The Echo II class carried eight anti-ship missiles near the top of the hull, stored in four water-tight compartments on either side. Interestingly, launching these missiles required the submarine to surface. The ship would be surfaced, the missile tube elevated 15 degrees, and the missile launched.
The submarine would have to remain surfaced for 30 minutes to launch all 8 missiles, as they could not be launched all at once, and they also required guidance from the submarine while in flight. Apparently the danger to hostile surfaced submarines was not as acute in the 1960's as it would be today.
Below: A November II Class submarine. Note the horizontal missile compartments (between the cutouts) on each side of the hull. It is possible these cutouts were to provide seawater quenching of the missile exhaust during launch.
K-431 did not meet her end in the depths of the ocean. She fulfilled her duties, but her career ended suddenly, right next to the pier. What ended her sea-going days was a pretty significant event that you probably never heard about, thanks to the Cold War.
In August 1985, K-431 was in the process of undergoing a refuelling at Chazma Bay near Vladivostok.
Fresh fuel had been installed in the ship, and the reactor vessel head was set in place. The head was not mating correctly to the reactor vessel, and the decision was made to lift the reactor vessel head partially with a crane to re-position it for correct alignment.
Re-aligning the head was complicated by the fact that the control rods for the reactor had already been attached to the control rod drive mechanisms (CRDMs), which are mounted to the reactor vessel head. This meant that when the reactor vessel head was lifted, all of the control rods would come out of the reactor core.
To help visualize the scenario, below is a diagram to help.
Below: A cut-away of a large power plant reactor. The reactor vessel head is the dome-shaped shell at the top that is bolted onto the reactor pressure vessel. The control rods are operated by motors that will later be installed on the CRDM nozzles.
Disconnecting each of the control rods to re-position the reactor vessel head slightly would have taken a great deal of time. Rather than disconnect each control rod to re-position the head, a calculation was performed to determine how high the reactor vessel head could be safely lifted. It was possible that the reactor could become critical as the control rods lifted out of the core along with the head. After this calculation was made, a steel beam was installed above the reactor vessel head to prevent it from being raised too high by the crane.
The reactivity calculation was correct, however the placement of the steel beam was not. When crane lifted the reactor vessel head (and all of the control rods), the reactor became prompt critical. This instant burst of power caused an immediate release of heat, neutron and gamma radiation, followed by a massive steam explosion as the fuel vaporized the surrounding coolant. The explosion expelled the new load of fuel, destroyed machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the refueling shack, with the shack's roof falling 200 feet away in the water. A fire followed, which was extinguished after 4 hours.
Ten people were killed by the blast. Radiation injuries (burns) were observed in 49 others, mainly those who had fought the fire after the initial explosion. Ten people developed radiation sickness (vomiting, hair loss), but there were no other immediate casualties.
Fortunately, since this was fresh fuel (20% enriched U-235), it did not carry a load of highly radioactive split atoms in it. The only fission products generated were created by the brief criticality. After just four days, the ship could be approached safely and cleanup and repairs could begin. The nuclear fuel was gathered up and disposed of, and within six months radiation levels were down to background levels.
K-431 was decommissioned in 1987 and scrapped.
Below, a mothballed or neglected Echo II class submarine with the missile compartments removed.
Below: Allegedly this is K-431 after the accident and decommissioning. Note the flotation devices alongside. As noted before, submarines are not particularly buoyant.