The blaze that broke out on the Perle in June 2020 at the Mediterranean base at Toulon needed about 250 firefighters and support crew members to put out, but there was no damage to the nuclear reactor nor any casualties.

Investigators are still trying to figure out the cause of the fire, which may have been caused by plastic that was exposed to a hot work lamp.

The vessel was brought to the Naval Group shipyard in Cherbourg on the coast of the English Channel, where much of the bow was replaced by using parts from a recently decommissioned sub, the Saphire.

"The fix worked: We did a Lego operation, attaching the back part of the Perle with the front part of the Saphire," said Herve Grandjean, a defence ministry spokesman.

The operation extended the Perle's length by one and half metres, at a cost of 110 million euros ($128 million).

It is now en route for Toulon, where the scheduled maintenance will resume, including a fuel recharge for the nuclear reactor that will provide power for 10 years.

The vessel, which can dive to 300 metres (985 feet) with a crew of 70, is an attack submarine used for tracking ships, escorting aircraft carriers, carrying out coastal intelligence missions and deploying special forces.

In addition to its six nuclear-powered attack submarines, France also operates four submarines capable of firing nuclear ballistic missiles.

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