Solute concentration decision method and water quality management method for steam turbine plant

A solute concentration decision method for deciding a planned value CI of a concentration of a solute in a solution to be supplied to a first drum among one or more steam drums for temporarily containing steam generated in a boiler of a steam turbine plant, the method comprising:

• a step of setting a target concentration of the solute in the solution of the first drum;
• a step of obtaining a capacity coefficient of a drum unit including the first drum and an evaporator for generating steam contained in the first drum, on the basis of a specification of the steam turbine plant;
• and
• a step of deciding the planned value CI of the concentration of the solute in the solution to be supplied to the first drum, on the basis of the target concentration and the capacity coefficient.