A baffling leak has been discovered at one of the newest nuclear reactors in the U.S. The South Texas Nuclear Project, 90 miles southwest of Houston, was discovered to have leaked cooling water from its large reactor vessel, a problem that experts have never before encountered. “This is the first time it’s been seen, either here or abroad,” said Victor Dricks, spokesperson for the U.S. Nuclear Regulatory Commission. The plant’s operators have pledged to keep the reactor offline until the source of the problem is found and the leak fixed. Managers of the 103 commercial nuclear plants in the U.S. have been on the lookout for leaks since last year, when extensive cooling-water seepage was discovered at the Davis-Besse nuclear plant near Toledo, Ohio, which has now been shut down for 13 months.