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Decommissioned nuclear power plant

The Fort St. Vrain Nuclear Power Plant is a former commercial nuclear power station located near the town of Platteville in northern Colorado in the United States. It originally operated from 1979 until 1989. It had a 330 MWe High temperature gas reactor (HTGR). The plant was decommissioned between 1989 and 1992. In 1996, it was converted into a conventional natural gas powered generating station, which as of 2011 has a capacity of just under 1000 MW.

Fort Saint Vrain Generating Station was built as Colorado's first and only nuclear power plant, which operated from 1979 until 1989. It was one of two high temperature gas cooled (HTGR) power reactors in the United States. The primary coolant helium transferred heat to a water secondary coolant system to drive steam generators. The reactor fuel was a combination of fissile uranium and fertile thorium microspheres dispersed within a prismatic graphite matrix. The reactor had an electrical power output of 330MW (330 MWe), generated from a thermal power 842 MW (842 MWth). The Fort St. Vrain HTGR power plant was proposed in March 1965 and its application was filed with the Atomic Energy Commission in October 1966. Construction began in September 1968. Although system safety was not compromised, frequent operation problems were experienced throughout the reactor systems due to water and corrosion. Ice formed on chiller units in the gas cleaning system, reducing their efficiency. When the reactor output was cycled down and the system cooled, water would condense on equipment and instruments within the helium system. Control rod drives rusted, and rapid shutdowns failed to operate correctly. The reserve shutdown system, which released borated graphite spheres into the core in the event of an Anticipated Transient Without Scram (ATWS), was sometimes unavailable because water had leached the boron to form boric acid, which softened the graphite spheres and caused them to clump together. Steel "tendons" within the PCRV experienced corrosion due to chloride precipitation and were weakened. Rust flecks migrated into the coolant and lodged in critical machinery, including control rod drives. Steam generator leaks also occurred due to corrosion.

The plant electrical system was challenged on numerous occasions, and the resolutions were frequently expensive. Transformers experienced faults. Backup generators sometimes failed to engage when activated, and on other occasions, side channel issues occurred during operation, preventing them from generating power. Failure of backup power also led to some of the moisture infiltration problems, by variously disrupting the logic of the bearing water injection systems and the helium circulator trip logic. Failures of transformers and consequent failure of backup power occurred on at least one occasion due to moisture infiltration into electric cables and subsequent ground faulting when the plant was at low power to remove water from previous moisture infiltration issues. It is believed that this electrical fault led to further moisture infiltration.

Following the reactor decommissioning, Fort St. Vrain was converted to a conventional natural gas powered combustion facility. The first natural gas combustion turbine was installed in 1996. Two more turbines were installed by 2001. Heat recovery steam generators (HRSGs) allow the plant to operate in combined cycle mode, in which waste heat recovered from combustion turbine exhaust gases is used to make a second stage of steam capable of driving the facility's original steam turbine and generator. As of 2011, the nameplate generating capacity of the plant is 965MW.