What is the primary source of supply for the MFP warm-up line?

The primary source of supply for the main feed pump (MFP) warm-up line is indeed the condensate pump discharge. This warm-up line serves to ensure that the main feed pumps can properly operate and reach their designated temperatures before they are engaged in pumping duties.

Using condensate water, which is relatively hot and comes from the steam cycle, helps to prevent thermal shock to the pump components. This practice not only protects the equipment but also ensures efficient operation by preheating the system to reduce wear and tear. The condensate from the pump discharge is already treated and consistent with the quality required for the internal components of the main feedwater pumps.

The other options, such as demin water, cooling water, and auxiliary feedwater, do not serve the same purpose as the warm-up source. Demin water is used for other applications where ultra-pure water is necessary; cooling water typically circulates for temperature control in various systems; and auxiliary feedwater functions in a different capacity to provide backup water supply to the steam generator or boiler, particularly if the primary feedwater supply is compromised. Thus, condensate pump discharge is the most optimal source for the MFP warm-up line, making it the correct answer.