One year after the tragedies of Sept. 11, how safe is the U.S. from terrorist strikes against its water supply and nuclear power facilities? Quite safe, according to industry and government studies commissioned in the wake of last year’s attacks. The Nuclear Energy Institute, an industry group, found that a Boeing 757 would not be able to penetrate the 4-foot-thick concrete walls and steel bars protecting nuclear reactors or the even thicker walls of nuclear-waste storage facilities. Moreover, the Nuclear Regulatory Commission determined that even a radioactive leak probably would not produce damage “as catastrophic as one might imagine.” Meanwhile, the nation’s water-treatment plants, reservoirs, and dams now boast a wide range of new safety measures, from round-the-clock security guards to surveillance cameras to more frequent water testing. Although experts said that physically poisoning the nation’s water supply was next to impossible, they acknowledged that even a minor attack could cause the public to lose faith in water purity.