Virginia Power officials said they may be closer to finding the cause of July's steam pipe rupture at the North Anna nuclear plant now that a section of broken pipe has been removed for analysis.

A 31-foot length of pipe has been lifted from a North Anna steam generator and taken to a laboratory, Virginia Power spokesman Carl F. Baab said. Baab said he expects preliminary results from the analysis in about a week, but final results will not be available for several weeks.

"Getting it out and looking at it in a laboratory we considered vital in determining a cause," Baab said.

The July 15 rupture forced the utility to shut down Unit 1, one of the plant's two 915-megawatt nuclear units, and led to the release of a small amount of radioactive gas into the atmosphere.

Virginia Power believes the break was caused by a phenomenon called "fatigue-assisted cracking" or "denting," which weakens the tube from the outside. The tube has a 3/4-inch diameter and is made of a stainless steel alloy called inconel.

Virginia Power plans to return Unit 1 to service Sept. 25. The daily cost of lost service was estimated at $200,000 to $500,000.