By Ros Krasny

BOSTON (Reuters) - A 24-year-old civilian worker was arrested and charged on Monday with setting the fire on a U.S. nuclear submarine at Maine's Portsmouth Naval Shipyard in May that caused an estimated $400 million in damage.

Casey James Fury started the blaze on the USS Miami "to get out of work," a Navy investigator said.

Fury, a painter and sandblaster, was charged in federal court in Portland, Maine, with arson for the May 23 blaze. The Miami was in the shipyard in Kittery, Maine, for repairs and retrofitting.

Authorities also charged Fury, of Portsmouth, New Hampshire, with setting a second, smaller fire at the shipyard in June, in the dry dock cradle on which the Miami rests.

If convicted on either charge Fury could face life in prison.

Charges came after an investigation by the Naval Criminal Investigative Service. They state that Fury admitted, following a lie-detector test, to using a cigarette lighter to ignite a vacuum cleaner and a pile of rags in a stateroom about the Miami.

"The reason he set the fire was in order to get out of work," said NCIS special agent Jeremy Gauthier.

He added that Fury told investigators he was taking a variety of medications for anxiety, depression, allergies and insomnia at the time of the first incident.

Fury also said he was anxious about a text conversation with his former girlfriend and wanted to leave work when he set the second blaze.

The fire on the Miami, a Los Angeles Class attack submarine, took about 12 hours to extinguish. Seven firefighters sustained minor injuries.

The vessel's nuclear propulsion plant had been shut down for more than two months during the repairs, and remained in safe and stable condition throughout the event. There were no weapons on board.

The Navy has yet to determine if it will repair the $900 million Miami or scrap it.

(Editing by Mohammad Zargham)