nuclear reactors are expensive to build because of the many safety features they need to have. there is also a problem with the huge amount of radioactive waste from the reactors. however they produce electricity cheaply, and do not pollute the air. there have been serious accidents at several nuclear reactors: windscale (uk) 1957, mayak (ussr) 1957, three mile island (usa) 1979, chernobyl (ussr) 1986 and fukushima (japan) 2011. concerns about safety have limited the growth of nuclear power. there are about 437 reactors around the world which provide about 5% of the world's electricity.