The Russian Navy suffers a high number of major nuclear submarine fires compared to other nuclear navies. See the April 2015 fire of Oscar class SSGN Orel. Fire was also a part of the August 2000 Oscar class submarine Kursk disaster.
Kyle Mizokami for Popular Mechanics on February 28, 2018 reported an earlier even more dangerous, December 2011, fire that involved Russian Delta class SSBN "Ekaterinburg" still carrying highly flammable torpedoes (with propellant), SLBM rocket fuel, nuclear warheads and of course its two VM4-SG nuclear reactors.
Following is a portion of Kyle's article https://www.popularmechanics.com/military/navy-ships/a18757426/in-2011-a-russian-submarine-fire-nearly-cause-a-nuclear-disaster/
"Russian Deputy Prime Minister Dmitri Rogozin has admitted that a December 2011 incident involving a nuclear missile submarine almost became one of the worst nuclear weapons disasters ever recorded.
The Ekaterinburg, a ballistic missile submarine, caught fire in drydock, threatening its load of liquid-fueled nuclear missiles. If the missiles had caught fire, then the resulting explosions would have spewed radioactivity over a wide area, threatening a nearby town of 300,000 people.
The nuclear-powered ballistic missile submarine K-84, also known as Ekaterinburg, had made a quick stop in drydock before heading out on a missile patrol. The ship was surrounded by wooden scaffolding for a welding job on the bow when a fire broke out. The fire quickly spread to the submarine’s rubber anechoic coating, which is designed to lower the sub’s acoustic signature underwater. The fire burned for nearly a day as firefighters struggled to contain the blaze..."
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Also see. Ekaterinburg, commissioned in 1985, is still in active service.