Concept for Fort Saint Vrain
Today we are highlighting a reactor that was a pioneer of its kind: the Fort Saint Vrain Nuclear Power Plant, located in Platteville, a city in Northern Colorado. Though the plant has since been converted to a natural gas generating station, Fort Saint Vrain was one of the first thorium-powered reactors in the United States.
First conceived in the 1960s and named after a trading post, Fort Saint Vrain operated as a high-temperature gas reactor, or an HTGR. HTGRs are nuclear reactors that use a graphite moderator with a once-through uranium fuel cycle, and can conceptually reach high outlet temperatures (up to 750°C). Coated fuel particles containing fuel kernels, usually made of uranium dioxide, are used for fuel. The particles are either dispersed in a pebble for the pebble bed design or molded into compacts/rods that are then inserted into hexagonal graphite blocks. As an inert gas, helium is the ideal choice for a coolant. Additionally, exposing helium to neutron radiation does not make it radioactive, unlike most other possible coolants. The molten-salt-cooled variant of an HTGR, however, uses a liquid fluoride salt for cooling, much like a LFTR, in a pebble core. Pebbles are injected into the coolant flow to be carried to the bottom of the pebble bed, and are removed from the top of the bed for recirculation. This design has many favorable features, including efficiency at high temperatures, low-pressure operation, high-power density, better electric conversion, passive safety systems, and better retention of fission products in the case of an accident. Using helium, the reactor at Fort Saint Vrain cooled its 1,400-degree core to transfer heat to the steam generators that drove steam turbines, which created electricity. There have only been two other HTGRs built elsewhere in the world since Fort Saint Vrain, which are similar in design, but not identical. During the 10 years it was operational from 1979 to 1989, the HTGR at Fort Saint Vrain was the first and only commercial nuclear power plant in Colorado.
The Fort Saint Vrain Information Center.
Though the plant was highly effective, inspections revealed expensive corrections were necessary, and with around $240 million already swallowed in investments, the Public Service Co. of Colorado decided it was time to call it quits. On August 5, 1997, the reactor was officially decommissioned. PSC, wanting to make the best use of their losses, made plans to recommission the site as a natural gas-powered electricity plant. Though the reactor building now sits empty and radioactive material-free, the rest of the site has since found its new purpose. The resulting Fort Saint Vrain Natural Gas Generating Station is operated by Xcel Energy and has a capacity of 965 MW, making it the largest gas-fired generating station in Colorado.
The spent fuel from the nuclear power plant, uranium and thorium, was transferred by the PSC to the newly-built Independent Spent Fuel Storage Installation next to Fort Saint Vrain. The building, which has 3-foot-thick concrete walls and is guarded 24 hours a day, holds 822 kilograms of spent uranium-235 and 13,903 kilograms of spent thorium, both of which are stored in 300-pound graphite blocks. Ted Borst, who works to protect and monitor the fuel, says the fuel isn’t going anywhere, and hopes that it can still be recycled. We, too, search to make use of spent fuel, which is extraordinarily valuable to many of the nuclear reactors we hope to achieve in the future.
The Fort Saint Vrain Generating Station can be seen in the distance.