Nuclear Power is a method of energy generation. It uses nuclear fuel to produce heat, which is usually transformed into electricity. Nuclear power is considered the preferred energy source for most plans for medium- to long-term human expeditions to Mars.
Usage On Mars
The generation of electricity from nuclear fuel does not depend on weather conditions so would be useful for maintaining a reliable source of power on Mars. However, due to the unclear availability of radioactive resources on Mars and the difficulty in preparing the fuel (the nuclear enrichment process) any nuclear fuel will have to be brought from Earth for the foreseeable future. This would significantly prevent the settlement from being independent from Earth.
Molten Salt Energy Storage
Molten Salt Energy Storage  is a process used in Concentrated Solar Thermal  that allows storing large amounts of heat energy in the form of high temperature molten salt. This reserve can be tapped for direct use in colony heating, Evaporative water purification, and Rodriguez Wells.
Rodriguez Well (RODWELL)
RODWELLS Are a form of well melted into Antarctic Ice to provide a constant source of water for use on a base, this lowers a heat source deep into the ice melting an area of ice that is partially pumped out as the ice cave grows 
Heating greenhouses and other buildings may be done indirectly by the heat of the nuclear fission. The heat can be transported in pipes from the reactor to the buildings. Heat exchangers avoid nuclear pollution of the buildings.
Types of Nuclear Generation
Radioisotope thermoelectric generators (abbr.: RTG) are simple devices. They produce a heat difference, transformed by a thermocouple to electrical energy. The maintenance effort is low. However, RTGs do not provide enough power for a base.
Nuclear Heatpipe Reactor
Megawatt Level Heat-Pipe Reactors  offer stable, safe power that requires no outside support system or personnel and is immune to meltdown, it can be scaled from .5kw to 50mw for remote bases, small cities, and forward operating bases here on Earth, these reactors could power a Colony or a Spaceship for between 5 and 40 years with no maintenance, with no in-situ resources needed.   
- What sort of nuclear fuel is needed?
- How long can the described nuclear power stations work without replenishment of nuclear fuel?
- What is known about nuclear resources on Mars?
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- Idaho National Labs, Dr. K.P Annath, Dr. Michael Kellar, Mr. James Werner, Dr James Sterbentz Portable Special Purpose Nuclear Reactor (2 MW) for Remote Operating Bases and Microgrids , https://ndiastorage.blob.core.usgovcloudapi.net/ndia/2017/power/Ananth19349.pdf, May 2017.
- NASA Kilopower Project, Dr. David Poston Small Nuclear Reactors for Mars - 21st Annual Mars Society Convention , https://www.youtube.com/watch?v=NLE5YFuCmhw, Sep 2018.