Last August at Unit 3 of the Browns Ferry (AL) nuclear power plant, operators had to manually shut down the reactor after two water recirculation pumps failed. A subsequent investigation has found that water pump controllers had locked up following a "data storm" – a flood of data that overwhelmed computer processors - on the plant's internal control network.
A separate malfunctioning control device was found to be responsible for the storm, and the plant's internal network is not supposed to be connected to any outside network, but discussions held between the House Homeland Security Committee staff and Nuclear Regulatory Commission representatives have suggested that it is possible that this incident could have come from outside the plant. If such a claim could be substantiated, it would become one of the first solid public cases of direct external manipulation of a critical infrastructure control system. In plain English: hackers and/or terrorists turning off the power.