WASHINGTON, April 18 — A nuclear reactor in Texas is leaking cooling water from the bottom of its giant reactor vessel, a development that experts view with concern because they have never seen it before, the Nuclear Regulatory Commission said today. Technicians at the South Texas Nuclear Project, about 90 miles southwest of Houston, have found residues indicating that cooling water leaked from the vessel through two penetrations where instruments are inserted into the core, according to the company that operates the plant. Operators at all 103 commercial nuclear reactors have been giving closer attention to their reactor vessels since...