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updated 6/20/2011 11:27:09 AM ET 2011-06-20T15:27:09

It didn't take long for the nuclear industry to figure out that one essential type of safety valve leaked.

These parts, known as main steam isolation valves, block radioactive coolant in boiling water reactors as it leaves the reactor core on its way to the turbine building. During an emergency like an earthquake, this prevents a radioactive release into the environment from a possible break in a pipe outside the reactor building.

Given the importance of these valves, many plants were long limited to a relative trickle: 11.5 standard cubic feet per hour for each valve.

As early as 1976, a federal regulatory guide said euphemistically that the leakage limits "have not always been maintained."

In 1982, U.S. Nuclear Regulatory Commission staff reported that reactor after reactor was failing leak tests, and backup systems weren't always working.

The next year, the NRC staff voiced concern about risk to the public.

The industry went to work. In 1986, it came up with a solution: Owners wanted to raise leakage limits. An industry study concluded that with mitigating measures, the limits could be set as high as 200 standard cubic feet per hour for all four steam lines combined — four times higher than before.

In 1999, the NRC fell in line. An agency report justified the change by saying that "the specified low leakage is difficult to maintain." Allowances could be made partly on the assumption that some radiation would be confined to the turbine building, even though that structure could be damaged in an accident.

More recently, plants have been allowed to operate with limits at least as high as 300 standard cubic feet per hour for all four valves combined.

Even so, some violate their higher limits. In 2002, Susquehanna Unit 1 in Allentown, Pa., surpassed its 300 scfh limit by more than three times with combined leakage of 1,051 scfh.

In November, personnel at the Dresden unit 3 reactor west of Chicago discovered that three of four valves were leaking beyond their allowed limit.

The operator reported fixing the problem, which it blamed on worn valve seats. The seats are made of special alloys hardened to withstand chafing from closing plugs. The company, though, acknowledged that their breakdown is a "normal occurrence."

Yet talk within industry and government isn't centered on replacing the valves more frequently. Instead, they are discussing lifting limits again.

Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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