In a letter to New York state, the Federal Emergency Management Agency also says the new sirens, which have not been placed into service, are blocking some sound from the existing sirens.

The new system "is not adequate" and cannot be approved until major changes are made, the letter said.

The sirens are meant to alert residents within 10 miles of the plant's two reactors, in Buchanan on the Hudson River 35 miles north of midtown Manhattan, when there is a major problem.

Plant owner Entergy Nuclear has missed three deadlines for the new system, which was ordered by Congress, and is already facing the prospect of large daily fines from the Nuclear Regulatory Commission as a result. FEMA approval is required before the system is considered finished.

Entergy spokesman Jim Steets said the company disagreed with some key points of the FEMA letter and would have to consult acoustics experts about others, but he added, "We are dedicated to cooperating with the agency to resolve this issue and assure that a state-of-the-art emergency alert system is placed into service and working effectively and reliably."

The letter said the new sirens did not meet the loudness criteria originally set by Entergy -- 60 decibels in low-population areas and 70 decibels in high-population areas. It said Entergy had recently tried to change the standard to 10 decibels above background noise.

"Rather than take suitable corrective action, Entergy is willing to have a system that does not meet its own emphatically stated design objectives, as long as FEMA will allow them to," the letter says.

It also said the sound from the sirens wavers too much instead of wailing steadily.

NRC spokesman Neil Sheehan said the commission was still evaluating the letter and would hold back on any penalties until a new system is approved and operating.