The VM-4 reactor is a nuclear fission reactor using 20% enriched uranium-235 fuel to produce 70–90 MW (94,000–121,000 hp) of power. It is used:
singly to power the Project 670 Скат and Чайка-Б (Charlie) submarines
in pairs to power the Project 671 Ёрш and Щука (Victor), Project 667 Мурена, Кальмар, and Дельфин (Delta), and Project 667 Навага, Налим, Груша, and Андромеда (Yankee) second-generation submarines.
The VM-5 reactor was the nuclear fission reactor used in a pair to power the Soviet Navy's Project 661 Анчар - Anchar (Papa) second-generation submarine. It was a pressurized water reactor (PWR) using enriched uranium-235 fuel to produce 177 MW (237,000 hp) of power.