The country with the largest nuclear power plant in terms of net electrical output is Japan. The Kashiwazaki-Kariwa Nuclear Power Plant, located in the Niigata Prefecture of Japan, is currently the world's largest nuclear power plant. It has a total net electrical output of about 7,965 megawatts (MW) from its seven reactors.
However, it's important to note that other countries have large nuclear power capacities spread across multiple plants. The United States, for example, has the largest number of nuclear reactors and the highest total nuclear power capacity in the world.