Nuclear power is currently the single largest carbon-free source of electricity in the United States. The climate mitigation cost savings of the existing U.S. nuclear fleet is denominated in hundreds of billions of dollars [net present value (NPV)] based on an integrated assessment modeling of the U.S. energy system within a globally consistent framework. Lifetime extensions of the existing nuclear fleet from 40 years to 60 and 100 years resulted in $330 billion to $500 billion (all figures are in U.S. dollars) (NPV) of mitigation cost savings for the United States under a deep decarbonization scenario consistent with limiting global temperature change to 2°C. The addition of new nuclear deployments in the United States increased the total U.S. mitigation cost savings of the 2°C climate goal by up to $750 billion (NPV). Immediate actions are required in the United States and globally to achieve net-zero carbon emissions by mid-century, and once achieving net-zero emissions, they must remain at net-zero indefinitely. Lifetime extensions of the existing nuclear fleet, in the United States and globally, support urgent near-term emissions reduction goals. Additionally, the longevity of nuclear power technologies reduces the need for new capacity additions of all carbon-free electricity sources and supports long-term actions necessary to maintain net-zero emissions.