SEOUL (Reuters) - Computer systems at South Korea’s nuclear plant operator have been hacked, but only non-critical data has been lost and there is no risk to the safety of nuclear installations including the country's 23 atomic reactors, the company and the government said on Monday. The attacks come amid concerns that North Korea may mount cyberattacks against industrial and social targets after accusations by the United States that Pyongyang was responsible for a devastating hacking assault on Sony Pictures.