The Tennessee Valley Authority soon will replace the private security company that has protected its nuclear plants for the past 11 years and brought nearly 500 security officers to the TVA payroll, according to an employee memo released Monday.

TVA Nuclear Chief Bill Campbell said he decided to terminate the utility’s contract with Pinkerton Government Services this fall and return to in-house nuclear security to “have more direct and effective management controls and lines of communication” at TVA’s three nuclear plants.

Under its current six-year contract, TVA is paying Pinkerton $159 million, or $26.5 million a year, for security at the Sequoyah, Watts Bar and Browns Ferry nuclear plants from October 2002 through October 2008, TVA spokesman John Moulton said. TVA did not solicit bids for a new security contract, and Mr. Moulton said the change is not being made just to save money.

TVA officials said the agency evaluated nuclear security operations over the past year and will work with Pinkerton in coming weeks “to ensure a transition for the use of in-house security forces” at the three nuclear plants.

The move reverses TVA’s 1997 decision to outsource most of its nuclear plant security and comes nine months after the nation’s biggest nuclear utility, the Chicago-based Exelon Corp., terminated its private security contractor, Wackenhut Corp.

After a video showed Wackenhut guards sleeping at the Peach Bottom Atomic Power Station in York County, Pa., Exelon fired Wackenhut last year and brought its security staff in-house at the corporation’s 10 nuclear power plants.

The Nuclear Regulatory Commission, which regulates nuclear sites, received reports from TVA during 2007 of at least two incidences of “inattentive guards” at TVA nuclear plants.

In 2006, a government watchdog group — the Project on Government Oversight — disclosed another incident in which 30 M-4 assault rifles were found improperly stored inside the Sequoyah nuclear plant near Soddy-Daisy. Peter Stockton, chief investigator for POGO, said the weapons “could have been delivered to an insider planning a hostage situation.”

The NRC conducts periodic drills of such terrorist attacks, known as “force-on-force attacks.” But the results of such tests are not disclosed for security reasons, NRC spokesman Roger Hannah said.

But Mr. Hannah said TVA is in full compliance with all security requirements and no enforcement actions have been taken against the federal utility for any plant security problems.

Mr. Campbell said he has “a high level of confidence in the officers who protect our plants.” Pinkerton Government Services CEO Kevin Sandkuhlers said TVA gave Pinkerton high performance grades over the past couple of years.

“I think our record has been outstanding at the TVA plants,” Mr. Sandkuhlers said.

Nationwide since Sept. 11, 2001, the number of nuclear plant security personnel at the nation’s 104 reactors has jumped from about 5,000 to more than 8,000 “and billions of dollars has been spent to enhance physical barriers to make plants even safer,” according to Mitchel Singer, a spokesman for the industry trade group, the Nuclear Energy Institute.

“Nuclear plants are the most secure of any type of industrial site,” Mr. Singer said.

But Stephen Smith, executive director for the Southern Alliance for Clean Energy, which opposes nuclear power, said the high level of security at such facilities “underscores the risk, costs and uncertainties upon nuclear power that we simply don’t have with clean energy sources like solar, wind and biomass.”