Cities: Skylines

The Fusion Power Plant monument will be available to build once you've unlocked 6 unique buildings. Each unique building has requirements you must meet before they unlock. The buildings and requirements are as follows:

Plaza of the Dead - Fill 3 instances of cemetery.

Fountain of Life and Death - One full lifespan lived in the city.

Tax Office - Construct a water treatment plant, 3 instances of water pumping stations, and electricity production of 1,000 MW.

Observatory - 1,000 abandoned buildings in the city, maintain for 5 weeks

Science Center - 5,000 full lifespans lived in the city

MAM Modern Art Museum - 50% of the population highly educated

Once those buildings have been placed in your city, the Fusion Power Plant will become available. It costs $1,000,000 to build and $8,000 a week for upkeep. It needs a massive amount of water, but it well worth it as it provides 16,000 MW of power. By comparison, a nuclear power plant provides 640. This will easily provide all of your power.