A leaking weld discovered by plant personnel prompted operators to shut down a reactor at the Diablo Canyon nuclear plant Tuesday night.An inspection of unit 1 revealed a through-wall leak in a socket weld, according to a report filed with the Nuclear Regulatory Commission. The weld, located inside the containment vessel, is near a relief valve for the residual heat removal (RHR) system, rendering it inoperable under plant guidelines and requiring a shutdown for repairs.A Pacific Gas and Electric release reported by California media added that the inspection showed a small buildup of boric acid on the area in question. The utility noted that the unit is in safe condition, and unit 2 at the plant continued to operate at full power. Both are large Westinghouse pressurized water reactors first licensed in the mid-1980s. They are located on the coast between Los Angeles and San Francisco.