The reactor used molten lead-bismuth for coolant, which meant it turned into a brick if it stopped circulating, but gave big power for its size.
When it comes to nuclear fast attack submarines (SSNs), it doesn’t get more hot-rod than the Soviet Alfa class. With a hull built of titanium, a highly automated control system run by a skeleton crew, and a reactor that was cooled by molten lead-bismuth, the Alfa class was absolutely cutting-edge when it debuted in the early 1970s. Able to hit speeds in excess of 40 knots while submerged, the Alfas were rocketships of the undersea world, with only the one-off K-222 “Golden Fish” submarine eclipsing their speed capabilities. They were also remarkably small, partially due to their reactor design, which had many advantages, but if its molten metal coolant stopped circulating, it turned into a mummified brick. Now, pictures from the removal of one of these reactors from a decommissioned Alfa class boat give us an idea of just how compact the class’s exotic reactors really were.
Before you continue on, you can read and should read all about this marvel of a submarine in this past feature of mine. It’s essential to get a better understanding of just how remarkable these submarines were when they first emerged and the many tradeoffs in their design that truncated their fleet size to just seven boats.