The KLT-40S is a pressurized water reactor based on the commercial KLT-40 marine propulsion plant, and is an advanced variant of the reactor plants that power nuclear icebreakers. The construction of a small-size float­ing nuclear cogeneration plant with two KLT-40S reactors is currently under way in Russia. The KLT-40S is a modular design in which the reactor, steam generators and main circulation pumps are connected with short nozzles. It is a four-loop system including forced and natural circulation of the primary coolant, with a once-through coiled steam generator, an exter­nal gas pressurizer system and passive safety systems.