 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. 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 is 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% of the fissile isotopes either uranium-235 or plutonium-239 whereas reactor fuel is only about 5% 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 would 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 is usually some sort of a core of uranium-plutonium in pieces and around that is a bunch of explosive that spashes 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 the sphere of uranium with the explosive and got it 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 had 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 then reaction stops. Without the proper physics and design which has taken 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 because 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 let's say the terrorists got past 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 5% 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 the fuel pins and the 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 everyone 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 absorbs 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