When a nuclear accident involving a loss of coolant occurs, like it did aboard K-19 (and, eighteen years later, at Three Mile Island), the outcome can be fatal. The reactor needs coolant - quickly - to keep it from overheating and commencing an uncontrolled chain reaction. But how could that happen when K-19 had no built-in system to deliver the critically needed coolant to the primary cooling loop? (Note: the link depicts the system in a pressurized water reactor which is different from the two liquid metal reactors aboard the Soviet submarine. Notwithstanding those differences, the importance of the primary cooling system - which contains radioactive material - is explained in the link.)
The captain and crew needed to improvise. The leak had to be fixed or none of the men would live. Worse, a total core meltdown would likely result in the loss of the ship through a thermonuclear explosion. Were that to occur, what would happen to the submarine’s ballistic missiles?
Quick-thinking, risk-taking men (both officers and midshipmen) had to work under severe radioactive conditions. Working in the more remote areas of Compartment Six, self-sacrificing sailors were exposed to noxious gases and steam sure to cause radiation sickness.
Captain Zateyev’s memoirs reflect the horror of K-19's predicament and the courage of his crew:
While working inside the reactor compartment, of course, they were alarmed, but they walked into that compartment without hesitation, ready for hard work. I saw the same calm, the same self-possession in Ryzhikov, Kashenkov, Penkov, Kharitonov, Savkin and Starkov. (K-19, page 130)
Lt. Boris Korchilov specifically asked for permission to help. Zateyev responded: "Boris, do you know what you’re asking?"
It took hours for the men to successfully weld the two pipelines together, thereby preventing a total catastrophe, but even those repairs had to be fixed. The ship’s executive officer Vladimir Yenin (referred to as "Mikhail Polenin" in the film), volunteered for that job. Zateyev recalled:
At one point a leak did develop in the jury-rigged piping. My starpom [second-in-command] Yenin; Chief Petty Officer Ivan Kulakov; and Seaman Leonid Berezov (the commander of the missile operators’ department) repaired that leak and received heavy doses of radiation in the process. (K-19, page 135)
Reaching Compartment Six, where the nuclear reactor was located, Zateyev saw Korchilov leave after his work was completed:
...the bulkhead door opened and Korchilov emerged from the compartment. He ripped off his gas mask and immediately began vomiting a white and yellow foam...All the men who had been in Compartment Six were exposed to massive amounts of radiation, the doses they sustained going far beyond permissible levels. (K-19, page 131)
In fact, some of the men were already dying.