The St. Louis-based utility filed an 8,000-page license application with the U.S. Nuclear Regulatory Commission for a reactor adjacent to Ameren's existing nuclear facility, the Callaway Plant, outside of Fulton.
Ameren said it has not decided to build a second nuclear plant, but wanted to preserve that option if the need arose.
Also, applying now puts Ameren in a position to seek federal loan guarantees and production tax credits created by the federal energy law of 2005.
Ameren said its 1,190-megawatt electric generating plant at Callaway accounts for 19 percent of the utility's total generation. It came online in December 1984.
AmerenUE president Thomas Voss said greater demand for power in Missouri in the next two decades will require the utility to have a large generating plant online by 2018. The company said it hopes to decide by 2010 whether to proceed with the plant.
Voss said Ameren will continue to encourage development of renewable energy sources and help customers reduce consumption and demand. "But going forward, we will also need nuclear energy from our existing Callaway unit and possibly from a second unit at Callaway," he said.
Ameren said it wants to pursue a possible nuclear plant because it wouldn't produce greenhouse gases, which are believed to cause climate change.
But some environmentalists say the benefit is offset by the problem of safely storing the process' nuclear waste.
Ameren spokeswoman Susan Gallagher said the Callaway plant's waste is safely stored in secure pools onsite. She said some European nuclear plants reuse the waste to generate more power.
Commercial nuclear power plants now produce some 20 percent of U.S. electricity, but concern about waste disposal has hampered the industry's growth.
Nevada's Yucca Mountain, billed as the nation's first nuclear waste repository, originally was supposed to open in 1998 but has been beset by lawsuits and political and scientific controversies, and cost overruns. The best-possible opening date is now 2020.
Earlier this month, a second nuclear reactor at AmerenUE's Callaway plant was the focus of an NRC-hosted public forum in Fulton.
Advocates touted Callaway's safety record and a jobs windfall from a second plant.
Opponents said they want Ameren Corp., the utility's corporate parent, to more aggressively pursue alternative and renewable energy options.
The company then known as Union Electric initially planned a second nuclear reactor at the Callaway County site. That plan was scrapped after a grass-roots effort opposing the Callaway project led Missouri voters in 1976 to decisively approve a law prohibiting state utilities from charging customers for power plants while they're being built.
Persuading state lawmakers to overturn that restriction is a top priority for Ameren in the next legislative session. Should that fail, the company likely won't build a second reactor but instead pursue more costly natural gas generators.
Ameren expects the new reactor to cost at least $6 billion, or $9 billion with financing roughly the entire value of the parent corporation.
Scott Burnell, spokesman for the NRC, said it will take a month for staff to ensure Ameren's application is complete.
Once the application is accepted for review, the NRC will alert the public that it can raise challenges within 60 days. Ameren's application will be reviewed on technical and regulatory grounds. The whole process takes roughly 42 months, Burnell said.