A low-carbon world needs a replacement for the combined-cycle gas turbine to provide variable heat and electricity. We describe a nuclear combined cycle gas turbine (NACC) with a thermodynamic topping cycle where the high-temperature heat is provided by an electrically-heated thermal energy storage system or hydrogen. During base-load operations the NACC compressed air is heated up to 700°C with heat from a high-temperature nuclear reactor to produce electricity. At times of low electricity prices, electricity from the NACC and the grid is converted into high-temperature stored heat in the form of hot firebrick. At times of high electricity prices compressed air is heated up to 700°C by the reactor and then further heated by sending the compressed air through hot firebrick to heat the air up to 1427°C before going through the turbine and heat recovery steam generator to produce added peak electricity. The round-trip electricity-to-heat-storage-to-electricity efficiency is above 70%; that is, if one unit of electricity is sent to heat storage, more than 0.7 units of peak electricity are returned. The enabling technology is a new doped electrically-conductive firebrick that enables conversion of electricity to very-high-temperature stored heat. Stored heat addresses daily variations in demand. Hydrogen can be used for long-term seasonal energy storage but at lower peak electricity efficiency because the efficiency of the electricity-to-hydrogen conversion step is near 75% versus near 100% for electricity-to-heat.