When a fire broke out aboard a deep-diving nuclear spy submarine last month, it killed 14 Russian sailors. Now Moscow is moving to fast-track the repair of the radioactive boat to ensure its secret mission is completed.

Recent reports in Russian media cast fresh light on the secrecy-shrouded accident as well as reveal the extent of damage suffered by the AS-32 "Losharik".

Did incompetence cause the disaster? A technological fault? Or sabotage?

Kremlin investigators say they are inclined to believe the latter.

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"It is evident that the fire has done serious damage and that the vessel's radio-electronic equipment, automatics, acoustics and navigation equipment, as well as the life support system, needs to be replaced," a report by the state-controlled TASS agency says.

Fourteen high-ranking officials lost their lives in the Losharik submarine disaster. Photo / AP
Fourteen high-ranking officials lost their lives in the Losharik submarine disaster. Photo / AP

Whether or not the submarine's titanium hull and nuclear reactor were damaged is not clear.

But Moscow's determination to retrieve the deep-diving vessel, believed capable of tapping into undersea intercontinental internet cables, is not in doubt.

Despite the threat of contamination, the Losharik will soon be towed from its haven on the Kola Peninsula to Severodvinsk — a secured military city near Arkhangelsk — for repairs. That's the site of another nuclear accident earlier this month, likely involving one of President Vladimir Putin's "super weapons" — a nuclear-powered cruise missile.

DEADLY RENDEZVOUS

On the evening of July 1, a fire ripped through several of the Losharik's seven interlocked pressure-resistant spheres. These are arranged caterpillar-like within the vessel's 70m-long streamlined hull, giving it the enormous strength necessary to dive to a depth of some 2km.

The nuclear reactor compartment was reportedly untouched.

But among the 14 crew deaths were two highly decorated "Heroes of Russia", seven "first rank" and three "second rank" captains. The highly qualified nature of the casualty list indicates the importance and technical challenges of the missions the experimental submarine undertakes.

It was the worst accident experienced by the Russian Navy since 2008.

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According to recent reports, the experimental submarine had been completing exercises in Motovsky Bay in the Barents Sea. Losharik was on the brink of docking with its much larger Delta IV Delfin class "mothership" BS-64 Podmoskovye when the fire broke out.

It had reportedly just completed the last of a series of training exercises before a planned "active service" deployment. But something appears to have sparked a fire in Losharik's battery compartment and caused the flooding of several of its circular modules.

According to Russian news service Kommersant, six passengers escaped. But the remaining 10 crew followed strict Russian navy procedural requirements. They were required to stay at their posts in order to combat the blaze.

'FOUGHT TO THE LAST'

All of the remaining would have had to don portable breathing units offering up to 30 minutes of oxygen to avoid being overcome by the smoke.

"The submariners acted heroically in the critical situation," Russian Defence Minister Sergei Shoigu said. "They evacuated a civilian expert from the compartment that was engulfed by fire and shut the door to prevent the fire from spreading further and fought for the ship's survival until the end."

Russia's military has been plagued by disasters in recent months. Photo / AP
Russia's military has been plagued by disasters in recent months. Photo / AP

According to Kommersant, the submariners "fought to the last". They battled the blaze for more than 40 minutes — swapping their portable breathing units for fresh ones and switching to fixed emergency breathing units.

Only once all fire extinguishers had been exhausted, and crew began to collapse from smoke inhalation, did the Podmoskovye mothership captain agree to an evacuation.

He reportedly sent four of his own crew to assist the Losharik. But these failed to return.

"Unfortunately, no one can tell how the fire developed on the AC-31 since all those on-board died," a source reportedly told Kommersant. But, according to a top Russian naval officer who spoke at the time of the accident, the crew's bravery "prevented a planetary catastrophe".

The full circumstances of the tragedy remain untold. But unconfirmed reports suggest one of the blazing batteries eventually exploded, sealing the fate of all on-board.

FORENSIC ANALYSIS

According to TASS, the damaged submarine will be gutted and repaired in the Zvezdochka shipyard.

"In the first phase, water will be pumped out of the vessel and fault detection will be conducted with the titanium hull, as well as the inside compartments, after which a technical plan for repair and restoration will be made," a source told the news agency.

"We can say that the fire has seriously damaged radio-electronic equipment, automation, acoustic and navigation equipment. Life support systems are to be replaced."

Kommersant reports Losharik had recently been refitted with Russian-made lithium-ion batteries, replacing the silver-zinc batteries initially installed more than a decade ago.

But lithium-ion batteries are problematic.

Poor quality control, or damage from accidents, can cause them to overheat and ignite. This has been a significant source of concern for the world's submarine fleets, including Australia's next-generation Attack class.

Kommersant says Kremlin officials have deliberately exploded several similar battery packs since the accident to determine what components failed — or if the Losharik had been sabotaged.

"As the experts explain, the (investigative) commission traditionally considers three options in the investigation of any catastrophe — equipment failure, crew error and malicious influence," the Kommersant article reads. "Some investigators doubt that chemical reactions in lithium-ion cells could cause such a significant explosion."

ACCIDENT-PRONE

Fires and fatal accidents are plaguing Russia's military.

Early in August, a large ammunition depot in Siberia exploded — sending a mushroom cloud billowing into the sky and a shockwave through surrounding communities. The resulting fire took days to extinguish.

Just days later, automated sensors detected a spike in radiation levels in the northern city of Severodvinsk. This was quickly linked to an explosion at a nearby secret Russian naval facility and Mr Putin's controversial nuclear-powered extreme-range cruise missile project.

But the disasters don't end there.

Last year, Russia's only floating dock facility capable of servicing the aircraft carrier Admiral Kuznetsov accidentally flooded and sank — damaging the ship in the process. And several dockyards and naval bases have experienced devastating fires.

In the case of Losharik, however, Moscow appears to have made it a high priority to return the submarine to service.

The top-secret vessel is operated by the GUGI (Directorate of Deep-Sea Research). It's more accurately termed Russia's main naval intelligence unit.

The small nuclear-powered craft was built to dive deep and recover — or manipulate — seabed objects after being carried into the vicinity by a "mothership" submarine. Such capability would be invaluable in recovering lost sensitive equipment or snagging samples of opposition technology — such as a crashed F-35 stealth fighter.

Then there's its intelligence-gathering capacity, such as planting surveillance equipment or tapping or sabotaging seabed cables.

Giving the immensely tricky and specialist nature of these missions, it is little wonder Moscow has ordered the Losharik's rapid repair.