The heat from nuclear fission can be used to generate electricity. There are approximately 400 nuclear reactors in operation globally. The lifecycle greenhouse gas emissionsGreenhouse gas emissions of nuclear power are low; the cost of nuclear power has remained high in the U.S. and other countries. Nuclear waste disposal remains a work in progress, with two countries having identified permanent disposal sites. Production of nuclear fuel and management of high-level nuclear waste provides opportunities for nuclear weapons production; the Nuclear Non-Proliferation Treaty has aimed to manage the expansion of nuclear power while restricting the spread of nuclear weapons technology.