Miscellaneous Mechanical Components
The cost of corrosion and radioactive contamination caused by poor water quality
in nuclear facilities is enormous. Demineralizers are an intricate part of water
The chemical theory of demineralizers is detailed in the
Chemistry Fundamentals Handbook. This chapter will address the mechanics of
how demineralizers operate.
STATE the purpose of a demineralizer.
Purpose of Demineralizers
Dissolved impurities in power plant fluid systems generate corrosion problems and decrease
efficiency due to fouled heat transfer surfaces. Demineralization of the water is one of the most
practical and common methods available to remove these dissolved impurities.
In the plant, demineralizers (also called ion-exchangers) are used to hold ion exchange resins and
transport water through them. Ion exchangers are generally classified into two groups: single-
bed ion exchangers and mixed-bed ion exchangers.
A demineralizer is basically a cylindrical tank with connections at the top for water inlet and
resin addition, and connections at the bottom for the water outlet. The resin can usually be
changed through a connection at the bottom of the tank. The resin beads are kept in the
demineralizer by upper and lower retention elements, which are strainers with a mesh size
smaller then the resin beads. The water to be purified enters the top at a set flow rate and flows
down through the resin beads, where the flow path causes a physical filter effect as well as a
chemical ion exchange.
A single-bed demineralizer contains either cation or anion resin beads. In most cases, there are
two, single-bed ion exchangers in series; the first is a cation bed and the second is an anion bed.
Impurities in plant water are replaced with hydrogen ions in the cation bed and hydroxyl ions
in the anion bed. The hydrogen ions and the hydroxyl ions then combine to form pure water.
The Chemistry Handbook, Module 4, Principles of Water Treatment, addresses the chemistry of
demineralizers in more detail.