The Gundremmingen plant, an atomic power plant in Germany, was reported to be infected by computer viruses.
According to the official comment, influence of the malware was minimal because the mainframe of the plant was isolated from the internet. It is relieving that this plant has been administered based on the standard safety control policy.
The viruses found in the computer of this plant are infamous so that millions of Windows PC in the world has been infected previously.
Reuters: German nuclear plant infected with computer viruses, operator says
In general, atomic power plants are deliberately protected physically and electronically from external threats. Although several incidents occur in the plant as common, they would not be critical.
If terrorists target the plant, the outcome of the conflict between the administrator and cracker depends on the level of technique and resource of each side. However, most crackers cannot overcome the security of the plants. And, if the purpose of the terrorists is to conquer a nation, it is no good to make the plant uncontrollable. Considering these facts, the risk of the meltdown caused by external power is minimal.
By the way, Germany has decided to shut down all of the atomic power plants. But the process is not so smooth. It is a challenging task to provide all of the electricity in the nation without atomic power. Because of the policy change in Germany, specialists of nuclear power will be shortening. It means that the risk of accident in every aspect will be increased until all of the plants are completely disabled.