A plumbing problem called "denting" will cost the nuclear power industry - and ultimately consumers - at least $600 million over the next year or so, with much more money at stake if a solution is not found, the Nuclear Regulatory Commission was told yesterday.

The problem, a form of corrosion that causes leaks in pipes that carry radioactive water, already has closed Virginia Electric & Power Co.'s Surry II plant near Newport News. Major repairs to correct the problem will take at least six months.

Surry I will close for six months as soon as Surry II reopens, and two Florida Power & Light Co. units, Turkey III and IV, will shut down later later this year.

Eleven other nuclear plants are affected to a "moderate" or "minor" degree, while 27 more are thought to be susceptible. The nuclear industry has launched a $40 million effort to find a solution before those plants have to close for repairs.

As it is, the Surry closures alone will cost about $230.4 million, according to power company estimates, the Turkey Point shutdown$380 million, and associated work to plug damaged tubes elsewhere an additional $20 million.

Critics of the nuclear industry are alarmed at the radiation releases that are occurring at Surry, and have filed petitions to stop the work. Their concern can be expected to grow as the denting problem does, adding to the beleaguered industry's headaches and expensive delays.

The problem, the NRC was told, involves the chemistry of the water in the steam generator system, the part that drives the turbines to produce electricity.

Minute amounts of chloride seem to trigger the formation of what the engineers call "green grunge," an iron oxide compound, in the steam generator water. Ti collects around the edges of holes in carbon steel support plates, pinching the thousands of pipes (carrying radioactive primary coolant water) that pass through the holes.

As the grunge collects, it bends and eventually cracks the pipes, causing them to leak their hot cargo into the sealed steam generator system. Ti also may enlarge the support plates, behind them and causing small pieces to break loose.

"If you have chloride ions present, you get denting," said Darrel Eisenhut, deputy director of the NRC's division of operating reactors. "It's just a matter of time."

Until Surry II closed in February, utilities had been plugging the leaking pipes as they developed. The amount of radioactive water permitted to leak without being plugged is controlled by the NRC at between 0.3 gallons and 1 gallon per minute, and most of that water remains isolated in the steam generator system.

Since 1976, when denting was discovered, more and more leaks have caused up to 25 percent of the pipes to be plugged in some plants, including Surry II, which means that the inside of the steam generators has become progressively more radioactive. This has immensely complicated the repair process.

Workers must cut the damaged steam generators in half - severing their piping, removing the bottom havles from the reactor containment buildings and interring the hot carcasses in special cement tombs.

The NRC initially estimated that the workers would receive a total of 3,380 to 5,840 manrems of radiation doing the job at Surry II and the same at Surry I, while Vepco estimated the total at 2,070 manrems for each unit with extensive protective measures.

Vepco is bringing "hundreds" of workers in to do bits of the job until their maximum allowed radiation dosage is reached, and the NRC says that so far the dosage per worker is "within the ballpark" of Vepco's estimates. An antinuclear group called Potomac Alliance has petitioned the NRC to halt the Surry project, partly on the grounds that the radiation doses are excessive.

The industry is trying to figure out why some nuclear power plants that would seem likely candidates do not have denting, why the denting at San Onofre in California seems to have reached a severe level and then stopped, and why the Palisades plant on Lake Michigan has a lot of grunge buildup but no denting.

Officials are working with new support plate materials and designs, incorporating them into plants under construction on orders from the NRC. Westinghouse Electric Corp. has developed a system to replace only the damaged tubes and not the entire steam generator, which it says costs far less in both money and radiation dosage.

"It's not a foregone conclusion any more steam generators will have to come out," said Eisenhut after the NRC meeting. "But it's still not understood very well."