They are pressurized water reactors (PWRs) that use enriched uranium-235 fuel.
OK-150 specifications: Distilled water was used for heat transfer and as a moderator.
There was a biological shield made of concrete mixed with metal shavings.
OK-900A specifications: Three OK-150s were used to power the Soviet icebreaker Lenin at the time of its launch in 1957.
Later, after damage caused by nuclear accidents in 1965 and 1967, these were removed and replaced with two OK-900s.