In June Xcel Energy completed the six-year, $283 million repowering of the Fort St Vrain generating station near Platteville, Colorado. What used to be a high temperature gas-cooled reactor (HTGR), with a somewhat troubled history, has now been reborn as a natural-gas-fired combined cycle plant. The phased repowering has added 720 MWe of generating capacity to the company’s system. The project’s timeline is summarised on p 53.

Fort St Vrain is now the largest generating plant owned by Xcel Energy in Colorado. It has three natural-gas-fired turbine generating sets of approximately 140 MWe apiece. Each gas turbine is equipped with its own heat recovery steam generator. The steam from these three HRSGs is used to drive the plant’s original 300 MWe steam turbine, which has been retained from the original nuclear plant. The steam turbine is a GE D8, with HP, IP and LP sections. The old, nuclear, steam conditions were 2400 psig, 1000°F main steam and 1000°F reheat. The new, combined cycle, conditions are 1800 psig, 1000/1000°F. The old steam turbine was refurbished but no major modifications were needed.

Public Service Co of Colorado, a predecessor to Xcel Energy, successfully completed the nuclear decommissioning effort in September 1996 – the first decommissioning of a commercial nuclear reactor in US history and one of the first nuclear power plants in the world to be fully decommissioned (although load factors were low and the operating life relatively short so the levels of irradiation were not has high as might be encountered in other decommissioning projects).

Xcel Energy first officially filed plans to repower Fort St Vrain in October 1993. The repowering option’s key economic attraction was the opportunity to make use of the station’s existing $61.5 million steam turbine generator. By repowering the facility with natural gas, the company saw the potential to save millions in additional construction costs.

The three combustion turbines are all GE 7FA’s, with dry, low NOx combustion. The HRSGs were provided by Vogt. The main contractor for the repowering was Black & Veatch.

The first combustion turbine started simple cycle operation in 1996. Combined cycle operation, with HRSG and steam turbine, started in 1998. Several further stages of the incremental repowering project, carried out in the period to May 1999, resulted in about 470 MWe of electricity generating capacity. The final phase – completed in late May 2001 – brought the repowered facility up to its current 720 MWe, enabling the plant to match growing system load in its service area. The full repowering was completed on time and under budget.

The sorry saga of Fort St Vrain as a nuclear plant goes back as far as March 1965, when it was originally announced, as a demonstration commercial-scale high temperature gas-cooled reactor. Construction of the nuclear facility began in September 1968, and first electricity was generated in December 1976.

The plant operated sporadically as a commercial nuclear power plant from July 1979 until August 1989, when it was permanently shutdown. Just prior to the permanent shutdown, the plant had recorded some of its best operating performances, reaching 178 221 net MWh in July 1989. But concerns over nuclear fuel availability, operational difficulties, frequent stoppages and financial losses forced the eventual shutdown. The pioneering plant had been over-ambitious in its aims and represented too much of a scaling up from previous HTGR projects. It encountered a spate of technical problems, notably on the primary circulation system. Also, the regulatory issues associated with being the only HTGR in the USA made compliance very expensive. In June 1990, the company announced plans to pursue an early dismantlement strategy for the facility. Concerns over long-term regulatory and financial issues associated with an alternative, 50-year, decommissioning effort led to an agreement between the company, regulators and consumer groups to decommission the plant by the mid-1990s, at a cost of roughly $125 million.

In August 1997, following completion of the decommissioning process, the NRC approved termination of Fort St Vrain’s operating licence, bringing to a close a chapter in nuclear technology that the industry would probably like to forget.

Timeline of the Fort St Vrain plant


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