The Royal Navy revealed, during a recent parliamentary investigation, that, during the last 21 years, it's nuclear submarine suffered about one fire a month. Moreover, the subs suffered two collisions every three years. None of the fires or collisions caused serious damage. Less than ten percent of the fires required more, than the sailors in the vicinity, to deal with. Three of the fires occurred while the ships were in port. Most of the fires, and all the collisions, occurred while the subs were underway, and submerged. That is when the boats are most vulnerable to something going wrong, as this is when the most equipment is in use, and often under stress. Subs are built to deal with these equipment failures, and in the case of the Royal Navy boats, that worked.
The 237 fires and 14 collisions took place on 21 subs (13 SSNs and 8 SSBN) that were in service during that period. Nearly half those boats have since been retired.