PARIS — The ballistic missile submarine Téméraire has been taken to Ile Longue, the French naval base for nuclear boats, marking a key step in a major refit of the sub, shipbuilder Naval Group said in a July 23 statement.

That move, which comes “after 20 months of work,” marked “a significant step in the the last modernization and adaptation of nuclear ballistic submarines for the M51 missile,” the company said.

The submarine was towed Friday from dock 8 at Brest to the highly secure base on Ile Longue, a small peninsula in Brittany, northern France, where the French Navy maintains its fleet of four ballistic missile boats.

The overhaul and refit required the hull to be split open and new silos installed for the M51 missile, which is longer and wider than the outgoing M45 weapon. Work will continue some three or four months on the Téméraire on Ile Longue, with the boat expected to sail for a further nine years after returning to service.

“The adaptation to the new missiles is of primary importance,” said Maximilian Porter, program director at the Direction Générale de l’Armement procurement office, regional paper France Ouest Entreprise reported. Combat and navigation systems were also updated.

Téméraire is the third ballistic missile submarine to be refitted for the M51 weapon, following the Vigilant and Triomphant. That program of refits has taken some 10 years. The fourth ballistic missile boat, Terrible, was delivered equipped for the M51.

Naval Group was prime contractor on the major overhaul and refit, which takes some two years and four million hours of work on each boat.

Airbus Defense & Space is contractor for the M51 missile, which is assembled at its nearby Guenvénez plant and delivered to the Ile Longue base, where the atomic warheads are fitted.

The government’s Alternative Energies and Atomic Energy Commission is assembling and fitting the nuclear warhead on the missile on Ile Longue. Each of the Triomphant-class submarines carry 16 M51 missiles, which can carry up to six independent warheads.

Studies are being conducted for a third-generation nuclear ballistic missile submarine and future versions of the M51. The M51.3 version is due to enter service around 2025. Under the newly adopted 2019-2025 defense budget law, the government has earmarked €25 billion (U.S. $29 billion) for work on the nuclear deterrent, including seaborne and airborne weapons.