason Trinity was tested at all: the uranium gun needed no test, the plutonium implosion did.
The grade distinction
How long fuel stays in a reactor decides what kind of plutonium comes out.
Short irradiation produces plutonium that is mostly 239, with under about 7 % of 240 — weapons grade. Long irradiation, which is what a power reactor does because it is trying to extract energy, builds up 240 and produces reactor grade.
This is the practical basis of non-proliferation monitoring. A power reactor run economically produces plutonium that is difficult to weaponise. A reactor cycled frequently with short irradiations is doing something else, and the fuel handling is visible.
⚠️ Reactor grade plutonium is not impossible to use — a device can be built from it, with reduced and unpredictable yield. The distinction is one of difficulty, not of physics, and treating it as absolute has caused arguments in both directions.
What it is actually like
Plutonium is a strange metal even setting radioactivity aside. It has six allotropes at ordinary pressure, more than any other element, and they differ in density by up to 25 %. A casting can crack as it cools simply by changing phase.
It is a poor conductor of heat and electricity for a metal, it is brittle in some phases and malleable in others, and it corrodes readily in air.
Weapons designers stabilise it by alloying with a small amount of gallium, which holds the useful delta phase down to room temperature.
⚠️ It is also warm to the touch. A kilogram generates a couple of watts from its own alpha decay — enough to feel, and the origin of the description that a plutonium sphere feels like a living thing.
The toxicity question
Plutonium emits alpha particles, which travel a few centimetres in air and cannot penetrate skin. A sealed piece is safe to stand next to and can be handled with gloves.
Inhaled as fine oxide dust it is a different matter. The particles lodge in the lung and deliver alpha radiation to a small volume of tissue continuously, and plutonium is also a bone seeker once absorbed.
⚠️ The claim that a few kilograms could kill everyone on Earth appears regularly and is arithmetic, not toxicology. It divides a lethal inhaled dose into a mass and assumes perfect distribution, which no mechanism provides. Plutonium is genuinely among the more hazardous materials to inhale; it is not the most toxic substance known, and chemical toxins beat it comfortably.
The real hazards of plutonium are proliferation and the difficulty of keeping track of it — not a stray gram in a laboratory.