(405a) Progress in High Temperature Materials and Systems in the U.S. Doe Nuclear Hydrogen Initiative

Nuclear powered hydrogen production can be performed today using liquid water electrolysis. With this technique, the nuclear power plant supplies electricity to the electrolysis cell, which splits water electrochemically into hydrogen and oxygen. The cost of hydrogen produced by liquid water electrolysis, however, is not yet economically competitive with hydrogen produced from the reforming of fossil fuels, and other methods are sought that can take advantage of the nuclear power source (e.g., no greenhouse gas emissions) at a lower cost.

The other hydrogen production techniques currently being considered for nuclear powered hydrogen production can be broadly divided into three classes: high-temperature electrolysis, thermochemical methods, and thermochemical-electrochemical methods. Using these methods, thermal energy from the nuclear power plant is used exclusively or in concert with electrical energy to split water into its separate components. Common to all of these methods is the need for high operating temperatures temperatures (700-1000 degrees C) in order to achieve higher process efficiencies and lower operating costs. Since water splitting is an endothermic process, these hydrogen production processes are unable to supply their own heat, and the nuclear power plant must be the source of the high temperature thermal energy needed to drive the hydrogen production processes. Since the current generation of nuclear power plants operates at temperatures no higher than 300-350 degrees C, more efficient nuclear powered hydrogen production will certainly require the development of high temperature nuclear reactors, and such reactors are currently under study by the U.S. Department of Energy.

Having a high temperature nuclear reactor and a more efficient water-splitting process are not all that is needed to successfully produce less costly hydrogen using nuclear power. An essential component in the system is the thermal energy transfer loop that would connect the high temperature nuclear plant to the hydrogen production plant. The thermal energy transfer loop must be capable of transmitting a significant portion of the high temperature heat from the nuclear reactor to the hydrogen production plant, and must do so with a minimum of thermal and power losses. Development of this thermal energy transfer loop is the focus of the System Interface and Support Systems area under the U.S. Department of Energy Nuclear Hydrogen Initiative.

Significant technical barriers to the construction of such a thermal energy transfer loop exist in the areas of materials, heat exchanger design and construction, system design and control, and in loop operation. There are considerable materials demands made on the thermal energy transfer loop, because it must simultaneously contact the high-temperature coolant used in the nuclear reactor, and the relatively cooler yet more corrosive fluids used for the hydrogen production processes, using either a gaseous or liquid heat transfer fluid that may or may not have corrosive properties of its own. Advanced heat exchangers using appropriate materials must be designed and constructed that can reconcile the high-temperature strength and corrosion issues, while still keeping within cost goals. The thermal energy heat transfer loop must operate in a very stable manner, so that process upsets at the nuclear plant or the hydrogen plant are not readily communicated or propogated through the loop.

This report provides an overview of the technical progress made to date in the U.S. in the development of the thermal energy transfer loop under the U.S. DOE Nuclear Hydrogen Initiative in the area of high temperature materials and systems for nuclear hydrogen production. An integrated research program has been initiated in this area that involves the cooperation of national laboratories, universities, and corporations. Work has been accomplished in high temperature materials property measurements, corrosion, identification of heat transfer fluid candidates, heat exchanger designs, and the nuclear plant/hydrogen plant distance requirements. Near term research plans (1-3 year time frame) and research needs will also be identified.