Gundremmingen plant in southern Germany was found to be riddled with computer viruses, including those which would allow attackers remote access to equipment for moving nuclear fuel rods.
Viruses, known as W32.Ramnit and Conficker, were discovered at the plant, which is located 75 miles northwest of Munich.
W32.Ramnit has the potential to give an attacker remote control over a system when it is connected to the internet and is also designed to steal files from infected computers.
The virus could be used by groups such as Islamic State to obtain nuclear secrets, bringing them one step closer to building a radioactive bomb.
However, officials said the plant was isolated from the internet and was therefore not vulnerable to attackers gaining remote access.
Fears of an ISIS-inspired nuclear attack have grown in recent weeks after terrorists involved in the Brussels attack were found to be monitoring an official in charge of a Belgian nuclear plant.
Investigators discovered 10 hours of footage filmed by jihadis of the home of the research and development director of the Belgian Nuclear Programme.
Brussels bombers - brothers Ibrahim and Khalid El Bakraoui - hid their camera in the bushes near his home and police now believe they were planning to kidnap the senior nuclear official in a bid to force him to give the extremists access to the highly sensitive atomic site.
The latest security breach, at Gundremmingen, which included malware found on 18 removable data drives, mainly USB sticks, has prompted German utility company RWE to increase its cyber security measures.
IT specialists are now looking into the incident at Gundremmingen's B unit, where viruses were found in a computer system retrofitted in 2008 with data visualisation software.
After Japan's Fukushima nuclear disaster five years ago, concern in Germany over the safety of nuclear power triggered a decision by the government to speed up the shutdown of nuclear plants.
The terrifying prospect of an ISIS nuclear attack was confirmed earlier this month when police searching the flat of Paris attacks suspect Salah Abdeslam found documents relating to a German nuclear base.
Investigators discovered the cache of files, which related to the Juelich base, in his Molenbeek flat when it was searched on March 18.
The atomic site, near the Belgian border, is used to store radioactive waste.
Printed news articles from the internet and photos of the chairman of the nuclear site Wolfgang Marquardt were found in the apartment.
( Source )