Nuclear energy production and use has been a controversial topic over the years. Nuclear energy can supply a very large amount of electrical power for use in cities, but the nuclear materials that make the electricity can be very dangerous for thousands or millions of years after they have been used. Countries have made decisions to use nuclear power based on their current energy demands as well as environmental conservation demands. Countries could venture into nuclear energy production to help meet the rising energy demand, to help conserve the environment by avoiding air pollution, and also as a long lasting replacement for the depleting fossil fuels sources of energy, but would have to ensure the ongoing safety of production and waste storage. There have been upgrades to reactor design to avoid nuclear energy accidents like those of Chernobyl and Fukushima, but accidents still continue to occur and ageing reactors are a safety concern. Countries using the nuclear energy production process to make dangerous weapons of mass destruction is a related concern.