What is the primary purpose of demineralizer beds in a PWR water system?

• A. Remove chloride and sulfate from water
• B. Remove mineral impurities from feedwater by ion exchange
• C. Remove dissolved oxygen from feedwater
• D. Filter out particulates

Answer: (B)

Demineralizer beds are used to produce very pure water by removing dissolved minerals through ion exchange. Water from the feed system passes through a cation exchanger, which swaps metal ions like calcium and magnesium for hydrogen ions, and an anion exchanger, which swaps anions such as chloride, sulfate, and bicarbonate for hydroxide ions. When these two beds work together (often in a mixed bed), the hydrogen and hydroxide combine to form water, leaving the water with extremely low ionic content. This high-purity water is essential in a PWR to minimize corrosion, scaling, and the buildup of activation products in the steam generators and primary/secondary circuits. The beds are regenerated periodically to restore their exchange capacity.