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Texas nuke plant seeks answers to bottom cracks

06/06/03

John Funk
Plain Dealer Reporter

As engineers at the Davis-Besse nuclear plant prepare to test for suspected leaks in the reactor's bottom, their counterparts at a similar Texas power plant with confirmed leaks said yesterday that they did not know if their reactor's cracking problems are one of a kind or the beginning of a trend.

The Nuclear Regulatory Commission is concerned about cracks in the special alloy tubes that carry instruments through the bottom of pressurized water reactors like Davis-Besse near Toledo and the South Texas Project reactor near Houston. Even a medium-size bottom leak could drain coolant from a reactor faster than emergency pumps could resupply it, leaving the nuclear core uncovered.

In a six-hour presentation to NRC officials yesterday at the agency's Rockville, Md., headquarters, a South Texas engineering team recounted the results of extensive high-tech inspections of the inside of the bottom tubes - the first-ever such probes at a U.S. nuclear plant. The checks were made last month to determine the extent and cause of the cracking at the 15-year-old Texas plant.

"We have spared no expense and involved the industry," South Texas plant design manager Steve Thomas said of the work.

Inspectors in April found tiny deposits of boron - normally present in reactor coolant - around two of the 58 tubes, or nozzles, that penetrate the reactor bottom. Ultrasonic and electronic probes of all 58 nozzles confirmed cracks in two of them.

The instrument inspections also found that none of the cracks was circumferential - following the circumference of the nozzle's 1-inch-thick walls - as the NRC had feared. Such cracks could eventually cut a nozzle in half, leading to a possible ejection of the nozzle from the high-pressure reactor and a large leak.

As to the cause, the inspection probes did not find myriad cracks where an engineering analysis predicted they would be, said Thomas, disproving the hypothesis that a phenomenon known as "water stress corrosion" caused the cracking.

Stress corrosion cracking of the nozzles that penetrate the tops of reactors has become an industrywide problem, and NRC engineers worry the bottom nozzles might be next.

While the inspection evidence seems to discount water stress corrosion, it does not help engineers explain how or why the cracks appeared. "We may never know what caused the cracks," Thomas said. The NRC nevertheless expects the company to prepare a root-cause analysis.

Before doing that, engineers plan a kind of biopsy by scooping tiny metal samples from the inside walls of the damaged nozzles for a metallurgical analysis. "We have talked about a number of things that might be unique to this plant and to these nozzles," said Mark McBurnett, South Texas licensing and quality manager. "Once we get the analysis of the samples, that is a question that must be answered."

If bottom leaks are found, a decision would have to be made about repairs. Davis-Besse managers suggested earlier that they might just plug the nozzles and do without instrument readings from that section of the nuclear core.

However, South Texas engineers are proposing to cut out the bottom half of the damaged nozzles and weld new tubes made of a tougher alloy into place.

Such a pioneering effort may have an Achilles heel. Because the two halves of the repaired nozzles will expand when the reactor is running, engineers want to leave a tiny crack between the old and new sections. That could potentially expose the carbon steel in the reactor wall to the corrosive coolant, NRC engineers pointed out.

Rusting needs oxygen, responded the South Texas team, and because the section of new nozzle would be welded in place, rust could not occur even if the older section developed new cracks.

The NRC must still approve the repair. The company hopes to have the 1,250-megawatt plant ready for restart in late summer.

For full coverage of Davis-Besse, go to www.cleveland.com/davisbesse/

To reach this Plain Dealer reporter:

jfunk@plaind.com, 216-999-4138


2003 The Plain Dealer. Used with permission.
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