June 6, 2003 By MATTHEW L. WALD ROCKVILLE, Md., June 5 - The managers of a Texas nuclear plant with two leaks at the bottom of its reactor vessel told the Nuclear Regulatory Commission today that they were backing away from their top theory on how the leaks developed. But, they said, they were nearly ready to fix them anyway. The operators of the plant, called the South Texas Project, 90 miles southwest of Houston, also told the agency staff that some residue left by the leaks was more than four years old and possibly much older. Previously, they had said the residue was not visible when the reactor was inspected last year. The leaks are in 2 of 58 nozzles on the underside of the reactor, which were installed to let operators push monitoring instruments into the nuclear core. The leaks were discovered when the plant was shut down for refueling in late March. At first, plant managers were inclined to believe that the leaks had been caused by a phenomenon in which the cooling water attacks metal that is under strain. But careful examination of the metal found flaws in some areas that had not been exposed to the water. The nozzles cannot be fully replaced because they are welded from the inside of the vessel. The managers' repair plan involves cutting off the portion of each nozzle that has the cracks, inserting a length of replacement pipe and welding it to the outside of the vessel. Leaving a gap between the old part of the nozzle and the replacement part, though, will allow the boron-laden cooling water to get access to areas that are not protected against corrosion. The Nuclear Regulatory Commission has approved "half-nozzle repair" for some structures at nuclear plants, but not for a reactor vessel. David Lochbaum, a nuclear engineer at the Union of Concerned Scientists, a group that often criticizes nuclear operators, said the two tubes so far had not presented a major safety problem because they leaked, leaving obvious deposits of boron, before they could break. If the proposed fix eliminates leaks but accelerates corrosion, Mr. Lochbaum said, "the first sign of a problem will not be a white residue but a loud bang and the rapid loss of water from the reactor vessel." Copyright 2003 The New York Times Company