This is the fifth information leak in over a week regarding the South Korean Gori and Wolsong nuclear power plants. Previous tweets contained personal data of about 10,000 staff of nuclear power plant's operator, Korea Hydro & Nuclear Power Co (KHNP), according to Yonhap.
Earlier in the day, South Korean authorities expressed concern over the leaked data. South Korean President Park Geun-hye pointed out that the activities of nuclear power enterprises could directly affect the lives and security of people, according to media reports.
"A grave situation that is unacceptable has developed when there should have been not a trace of lapse as a matter of national security," Park Geun-hye said at a cabinet meeting in Sejong, South Korea.
As a counter measure, South Korea reportedly launched a two-day safety drill on its four nuclear plants on Monday, to confront cyber-attacks.
Since December 15, the hacker started posting internal KHNP information, claiming that if the reactors are not shut down by Christmas, 100,000 pages of data would appear on their Twitter account.
In response, the nuclear operator claimed that the revealed information did not concern the core technologies and thereby, did not endanger the reactors' security. All 23 nuclear power reactors in South Korea are operated by the KHNP, providing 30 percent of total country's power supply, according to the company's official website.
The incident with the KHNP files took place shortly after the United States accused North Korea of a cyber-attack on the Sony Pictures Entertainment company.
However, in this particular case, the South Korean president did not mention North Korean involvement in the nuclear plants data leakage.