1. “Nuclear weapons are the same as nuclear power reactors.”
There are 2 types of nuclear weapons: atomic and thermonuclear.
Atomic weapons assemble a supercritical mass of uranium with an uncontrolled rate of fission. In order to accomplish such a rate, uranium-235 needs to be enriched to at least 90%. This is far beyond that of natural uranium. Reactor fuel is a combination of uranium-235 enriched to around 5% and uranium-238 makes up the rest of the fuel. This makes using spent fuel in nuclear weapons with a favorable outcome is nearly impossible.
Thermonuclear weapons use both fission and fusion in the same weapon. Obviously, nuclear fusion is an underdeveloped technology, making thermonuclear weapons and nuclear reactors very, very different.
That being said, the rate of reactivity in a reactor is controlled by its moderator and coolant, as well as several control and safety rods that are lowered into the reactor core automatically if any deviation from normal operating conditions occur. These rods can slow the reactor’s chain reaction down significantly or can stop it all together. Once a reaction in a nuclear weapon has started, there’s no going back. These explosions occur within a matter of seconds. Chernobyl’s meltdown has often been used as “proof” that nuclear reactors can explode like nuclear weapons, but explosions in the reactor were the product of a deadly combination of nuclear fuel (soaring temperatures caused fuel claddings to rupture) and coolant, causing the water to violently flash to steam. Even then, this is not a fault of nuclear fission itself.
1. For an atomic weapon to detonate successfully, uranium-235 needs to be enriched to at least 90%. Uranium-235 is fissile, meaning it can sustain a chain reaction.
2. Most nuclear fuel is composed of uranium-235 that is enriched to at most 5% and uranium-238 makes up the rest of it. When the uranium-235 has been depleted, the fuel is no longer usable because only uranium-238 remains. Uranium-238 is fertile (can be fissioned by fast neutrons) but is not fissile (cannot sustain a chain reaction).
3. An uncontrollable nuclear reaction is required for a nuclear weapon to detonate. The rate of fission in nuclear reactors is constantly monitored and controlled with a moderator and control/safety rods. A runaway chain reaction in a reactor isn’t likely to occur unless a loss of coolant accident takes place.