Two nuclear submarines, one French and the other British, collided in mid-Atlantic earlier this month, reports in the British and French news media said on Monday, quoting sources in the two defense ministries.

Both submarines were damaged extensively but have returned to their home ports since the collision on the night of February 3, the reports said.

The French vessel, Le Triomphant, was said to have suffered severe damage to its sonar dome, housing equipment crucial to navigation and tracking of other ships. The British craft, H.M.S. Vanguard, was towed back to its home port at Faslane in Scotland with “very visible dents and scrapes,” the BBC reported.

The two submarines are at the core of their countries’ nuclear forces, each carrying a battery of intercontinental ballistic missiles equipped with multiple warheads. None of the media reports suggested there had been any risk of the collision accidentally triggering the missiles, which can be launched only after complex procedures designed to make the missiles fail-safe in the event of accidents.

But what little is known about the collision suggests that the two vessels were fortunate to have avoided a more serious outcome, including sinking. Although defense officials in London and Paris gave no indication of where the collision occurred, the fact that it took Le Triomphant three days to limp back to its home port after the collision, a detail disclosed by French officials, suggested the two craft had been far out at sea.

French officials quoted in The Daily Telegraph, a British newspaper, played down the incident. “The collision did not result in injuries among the crew and did not jeopardize nuclear security at any moment,” one official was quoted as saying. But he also said that the crew of 101 sailors “neither saw nor heard anything” before the impact, indicating a failure in the submarine’s systems for tracking other vessels.

The British Defense Ministry refused to comment on the collision, though its statement implicitly confirmed that there had been an incident at sea. “It is MOD policy not to comment on submarine operational matters, but we can confirm that the U.K.’s deterrent capability has remained unaffected at all times and that there has been no compromise to nuclear safety,” a ministry spokesman said.

Defense experts said that one possible explanation for the collision was that the two submarines were involved in an exercise that involved the craft tracking each other at close quarters in the kind of war games that have played out in the Atlantic for decades between western and Soviet submarines, especially in the northern Atlantic between Scotland and Iceland.

Britain and France each have four missile-carrying nuclear submarines, constituting the core of their nuclear arsenals. According to military journals, Le Triomphant carries 16 ballistic missiles with a range of 5,000 miles, each with six warheads. H.M.S. Vanguard is a Trident-class submarine, 492 feet long and weighing 16,000 tons. It, too, carries 16 missiles, each with three nuclear warheads.

(Source: NY Times)