Nuclear heat for hydrogen production: Coupling a very high/high temperature reactor to a hydrogen production plant

Abstract

Hydrogen has been dubbed the fuel of the future. As fossil fuel reserves become depleted and greenhouse gas emissions are reduced inline with the Kyoto protocol, alternative energy sources and vectors, such as hydrogen, must be developed. Hydrogen produced from water splitting, as opposed to from hydrocarbons, has the potential to be a carbon neutral energy solution. There are several methods to extract hydrogen from water, three leading candidates being high temperature electrolysis, the SI thermochemical cycle and the HyS hybrid thermochemical cycle. All three of these processes involve a section requiring very high temperatures. The Very High Temperature Reactor (VHTR), a gas cooled Generation IV reactor, is ideally suited for providing this high temperature heat. Nuclear hydrogen production is being investigated around the world. The four leading consortiums are the Japan Atomic Energy Agency (JAEA), PBMR/Westinghouse, GA, and AREVA NP/CEA/EDF. There are also many smaller R&D efforts focussing on the development of particular materials and components and on process flowsheeting. A nuclear hydrogen plant involves four key pieces of equipment: the VHTR, the hydrogen production plant (HPP), the intermediate heat exchanger (IHX) and the power conversion system (PCS). The choice of all four items varies dramatically between programmes. Both pebble bed and prismatic fuel block VHTRs are being developed, which can be directly or indirectly coupled to a HPP and PCS placed either in series or parallel. Either a Rankine steam cycle or a Brayton gas turbine cycle can be employed in the PCS. This report details the choices made and research being carried out around the world. Predicted process efficiencies and plant costs are currently at a preliminary stage and are very similar, regardless of the options chosen. The cost of hydrogen produced from water splitting using nuclear technologies is around $2/kg H 2. This is competitive with hydrogen produced by other methods, particularly if carbon emissions are regulated and costed. The technological feasibility and testing of key components will be one of the determining factors in plant viability.