It is a liquid metal cooled reactor (LMR), using highly enriched uranium-235 fuel to produce 73 MW of power.
K-27 was a November-class first generation nuclear submarine, and the only one of its class fitted with liquid metal cooled reactors.
However the seven-member Alfa class were subsequently fitted with liquid metal cooled reactors.
Its usage in the K-27 led to a nuclear incident with 9 fatalities.
[1] It was developed by OKB Gidropress in cooperation with IPPE.