An auxiliary water pump failed to start during testing on May 25 at the Callaway nuclear power plant in May. Utilities provider Ameren UE owns the plant and fixed the problem manually, saying there were no safety problems.

The Nuclear Regulatory Commission will investigate how Ameren handled the problem and will write a report in 30 to 45 days after completing inspection. Ameren spokesman Mike Cleary is optimistic about the report, citing that Ameren handled the problem properly.

"Somewhere between May 4 and May 25 it became inoperable," said Cleary. "We would just like to find out when it happened and what the circumstances were."

The pump in question supplies water to the plant's steam generators.

"We have lots of redundant systems to keep the plant safe," Cleary said. But when you have one of those pumps out of service it just means that the overall level of safety has declined somewhat, and we don't want that to happen."

The plant supplies 19 percent of Ameren customer's power, and is the only nuclear power plant in the state. Ameren planned to build another plant, but a recent law bans utilities from charging customers the costs of a new power plant before it begins producing electricity. It would have cost at least six billion dollars, but the plans to build a new plant have been scrapped.