The design of the Fort St. Vrain steam generators

Abstract

The steam generators for the Fort St. Vrain nuclear power plant are the first application in the United States of once-through boiler design coupled with a high temperature gas-cooled reactor. They contain many design features which are unique for this type of component. Since they are an integral part of the primary system and completely enclosed by the prestressed concrete reactor vessel, they must be removable as well as fit the space available for penetrations. These requirements made the once-through boiler principle a logical choice. Multi-start helically wound tubes supported by perforated plates in a star-shape arrangement resulted in an extremely compact design. The helium inlet temperature of 1427°F and steam temperatures of 1005°F main and 1001°F reheat required unique solutions in terms of flexibility and cooling of support systems and selection of insulation materials and design. Operation in a helium atmosphere without a protective oxide layer called for materials with good wear protection characteristics where parts may experience relative motion. Stabilizing orifices, externally adjustable at the steam generator inlet, plus essentially equal tube lengths for each of the many parallel circuits are utilized to balance circuit performance. To minimize gas bypass flows, special gas seals are provided around individual tube bundles. Field erection time was minimized by developing an upper and a lower module assembly and joining them after erection in the reactor vessel.