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05/01/2006 10:19 AM Many reactor types were considered initially; however, the list was downsized to focus on the most promising technolgies and those that could most likely meet the goals of the Gen IV initiative. Three systems are nominally thermal reactors and three fast reactors. The VHTR is also being researched for potentially providing high quality process heat for hydrogen production. The fast reactors offer the possibility of burning actinides to further reduce waste and of being able to breed more fuel than they consume. These systems offer significant advances in sustainability, safety and reliability, economics, proliferation resistance and physical protection. Very-High-Temperature Reactor (VHTR) The Very High Temperature Reactor concept utilizes a graphite-moderated core with a once-through uranium fuel cycle. This reactor design envisions an outlet temperature of 1,000°C. The reactor core can be either a prismatic- block or a pebble-bed design. The high temperatures enable applications such as process heat or hydrogen production via the thermo-chemical iodine-sulfur process. It would also be passively safe. Supercritical-Water-Cooled Reactor (SCWR) Supercritical-Water-Cooled Reactor (SCWR) The Supercritical water reactor (SCWR) is a Ml a = be concept that uses supercritical water as the working fluid. SCWRs are basically light water reactors (LWR) operating at higher pressure and temperatures with a direct, once-through ee cycle. As most commonly envisioned, it would | | operate on a direct cycle, much like a Boiling Supercritical-Water-Cooled Water Reactor (BWR), but since it uses Reactor (SCWR) supercritical water (not to be confused with critical mass) as the working fluid, would have only one phase present, like the Pressurized Water Reactor (PWR). It could operate at much higher temperatures than both current PWRs and BWRs. Supercritical water-cooled reactors (SCWRs) are promising advanced nuclear systems because of their high thermal efficiency (i.e., about 45% vs. about 33% efficiency for current LWRs) and considerable plant simplification. Page 2 of 5 Generation IV reactor - Wikipedia, the free encyclopedia Reactor types Thermal reactors Very-High-Temperature Reactor (VHTR) Main article: Very high temperature reactor Main article: Supercritical water reactor The main mission of the SCWR is generation of low-cost electricity. It is built upon two proven technologies, http://en.wikipedia.org/wiki/Generation_IV_reactor