A hacker who claims to have stolen sensitive data belonging to South Korea’s nuclear power plants is demanding money to keep the information private, reports the International Business Times.

The hacker took to Twitter to post documents relating to an advanced power reactor on Thursday, using an account under the name of the president of an anti-nuclear group in Hawaii. The state-run Korea Hydo Nuclear Power Co. (KHNP) has not been able to identify how sensitive the documents are, but assured they could not include classified information because its server has been isolated since last year.

The hacker did not put a price on the ransom, but claims many countries are interested in buying information about the nuclear reactors.

"Need money. Only need to meet some demands... Many countries from Northern Europe, Southeast Asia and South America are saying they will buy nuclear reactor information. Fear selling the entire information will undermine President Park (Geun-hye)'s efforts to export nuclear reactors," the hacker said in a post.

This is the fifth posting of this kind since December 15, reports the Korea Times, including a post which threatened to “bring destruction” unless the country’s nuclear reactors were shut down before Christmas last year.

"Since the so-called anti-nuclear group made its fifth release of information on Dec. 23, no cyber-attack or information leak has taken place while the documents released today appear to have been obtained long before," KHNP said in a press release, according to Yonhap.

In the most recent post, the hacker threatened to attack the company with 9,000 viruses, after an additional 7,000 have reportedly already been found.