Liquid hydrocarbons made from crude oil serve many functions: (1) a dense, easy-to-store, easy-to-transport energy source, (2) a method for daily-to-seasonal energy storage, (3) a chemical feedstock, (4) a chemical reducing agent and (5) a method to enhance high-temperature heat transfer in many furnaces and industrial processes. There are multiple methods to produce and use liquid hydrocarbons without increasing atmospheric carbon dioxide levels including (1) negative carbon emissions to balance carbon dioxide releases from burning crude-oil products and (2) producing liquid hydrocarbons from nonfossil feedstocks such as carbon dioxide or biomass. Understanding liquid hydrocarbon demand is the starting point in assessing options for producing and using liquid hydrocarbons without increasing atmospheric carbon dioxide levels. Our assessment is that U.S. demand for liquid hydrocarbons is unlikely to go below the equivalent of 10 million barrels per day of crude oil. The costs to replace liquid hydrocarbons increases rapidly at lower liquid hydrocarbon consumption rates. Hydrocarbon biofuels from cellulosic feedstocks can meet such demands but options based on more limited feedstocks (bio oils, sugars, etc.) can't meet such demands.

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We describe a roadmap, based on a series of workshops and studies, to use base-load nuclear reactors to replace fossil fuels in a low-carbon world that integrates nuclear, wind, solar, hydro-electricity and biomass energy sources. Nuclear reactors with large-scale heat storage enable variable electricity to the grid with nuclear plants that both buy and sell electricity. The low-cost heat storage and assured generating capacity enables efficient use of largescale wind and solar. Nuclear hydrogen production facilities at the scale of global oil refineries produce hydrogen to replace natural gas as a heat source. Nuclear heat and hydrogen convert plant biomass into drop-in hydrocarbon biofuels to replace gasoline, diesel, jet fuel and hydrocarbon feed stocks for the chemical industry. The external heat and hydrogen greatly increases the quantities of biofuels that can be produced per unit of feedstock. The system can produce variable quantities of biofuels and sequestered carbon dioxide that enables negative carbon dioxide emissions and increases revenue if there is a market for removing carbon dioxide from the atmosphere.

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Fossil fuels are the primary energy source because of their (1) low cost, (2) ease of storage, (3) lowcost transport and (4) economic dispatchability. Because the capital cost of power plants, furnaces, and boilers is small relative to the cost of the fuel, it is economic to meet variable energy demand by operating fossil plants at part load. Nuclear, wind, solar and hydrogen production plants have high capital cost; thus, operating these facilities at half capacity can almost double energy costs. A low-carbon system is defined that enables high-capital-cost lowoperating-cost technologies to operate at high capacity while providing variable heat, hydrogen and electricity to the customer. This minimizes total costs. In the U.S., over 80% of all energy used is in the form of heat; thus, heat production and storage is central to a low-carbon economy. Nuclear power is the primary low-carbon low-cost heat producing technology.