Why must power plants spend so much money and be so cautious in dealing with these materials?

If you are referring to nuclear power plants and radioactive materials, then there is a high necessity to be cautious and stay safe because only by being safe you can prevent accidents like those caused in Ukraine and Japan. Unlike other types of power plants whose destruction in case of an accident can relatively easily be contained, nuclear power plants are extremely difficult to manage if things go out of hand since you can destroy the air around it, the water, the food by ruining the soil, and many other things.