Every energy source has environmental impacts—positive and negative. Nuclear power is a carbon-free source of energy that can reduce CO2 emissions by displacing the use of fossil fuels. The present level of carbon displacement is approximately 0.5 gigatonnes of carbon per year (GtC/year), compared to the nearly 8 GtC/year emitted by the use of fossil fuels. However, there are three major negative environmental impacts of nuclear power: catastrophic accidents, nuclear weapons, and nuclear waste. The last two, weapons and waste, are directly tied to the type of nuclear fuel cycle (Figure 4 in the main nuclear article by Raj et al. in this issue). The different fuel cycles refect different strategies for the utilization of fssile nuclides, mainly 235U and 239Pu, and these different strategies have important implications for nuclear waste management and nuclear weapons proliferation.

Raj et al. describe the promise of nuclear energy as a sustainable, affordable, and carbon-free source available this century on a scale that can help meet the world's growing need for energy and help slow the pace of global climate change. However, the factor of millions gain in energy release from nuclear fssion compared to all conventional energy sources that tap the energy of electrons (Figure 1) has also been used to create explosives of unprecedented lethality and, hence, poses a serious challenge to the expansion of nuclear energy worldwide. Although the end of the cold war has eliminated the threat of annihilating humanity, the likelihood of a devastating nuclear attack has increased as more nations, subnational groups, and terrorists seek to acquire nuclear weapons.