It looked like a normal, boring metal ball.

Smooth, shiny, and a bit bigger than a grapefruit. It didn’t glow, it didn’t tick,  nothing to give away the danger within.

But within the span of a year, it took the lives  of two men, without either one even touching it. This metal ball held enough power  to eliminate an entire city,   but neither death involved any explosions. The first man died three  weeks after dropping a brick.

The second died nine days after  the slip of a screwdriver. Different tools, same deadly outcome. And it was all because this ball,   which eventually got nicknamed the  Demon Core, was made of plutonium. [♪ INTRO] To tell the story of this small plutonium  ball of death, we also need to talk about a   different,and perhaps much more famous, element  just two squares over on the periodic table.

Because while plutonium technically exists  in nature, it’s in very tiny amounts. So if you want enough for any kind of practical  purpose, you have to start with uranium. Uranium sits in the Earth’s crust,   just waiting to be dug up and refined  for nuclear power or nuclear weapons.

But if you have a lump of uranium atoms,   you can turn some of them into plutonium  atoms by bombarding them with neutrons. It’s a multi-step process, since you  actually need to change the number   of protons in an atom’s nucleus to  turn it into a different element, but we don’t need to worry about  those details in this video. Just like its predecessor, plutonium is useful  because it’s fissile, meaning the atoms can be   split, or fissioned, into smaller atoms,  releasing a ton of energy in the process.

But there are some important  differences between the two: While both emit the same kind of radiation,  if you have two balls of equal mass, one   uranium and one plutonium, the plutonium ball will   emit more radiation in the same amount  of time, making it riskier to handle. And most importantly for our story,  it takes less plutonium to reach   something called criticality, which is when  things get interesting…if not disastrous. We will come back to that in a bit.

In the 1940s, the United States’ effort to  build a nuclear bomb focused on uranium. But even though uranium is present in reasonable  quantities in the Earth’s crust, there’s a catch. The isotope of uranium that’s  fissile is uranium-235.

As the name implies, uranium-235 has a combined  total of 235 protons and neutrons in its nucleus. But over 99% of all the uranium  mined on Earth is uranium-238,   which is still radioactive, but not fissile. In other words, 99% of all  the uranium we find can’t be   used for either nuclear energy or nuclear weapons.

There’s also the problem that these  isotopes always come mixed together. So scientists had to develop ways to separate  the 235 from the 238 bulk…which is about as   easy as trying to differentiate decaf  coffee from regular just based on smell. Different isotopes of the same  element behave the same chemically,   so they are really hard to separate.

However, physicists at the University of  California Berkeley discovered that if you   bombard those “useless” uranium-238 atoms with  neutrons in a reactor, you get plutonium-239. And plutonium-239 is not only fissile, it can  also be easily separated from the uranium. It was a total game changer, and  the Manhattan Project shifted gears.

They decided to build two kinds of bombs. The US dropped one of each  on the citizens of

Japan:   the uranium-powered Little Boy,  and the plutonium-powered Fat Man. But a second plutonium core  had been made, just in case. And when World War II came to a close, it remained  at Los Alamos National Laboratory in New Mexico,   where scientists could continue  their experiments on criticality. This is our Demon Core.

To get into that criticality, we’re going  to dig into some nuclear physics a bit. But don’t worry, we’ll keep  the metaphors to a maximum. When an atom undergoes fission,  it doesn’t just crack in half.

It also releases a number of lone neutrons that  aren’t bound to either of the new, smaller nuclei. These neutrons are then free to smash into other  atoms, causing them to undergo fission, and so on. But whether or not these neutrons will  actually do that depends on several factors,   like how many atoms you’ve got altogether,  and how closely packed those atoms are.

If, on average, exactly one neutron from each  fissioned atom causes another atom to split,   you have what’s called a critical reaction. It’s like a perfect row of dominoes, where  each one knocks over exactly one more. This is an ideal situation for nuclear  reactors used to produce energy:   a constant, steady stream of reactions  that put out a constant amount of energy.

Meanwhile, if the neutrons hit fewer  than one additional atom…again,   on average…the reaction  will eventually fizzle out. This is a subcritical reaction. And if more than one neutron from each  fission event causes another fission,   things can escalate quickly.

Now it's like each domino is knocking over  2 or 3 others, spreading exponentially. This is a supercritical reaction. Barring a few experimental exceptions  over the decades, this is what you don’t   want to happen in a nuclear reactor…but  you do want to happen in a nuclear bomb.

This is also where plutonium gets extra spicy. By which I mean extra dangerous. It releases more neutrons per  fission event than uranium does.

That means you need less of it  in order to go supercritical. But there’s more to criticality  than just how many atoms there are. It's also about how they’re arranged.

Imagine we have 100 dominoes in a long line. If one is even a little out of place, the  chain can break, stopping the reaction. But if you pack those dominoes  into a tight circle, it's much   harder to stop the toppling once it starts.

The same goes for plutonium atoms. In fact, the core of Fat Man was surrounded  on all sides by explosives that detonated inward…  compressing the core, increasing  the density of the plutonium atoms,   and all but guaranteeing  a gigantic chain reaction. But what ultimately doomed the men killed  experimenting on the demon core was yet   another way to help a ball of plutonium  hit criticality: neutron reflection.

As atoms in the Demon Core split, some of the  neutrons will escape into the surrounding air. It’s the end of the road for  their nuclear fission dreams. But if you can place something around the  core that reflects those neutrons back,   you give them another chance.

There’s no good domino analogy here, so let’s  say it's like a pinball machine with extra bumpers. This is exactly what our first victim,  Harry Daghlian was doing in 1945. He was trying to reflect neutrons back toward   the demon core by surrounding it with  bricks made out of tungsten carbide.

He was in the lab alone, late at  night…too impatient to wait for   the morning when his experiment  was scheduled to take place. As he added additional bricks around the core,   his equipment alerted him that the core was  rapidly approaching dangerous territory. He tried to remove his latest brick,   but it slipped from his hand  before he could clear the assembly.

When it fell back down, the extra  coverage reflected enough of the   core’s neutrons to push the deadly  orb into a supercritical state. Daghlian reacted immediately,  shoving the brick to the floor,   but the radiation released in just that  moment was enough to kill him within weeks. It also irradiated a lone security guard  who was stationed a few meters away,   but we’ll get to his fate later.

But before we get to that, we’ve got  to keep the lights on. So here’s an ad: This SciShow video is supported by Brilliant:  an interactive online learning platform. Brilliant’s app can help you  gain real knowledge in science,   programming, data, math; so many topics!

I know you’re constantly  picking up your phone every day. And if you’re anything like me, you feel  bad about the scrolling and time wasting. So one way to lean into that habit  and to turn it into something you’ll feel   good about is to open the Brilliant  app when you reach for your phone.

Brilliant helps you make a habit of learning a little every single day, all from your phone or computer. And it’s available to try for  free at brilliant.org/scishow,   the QR code onscreen, or the  link in the video description. You’ll also get 20% off an  annual premium subscription.

Now, it’s worth noting that the scientists at Los   Alamos could have used robots to  handle a core like this remotely. But they argued that these  robots weren’t reliable enough,   and preferred to do their work  by hand to avoid accidents. And yes, they did have certain  safety protocols in place.

But that doesn’t mean they were always followed. Enter, Louis Slotin. Instead of the tungsten carbide bricks,   Slotin’s experiments involved surrounding the  Demon Core with two hemispheres of beryllium.

Not only is beryllium a neutron reflector. It’s also a neutron multiplier. When it gets hit by a neutron,  the atoms spit out two in return.

If tungsten bricks are like extra bumpers in  pinball, beryllium is like having extra balls. To increase the amount of reflection,   Slotin simply needed to lower  the top hemisphere over the core. But instead of resting that  hemisphere on the mandatory spacers… so if it slipped out of his hand,  it wouldn’t cover the core too much… He held the two halves apart  with the end of a screwdriver!

Slotin, you know that this thing killed a man! You are not the protagonist of an action movie! You do not have plot armor!

In May 1946, Slotin was demonstrating his   experiment to a colleague who was set  to take over his role at Los Alamos. Six other people were in the  lab, including a photographer. And during the demo, the  screwdriver finally slipped,   completely encasing the core, and  triggering a burst of radiation.

Slotin managed to knock the hemisphere off  within seconds, but the damage was done. After removing the top hemisphere,  he said, “Well that does it”. He died nine days later from radiation sickness.

Across these two accidents, 10 people were  exposed to radiation from the demon core. But only two died as a direct result,  which might sound like a bit of a miracle. However, a lot of that just  comes down to just how quickly   your exposure drops the further  you are from a radiation source.

It follows an inverse square law,  meaning that as the distance doubles,   the amount of radiation is reduced by one fourth. But in the grand scheme of radiation doses,   distance isn’t the only thing  you have to worry about. Lethality also depends on the type  of radiation your body is absorbing.

The type that plutonium emits when it’s just  sitting around is, in large part, alpha radiation. These particles are essentially helium nuclei,  consisting of two neutrons and two protons. They’re large and slow, and can be stopped  by something as flimsy as a sheet of paper.

Now, if they manage to get inside you,   alpha particles will wreak absolute  havoc on your bodily tissues. So don’t go to places where there  might be plutonium dust hanging around. But when the Demon Core went supercritical,  it produced two other types of radiation: both super high energy particles of light,   better known as gamma rays,  and super high energy neutrons.

Both are way harder to block. Harry Daghlian received an estimated  200 rads of neutron radiation,   and 100 rads of gamma radiation, while Louis Slotin received an estimated 1000   rads of neutron radiation and  114 rads of gamma radiation. To put those numbers in perspective, if  you were getting a CT scan of your chest,   your thyroid would absorb  about 1 rad of x-ray radiation.

Now technically, these numbers don’t tell the  whole story about just how bad these guys had it,   because human tissues don’t respond to  each kind of radiation the same way. For example, neutron radiation can be anywhere   between 5 to 10 times more  damaging than gamma rays. But we’ll stick with what we’ve got  here, because we can compare them to   what the eight survivors absorbed, and what side  effects they encountered after the accidents.

Two men in the room with Slotin experienced  some mild symptoms, including Al Graves,   who was effectively standing right  behind Slotin when it all went down. That meant Slotin’s body shielded  him from most of the blast. Still, Graves received an estimated 166 rads  of neutron radiation and 26 rads of gamma.

He was hospitalized for over two weeks, and ended  up losing the hair on the left side of his head. Yep, the side that was not  blocked by Slotin’s body. None of the other men had any immediate symptoms.

Due to the extra distance from the Demon  Core, they received doses ranging from 7   to 51 rads of neutron radiation and  0.1 to 11 rads of gamma radiation. But that doesn’t mean they were spared  from all negative health effects. Two of the men, including the security  guard from Daghlian’s accident,   eventually died from leukemia.

While doctors couldn’t prove a causal  relationship between the radiation   and the cancer, they did conclude it was likely. Another two men died from health problems that   the accident may have caused, but  the evidence was less definitive. One man was killed in action in the Korean War,   so whether or not he would have manifested  any related symptoms is impossible to say.

Another refused to participate in  follow-up research, so his health   effects are less well characterized,  but he died in 2001 at the age of 81. Finally, Raemer Schreiber went on to build a new,   safer facility at Los Alamos  for handling plutonium. He designed it so that handlers and their   radioactive experiments were  at least a quarter mile apart.

He died in 1998, at the age of 88. As for the demon core, it was melted down and  reintegrated into the national nuclear stockpile. It never saw battle, but it still managed to earn   a place in history as one of the  most infamous pieces of metal.

And it's a pretty unforgettable reminder  that lab safety exists for a reason. [♪ OUTRO]