Nuclear fission happens in a nuclear reactor. Fission happens when a fast moving neutron is released in a reactor. When the neutron strikes the nucleus of uranium it splits making more neutrons and releasing a lot of energy.  This starts a chain reaction. This is why they use the name nuclear reactor

All of these reactors are housed in nuclear power plants.  After the 2001 terrorist attacks nuclear power plants have very strict security.  The employees are no longer allowed to bring their children to work.  Employees have to go through metal detectors and things like that.  If a terrorist got inside a nuclear power plant they could tamper with the reactors and create a nuclear meltdown, this would cover the world with radiation.  Many people think that scientists will go to far with nuclear power, and they might tamper with an unstable element and ruin the world.