Does plutonium exist in the world, apart from being manufactured?

Question

In reference to the Manhattan Project at Los Alamos, a mathematician, Peter Lax, described his time working there as "living science fiction". He said:

we were told essentially the basic thing: we are building an atomic bomb, that there are two bombs, one built of a special isotope of uranium, and a second bomb built of plutonium, which is an element that doesn’t exist in the world, except that they are manufacturing it.

Was this correct, that plutonium "is an element that doesn’t exist in the world, except that they [were] manufacturing it"?

Answer 1

The Royal Society of Chemistry states:

Plutonium was first made in December 1940 at Berkeley, California, by Glenn Seaborg, Arthur Wahl, Joseph Kennedy, and Edwin McMillan. They produced it by bombarding uranium-238 with deuterium nuclei (alpha particles). This first produced neptunium-238 with a half-life of two days, and this decayed by beta emission to form element 94 (plutonium). Within a couple of months element 94 had been conclusively identified and its basic chemistry shown to be like that of uranium.
To begin with, the amounts of plutonium produced were invisible to the eye, but by August 1942 there was enough to see and weigh, albeit only 3 millionths of a gram. However, by 1945 the Americans had several kilograms, and enough plutonium to make three atomic bombs, one of which exploded over Nagasaki in August 1945

The US effort to build a nuclear bomb got the name Manhattan project no earlier than 1941^*, so there is no contradiction there.

But plutonium does exist in nature (note that you are not asking about a specific isotope), as is e.g. shown in The Occurrence of Plutonium in Nature by Charles A. Levine and Glenn T. Seaborg (PDF avalable here):

Plutonium has been chemically separated from seven different ores and the ratios of plutonium to uranium determined. This ratio was found to be fairly constant (approx. 10^-11) in pitchblende and monazite ores, ...

In his autobiography (G.T. Seaborg and E. Seaborg - Adventures in the atomic age: from Watts to Washington), Seaborg says more about the naming of Plutonium:

It was so difficult to make, from such rare materials, that we thought it would be the heaviest element ever formed. So we considered names like extremium and ultimium. Fortunately, we were spared the inevitable embarrassment that one courts when proclaiming a discovery to be the ultimate in any field by deciding to follow the nomenclatural precedents of the two prior elements.
A new planet had been discovered in 1781 and, like the rest of the planets, named for a Greek or Roman deity-Uranus. A scientist who discovered a heavy new element eight years later named it after the planet: uranium. The planet Neptune was discovered in 1846, so Ed McMillan followed this precedent and named element 93 neptunium. Conveniently for us, the final planet, Pluto, had been discovered in 1930. We briefly considered the form plutium, but plutonium seemed more euphonious.

So the element has existed since the formation of the Earth (and maybe earlier), but a) it was not known before December 1940 and b) it did not have the name plutonium.

^{* I cannot find the exact date. As mentioned in this NY Times Article, that information is in the book The Manhattan Project: The Birth of the Atomic Bomb in the Words of Its Creators, Eyewitnesses, and Historians by Cynthia C. Kelly}

Answer 2

Two billion years ago, conditions in an ore body in present-day Gabon were suitable for the creation of naturally-occurring fission reactors, based on the fission of U-235, which at the time made up 3% of the uranium.

https://www.scientificamerican.com/article/ancient-nuclear-reactor/

(This article references a 40 year old SciAm article: see “A Natural Fission Reactor,” by George A. Cowan, July 1976])

These reactors operated on and off for several hundred thousand years, at an average power output of under 100 kilowatts.

As a part of the process, U-238 atoms in the reactor zone absorbed a neutron and converted to U-239, which then decayed to Pu-239.

During the lifetime of the reactor system, it is estimated that around 2 tonnes of Pu were created.

Virtually all this plutonium has disappeared, either through natural decay, or involvement in fission of the plutonium. So we also have a naturally-occurring breeder reactor...

Answer 3

I assume "the world" means Earth. It is likely that somewhere in the universe where a supernova (or a neutron star merger, see comment) has recently occurred during which the r-process prevailed, plutonium can be found.

First, note that the most stable isotope of plutonium (Pu-244) has a half-life of about 81 million years, whereas the Earth is about 4540 million years old. From this it follows that there is no primordial plutonium left on Earth. When Earth was very young, it probably existed, but all that has decayed by now.

As explained by DJohnM's answer, at least once in the natural history of our planet, plutonium was produced in a so-called natural fission reactor in Oklo, Gabon. However, the plutonium produced by these natural processes on Earth will have decayed by now. Because the ratio between uranium-235 and uranium-238 has been gradually changing, natural fission reactors cannot have occurred in "recent" times, so it is impossible that plutonium from natural reactors still exists today.

So the claim is largely correct. While uranium is a primordial element on Earth, discovered in 1789, plutonium is not naturally abundant.

However, to be precise, we have to define more precisely what we mean by "an element existing" in the world. It will happen by accident once in a while that one uranium nucleus fissions (spontaneously), producing neutrons one of which is slowed down and hits another uranium nucleus, producing the heavier uranium-239. After two beta decays, the latter turns into plutonium-239. Pu-239 has a half-life of about 0.024 million years. All this means that wherever U-238 exists, these nuclear processes will reach an equilibrium, and there will be an absolutely tiny trace amount of plutonium present. That is the 10^-11 fraction mentioned in Jan Doggen's answer.

It is debatable whether one part plutonium in 100 billion parts uranium qualifies as "plutonium existing in the world". One thing is certain: It is not practically possible to mine or extract plutonium from such sparse occurrences. This is important in politics when we want to limit the proliferation of nuclear weapons; the only feasible way to obtain plutonium is from access to nuclear reactor technology.

We can illustrate how tiny 10^-11 is by comparing the natural trace occurrence of plutonium to the amount of plutonium spread by the detonation of nuclear weapons since 1945. These two sources are comparable. In other words, if you encounter a plutonium atom in nature, it is at least as like to originate from one of the nuclear weapons used or tested since 1945, as to be of natural origin.