The rods are arranged into a fuel bundle which is then ready to be placed in special pressure tubes inside the reactor. The reactor vessel is called the calandria . Nuclear reactors can not explode like a nuclear bomb. Even under a worst-case scenario, with a core meltdown , a critical mass of fuel would not be present and the fuel would burn into the ground. Refuelling can be done by removing fuel bundles from the pressure tubes and replacing them with new bundles.
Heavy water contains deuterium , an isotope of hydrogen having one neutron in the nucleus.
Heavy water also transfers heat from the fuel into a heat exchanger which heats ordinary water to produce steam. The steam produced is used to turn turbines which are connected to electric generators. Condensers change the steam back into water so it can be cycled back to the steam generator. If excess heat builds up in the calandria, the heavy water can be drained out. This causes the chain reaction to stop, because the moderator is no longer present.