Articles, Blog

Nuclear Reactors vs. Nuclear Weapons

– A nuclear reactor and a nuclear weapon are about as similar as a
hamster and an alligator. They’re both made of cells, but one is clearly more
dangerous than the other. (light music) So, the thing in common
between nuclear weapons and nuclear reactors
is they’re both powered by fissile isotopes like
uranium 235 or plutonium 239. Fissile isotopes are special elements that break apart when neutrons hit them. When this happens, they
release more neutrons, and little atoms we call fission products, which carry off a lot of energy. In nuclear reactors,
this energy’s released over a long period of
time, whereas in weapons, it’s released all at
once, instantaneously. So, fissile isotopes
are the key ingredients in both nuclear reactors
and nuclear weapons, but that’s where the similarity stops. The difference, one of
the many differences, is that a nuclear weapon is
almost pure fissile material, it’s about 90 percent
of the fissile isotopes, either uranium 235 or plutonium 239, whereas reactor fuel’s
only about five percent. And, just getting a bunch of
uranium and plutonium together doesn’t make a weapon. If you lit a block of uranium 235 on fire, it would burn chemically,
there’d be a fire just like if you lit any other
flammable material on fire, but it would not be a nuclear explosion. You actually have to compress it so much that you get the atoms to slam together much denser than they’d normally be. So, a nuclear weapon, there’s
usually some sort of a core of uranium or plutonium in pieces, and around that is a bunch of explosive that smashes them into each other, and around that is some sort of a shell that contains the explosion
until it gets big enough. These are exceedingly difficult to make. If you took millions of sticks of dynamite and put them around a sphere of uranium and blew them all up
at the exact same time, the force would not be large enough to compact the uranium to make a weapon. If you got the right kind of explosive and surrounded this sphere
of uranium with the explosive and got ’em all to go
off at the same time, it still wouldn’t make a weapon. If you took a whole bunch
of bricks of uranium and put them all together and then drove a tanker
full of gas next to it and has a whole bunch of high explosive blowing up all around it, if you had enough uranium to
go super critical, it would, very, very briefly. You would get a quick pulse of neutrons, at which point everything
would heat up and blow apart, and once all the uranium’s
in different pieces too far away, the reaction stops. Without the proper physics and design, which has take countries
like ours years to do, the best you could do is a
Michael Bay style explosion, but you couldn’t actually
get a nuclear explosion. I’ve just described what
took the Manhattan Project about five years to do, and tens of thousands of
absolutely brilliant scientists, some of whom won the Nobel Prize. In order to use the MIT
reactor fuel to make a weapon, yeah, that would be difficult, that’s the definition of
easier said than done. Not only would you have to figure out how to put the weapon together, but you’d have to physically
get to the reactor’s fuel, and that’s basically impossible. The building is about four feet thick of rebar enforced concrete that’s actually been simulated
to take a loaded plane crash, ’cause that’s one of the safety criteria for reactors in this country, you have to be able to
fly a plane into it, and it shouldn’t break apart. That doesn’t even count the
security inside the building, let alone the SWAT teams
that would swarm the place if anyone even tried to break in, but lets say the terrorists
got passed all of that, then they’d still have to
get to the fuel itself, which is normally kept
behind a lot of shielding to keep it safe so we can work with it. So, if they took away the lead, the concrete, and the steel shielding, they’d be faced with the fuel itself, which is so radioactive that
it would kill them on contact. Even if somebody were to steal enough material to make a weapon, you can’t just put it
together and have a weapon, it doesn’t work. There’s so much other stuff in the fuel. Most light water reactor
fuels in this country are made of uranium dioxide. You put oxygen in the way
of those uranium atoms, it doesn’t work anymore. It works as fuel, but not as a weapon. Reactor fuel is only about
five percent uranium 235, and there’s other structural materials, there’s steel holding the thing up, there’s water surrounding it, there’s zirconium alloys
holding pins and fuel rods in, there’s all this other stuff that would have to be
chemically separated. Another difference between
reactors and weapons is the way the chain
reaction is controlled. A reactor is a tightly
controlled chain reaction with negative feedback, so if anything goes
wrong, the reaction stops. A nuclear bomb is an
uncontrolled chain reaction designed to get as hot as
possible as fast as possible. So, when you hear the word chain reaction, you might automatically think of something that’s out of control, but it’s actually really
hard to keep a reactor going. So, let’s say you had a certain number of uranium 235 atoms. Every time one of those
uranium atoms splits apart, it gives off two or three neutrons, which could cause another
uranium atom to split apart, but not every one does. Some of them leak out of the reactor and get absorbed by the shielding, some of them get absorbed by
other materials in the reactor, and some of them can get even captured by uranium 235 without inducing fission. As the fission reaction proceeds, more and more other stuff builds up, and that other stuff, which
we call the fission products, absorb some of those neutrons away, and makes them unavailable to
keep the chain reaction going. Also, when a fission reaction heats up, it causes the atoms to spread out, making it harder for
some of those neutrons to hit other fuel atoms and
keep the reaction going. In addition to that, there’s control rods. There are materials that
are really, really good at absorbing neutrons, like
boron, hafnium, or gadolinium. And you can simply stick those down into the reactor to
absorb away the neutrons and shut down the chain reaction. The same sorts of reactions are happening in a nuclear power plant
and a nuclear bomb, but in a nuclear bomb, they’re happening
quintillions of times faster, and it’s all over in a split second. The whole nuclear part of the explosion takes less than a second, and in a nuclear power plant, you’re releasing that same
energy over years or decades in a controlled way that we can harness.

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