Monday, November 10, 2008

Materials Challenges for the Supercritical Water-cooled Reactor (SCWR)

The Supercritical Water-cooled Reactor (SCWR) is the most promising evolution of the water-cooled reactor technology that currently dominates the commercial market for nuclear reactors. Essentially the idea is to increase the thermodynamic efficiency of the reactor type by going to higher temperatures and pressures, and thus also going to the supercritical state. Supercritical water also has a much higher specific heat, enabling a higher heat transfer per unit mass. By going to higher pressures and temperatures, it is also possible to avoid phase changes within the coolant loop altogether. This substantially reduces the requirements for pumps and compressors within the coolant loop, simplifying it considerably and introducing even greater economy in the whole power plant.

The merit of the supercritical coolant idea has been tested and the concept has actually been deployed for fossil fuel power plants (coal-fired) already, where thermodynamic efficiencies have considerably risen as a result. Some jurisdictions have also mandated that all future coal-fired power plants be of the supercritical type. The supercritical water coolant imposes more stringent requirements for plant materials used in fossil fuel-fired power plants as well, and this experience is relevant to the Supercritical Water-cooled Reactor (SCWR) concept.

However, what is different about nuclear reactors, of course, is the radiation dose that reactor materials will experience, in addition to the thermochemical stress that the supercritical water environment might impose. This is even more the case when new fast neutron spectrum fuel cycles are introduced, with radiation doses upto a hundred times higher than the thermal spectrum neutrons that current generation reactors use.

Superior irradiation creep strength, and superior thermomechanical behavior in general then becomes a a very desirable critical property to have for cladding materials. I discuss the Materials Challenges for the Supercritical Water-Cooled Reactor in my paper published in the Canadian Nuclear Society Bulletin, Vol. 29. No. 1 pp. 32-38 March 2008.

The supercritical water environment is also chemically different in that it dissolves organic species but not inorganic ones, the reverse of ordinary water. Thus the supercritical water environment is expected to create novel corrosion challenges as well, and these also must be studied and understood.

Materials such as Oxide Dispersion Steels (ODS Steels) have been proposed for use as structural and cladding materials in the Supercritical Water-cooled Reactor SCWR (as well as other Generation IV Reactor concepts). These materials appear to display very desirable irradiation and thermal creep properties, as well as desirable electro-chemical properties (corrosion resistance). Understanding the physical origin of these properties by developing suitable multiscale materials models (MMM) is the focus of my present research.