In 1939, the last naturally occurring element on Earth, Francium, was discovered. However, the periodic table of elements still wasn’t full.
The next year, a non-natural element was discovered: Plutonium.
This new element had fascinating properties which made it incredibly useful and incredibly dangerous.
Learn more about plutonium, how it is made, and what it can do, on this episode of Everything Everywhere Daily.
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To start a discussion of plutonium, we might as well start with what it is and where it comes from.
Plutonium has the atomic number 94, which means it has 94 protons. Its discovery is credited to Nobel Laureate Glen Seaborg who discovered ten different elements.
If you remember back to my episode on the element uranium, which is element 92, it was given its name from the then newly discovered planet Uranus….excuse me, Uranus.
Just months before the discovery of plutonium in 1940, element 93 was discovered by bombarding uranium with a cyclotron, and it was named Neptunium, the next planet after Uranus….excuse me, Uranus.
Then, later that year, Seaborg and his group at the University of California Berkley, bombarded uranium with deuterium, a hydrogen isotope, which created element 94. It was named after the planet after Neptune, or at least it was at that time, Pluto.
Only a few atoms of it were actually ever initially created.
The abbreviation for plutonium is Pu, even though it really should be Pl and there are no other elements with Pl as an abbreviation. Seaborg thought it would be funny to call it “Pu”, and the abbreviation stuck.
As I mentioned in the introduction, plutonium isn’t considered to be a naturally occurring element, however, that isn’t 100% true. There are actually extremely small amounts of naturally occurring plutonium on Earth.
A study published in May 2021 found an extremely trace amount of plutonium on the ocean floor which was believed to be residual from the formation of the solar system. Likewise, there are also very small amounts that are created through the natural radioactive decay of uranium.
In 1942, after the first controlled nuclear fission reaction took place by Enrico Fermi at the University of Chicago, enough plutonium was produced where its physical properties could actually be studied.
The process of nuclear fission is how almost all plutonium is created. There are 20 known isotopes of plutonium, but only two are widely created: plutonium-238 and plutonium-239.
Plutonium-239 is created when a Uranium-325 atom splits, ejecting a neutron which is captured by a uranium 238 atom, turning it into uranium 239. The neutron then engages in beta decay, turning into a proton and ejecting an electron, turning into neptunium 239, and then a second beta decay turning it into plutonium-239.
Plutonium 238 is created by uranium 238 capturing a deuterium atom, to become neptunium 238, and then a beta decay to become plutonium 238.
If you remember back to my episode on uranium, the two common isotopes of uranium, 235 and 238, behaved differently in nuclear reactions. In particular, U-235 which only constitutes 0.7% of all natural uranium, is the stuff that you need to make bombs and nuclear reactors. It is called fissile.
By the same token, Pu-238 and Pu-239 behave differently in nuclear reactions as well. Pu-239 is fissile like U-235. That means it can be used in bombs and reactors.
In fact, the atomic bomb dropped on Nagasaki, nicknamed “Fat Man”, was a plutonium bomb.
Pu-238 is not fissile, however, it is highly radioactive with a half-life of only 87.4 years. I’ll be talking more about this in a bit.
So what are the properties of Plutonium? What is it like?
Plutonium is a metal. Physically, it has a silvery appearance like most metals, however, it oxidizes quickly which can change its color.
It has the unique property of not being magnetic. It will also expand and shrink from heating and cooling far more than any other metal. It also is a very poor conductor of heat and electricity for a metal.
One of the other, and perhaps most important attributes for this discussion, is that plutonium is highly toxic. By toxic, I’m putting aside the fact that it can be highly radioactive. Just as an element, plutonium is very poisonous and it is about as toxic as nerve gas. It can accumulate in a person’s bones and it is something you really don’t want to mess around with.
When plutonium was first being produced in quantity during the Manhattan Project, no one really knew anything about it. One researcher, Donald Mastick, accidentally swallowed a small amount of plutonium chloride and it was detectable in his body for 30 years.
From 1945 to 1947, 18 people were actually had plutonium injected into their bodies for testing.
One many named Albert Stevens, a house painter from Ohio was injected with over 3.5 microCuries of plutonium without his informed consent. Astonishingly, he lived to the age of 79, 20 years after his injection, and died of a heart attack, not cancer. It is believed he received the highest accumulated radiation dose of any human in history.
On top of being poisonous and radioactive, it can also spontaneously burst into flame at room temperatures if left exposed to open air. There have been plutonium fires at factories that create nuclear weapons components.
…and, as if the poison, radiation, flames weren’t enough, Pu-239 can reach criticality at about ? the mass of U-235. This can result in criticality accidents where people handing enough plutonium can have massive amounts of radiation exposure. While this can’t result in an explosion, it can provide a lethal dose of radiation, and such accidents have happened almost 60 times.
If this stuff is so nasty, and it is, then what is the point of it? Why bother making it at all?
Well, as I mentioned, the initial use was for nuclear weapons. Pu-239 is the primary isotope used for nuclear weapons and it is much more fissile than U-235. Pu-239 is fashioned into what is called a pit, which is basically a small sphere. Neutron-deflecting substances coat the exterior, which lessens the amount of plutonium required.
Since the end of the cold war, the demand for plutonium for weapons use has decreased substantially.
The quality of the plutonium for reactors or weapons is determined by the amount of Plutonium-240 which is in it. Weapons-grade plutonium requires less than 7% Pu-240, and because the isotopes are chemically identical, you have to separate them via enrichment, just like you would enriching uranium, which is really hard to do.
Likewise, plutonium can also be used as a fuel in nuclear reactors. While it isn’t the primary fuel used in reactors, there are many experimental reactors that could use plutonium and consume it completely.
For both weapons and reactor fuel, there are alternatives to plutonium. However, there is one use for which really is no substitute and you pretty much have to use plutonium.
That use is for deep space probes.
As I mentioned before, plutonium-238 is highly radioactive. This isn’t the isotope that is used in bombs.
Because it has such a short half-life of only 87 years, it generates a lot of heat. Plutonium-238, if just left to itself, will glow red hot. Hot enough to boil water. Moreover, it will remain that hot, with no outside energy, added, for decades.
In theory, a coffee mug made out of Pu-238 would keep your coffee warm for your entire life. Granted, if you were drinking out of a plutonium mug your life might not be very long, but the point is it remains hot for a long time.
What does this have to do with deep space probes?
Solar panels can provide sufficient energy so long as you are sufficiently close to the sun. Even rovers and orbiters sent to Mars can get sufficient power from the sun.
However, beyond Mars, the light of the sun just isn’t strong enough to use solar panels. At Jupiter, the brightness of sunlight is only 4% of what it is on Earth.
So, how can you power spacecraft that far away?
The answer is with something called a radioisotope thermoelectric generator, or RTG. What an RTG can do is take advantage of something called the thermoelectric effect which can convert heat directly to electricity.
While there are 22 known isotopes of radioactive elements which could in theory power an RTG, there is only one that can actually practical power it, and that is plutonium-238.
The thermoelectric effect isn’t that efficient, so it is seldom used in applications on Earth as it is more efficient to boil water to turn a turbine. One terrestrial use of RTGs was nuclear-powered lighthouses created by the Soviet Union. These lighthouses had no staff and just operated on autopilot.
Another use, believe it or not, was plutonium-powered pacemakers. There are probably a few dozen people in the world that have these pacemakers still installed. The RTG was about the size of a watch battery. So long as the plutonium remained fully encased inside the container, the toxicity and radiation really aren’t an issue. Pu-238 is an alpha emitter which is a type of radiation that can be easily blocked with as little as a piece of paper.
In deep space, there aren’t really a lot of options, which is why almost every probe sent past the orbit of Mars, and several Mars landers, including the Viking landers and the Curiosity rover, have had plutonium-based RTGs as their power source.
There have been some RTGs used in satellites in Earth’s orbit and some were used on the moon, but it really isn’t necessary to use them anymore as solar cell efficiency is so much better now. Also, you don’t want to be using plutonium on something that might reenter the Earth’s atmosphere.
A single kilogram of Pu-238 is about the size of 2 marshmallows or a bit bigger than a golf ball and can give off over 500 watts of heat, continuously.
RTG fuel is usually in the form of pellets of plutonium dioxide, and the amount can range from a few kilograms to as much as 35.
One problem NASA had recently was a shortage of plutonium. The United States stopped making Pu-238 in 1988. NASA started buying it from Russia in 1993, a whopping 16.5 kilograms, but they also stopped producing it.
The US government actually started to make Pu-238 for NASA in 2015 for the first time in decades, albeit the amount produced each year is still quite small. It takes about 2 to 3 years of exposure to a nuclear reactor to make a batch of Pu-238.
Current production is only 400 grams per year, but they hope to triple that by the year 2025. The extreme cost and difficulty in the production of Pu-238 make it one of the most valuable substances on Earth.
Plutonium is serious stuff. It is extremely toxic and radioactive, but thankfully most of us will never encounter it in our lives.
Nonetheless, certain isotopes of plutonium have properties that no other isotopes of any other elements have. If it wasn’t for plutonium, we wouldn’t be able to explore the solar system.