The United States, like many industrialized countries, has pledged to reduce its net carbon emissions, and, like others, the U.S. has been boosting renewables, exploring the use of hydrogen for power generation, switching to natural gas and modernizing its grid. Now the Tennessee Valley Authority plans to add to the mix a powerful new source: small modular nuclear reactors, or SMRs, which can be deployed faster than conventional ones and at a lower cost per unit of output.