In 2003, an Internet worm was said to have entered the Davis-Base Nuclear Power Plant computer network located in Lake Erie, Ohio, disrupting the system for more than five hours. Though safety was not compromised because at the time the nuclear power plant was shut down, the event did show the potential for widespread disruption caused by the transmission of malicious code.
Wilson, C. CRS Report for Congress: Computer Attack and Cyberterrorism: Vulnerabilities and Policy Issues for Congress. [Online, October 17, 2003.] CRS Report Website. http://www.fas.org/irp/crs/RL32114.pdf.