Welche Aufgaben hat das Wasser im Primärkreislauf (Kernkraftwerk)?

Just the same as in many other power plants, such as a coal power plant. It is evaporated (in the case of a nuclear power plant, the heat comes from the nuclear fission, in the coal power plant due to the combustion of coal, etc.), then relaxes in a turbine, whereby a shaft is driven. In a generator, this rotational energy is then converted into electrical energy.

Sorry, I’m just seeing you writing primary. Of course that’s not true.

https://fank.fgov.be/de/dossiers/kernkraftwerke-belgien/funktionweise-eines-kernkraftwerke

I can’t explain it better