How is nuclear power dangerous?

Answer 1

Nuclear energy as it is used to generate power can be dangerous. The nuclear reactors used to heat water to generate steam to spin turbines to generate electricity must be operated by individuals who know what they are doing. If something goes wrong, the duty crew must make all the right decisions and make them first time, every time. Failure to do so can cause structural elements of the core to fail and release both nuclear fuel and waste into the coolant passages in the core. (The fuel rods are designed to hold everything inside throughout the life of the fuel bundle.) This is what happened at Three Mile Island. Both mechanical failure and the failure of the duty crew to react correctly caused a meltdown. Spent fuel presents its own special problems. Fuel bundles must be recovered from the reactor and taken away and stored for an extremely long period of time before radiation levels are low enough to try to do anything with them. Fission byproducts are highly radioactive, and remain so for tens of thousands of years. Links are provided for further reading.

Answer 2

Nuclear power is only dangerous if radio-activity from the nuclear plant is released into the environment. Nuclear power plants are designed in such a way that radio-activity is never released and remains contained whithin the plant, under normal conditions and also under certain extreme conditions (earthquakes, flooding, plane crashes etc.). The only radio-active material that ever leaves a nuclear power plant, is the "burnt-out" uranium, which is stored in special bunkers, where it can decay further until it is no longer radio-active. The problem is that this decay can take 1000s of years, so these bunkers are created in geologically stable places.

In a nuclear power plant, radio-active uranium is used to generate a nuclear chain reaction: energy is released and absorbed in the form of heat by a fluid. This fluid, which can be in direct contact with the radio-active uranium, is kept in a closed circuit. This means it stays in a closed loop of pipes, and never comes into contact with anything else. These pipes run through a reservoir, and the heat of the radio-active fluid in the pipes will heat up the fluid in the reservoir (much like how the hot fluid in a central heating system heats up a room: the fluid never leaves the pipes or radiators). The water in the reservoir is thus heated, and can be used to drive electrical turbines to generate electricity.