Water quality control method for nuclear power plant, fuel rod, fuel assembly, as well as nuclear power plant

In a fuel assembly, surfaces at the inside of a channel box in contact with water are constituted with a high potential material layer having a higher potential than mixed cruds deposited on the surfaces of a fuel cladding tube of a fuel rod (potential difference between reactor water and the layer is great). The high potential material layer is formed on the surfaces at the inside of the channel box in contact with water by plating a high potential material such as magnetite, platinum and magnesium oxide in the form of a film layer. Electric current flows from the high potential material layer at high potential through the surfaces of the fuel cladding tube, which is short circuited with the layer, to the mixed cruds at low potential. That is, an oxidizing circumstance is provided where electrons are discharged from the mixed cruds to the fuel cladding tube. This can suppress redissolution of radioactive nuclides such as radioactivated ferrite contained in the mixed cruds into reactor water. (I.N.)