If coolant water is lost, fission ceases because the unmoderated neutrons move too fast for nuclear reaction . . Most countries construct containment buildings surrounding the reactor core, to prevents the release of radioactive material if a problem arises. The reactor at Chernobyl , Ukraine, had no such containment building, and so when coolant was lost the radioactive material was released to the atmosphere. Another reactor design in development uses molten sodium as a coolant. The advantage of this design is that the liquid range of molten sodium is much broader than for pressurized water , and so if there is a problem, the molten sodium can withstand a much larger increase in reactor core temperature before boiling and causing catastrophic loss of coolant.
Fusion is the fusing of two light nuclei into a heavier atom.
Obviously, such reactions can't be conducted in a steel container. Breeder Reactors are another type of nuclear reactor. Some people think we may be getting low 235 U, and the breeder reactor is being studied to address this concern.