KOCHI, Japan - Former Japanese fishermen plan to seek compensation from the Japanese government for keeping undisclosed for decades records of their radiation exposure linked to a series of U.S. hydrogen bomb tests in the Pacific, sources close to the matter said Thursday.

About 20 people, including families of deceased fishermen, are expected to file a lawsuit with the Kochi District Court possibly next month, each seeking about 2 million yen ($18,000) in compensation, the sources said.

It is the first time that a state compensation lawsuit has been filed in Japan in connection with the hydrogen bomb tests the United States conducted on the Bikini Atoll in the Marshall Islands in 1954 that began with a test explosion code-named Castle Bravo on March 1.

Castle Bravo was the largest-ever nuclear weapon test the United States conducted, and it spewed radioactive fallout over a vast area. Japanese tuna fishing boat Fukuryu Maru No. 5 was doused with the fallout and one of its 23-man crew died about six months later.

As other Japanese ships, many from Kochi, were sailing in the vicinity around the time of the hydrogen bomb tests, calls later emerged that the state should disclose the results of radiation checks conducted on the ships at that time.

But it was only in September 2014 that the Health, Labor and Welfare Ministry released the documents, retracting its earlier position that the records no longer existed.

The ministry acknowledged that crew members on 10 ships, among some 500 vessels in the vicinity of the tests, received a certain level of exposure, but asserted that the doses did "not reach levels that could damage their health."

The former crew members plan to argue in the lawsuit that the state intentionally hid the records. As a result, they were deprived of the chance to exercise their right to seek damages from the United States and suffered emotional distress, the sources said.

They will also point out that the government failed to conduct any follow-up studies on the crews of ships other than the hardest-hit Fukuryu Maru No. 5, also known as the Lucky Dragon, and failed to offer compensation after Japan and the United States reached a political settlement in January 1955.

The United States, without admitting liability, paid $2 million as sympathy money to Japan over the issue. The funds, then worth about 720 million yen, were distributed mainly to crew members of the Fukuryu Maru and the fishing industry in general, which was seriously damaged by a public scare about contaminated marine products.

The former crew members and bereaved families are expected to form a group of plaintiffs early May to prepare for filing the lawsuit, the sources said.

Many of them also applied in February for workers' compensation for cancer and other diseases they say were caused by exposure to radiation from the U.S. hydrogen bomb tests.

==Kyodo