Maintenance of good chemistry of reactor system fluids is essential for minimizing corrosion of reactor system components and generation of acti­vation products that can give rise to high radiation fields on piping and equipment, resulting in increased radiation exposure of plant personnel. Reactor coolant and moderator water chemistry is generally maintained by circulating a part of the coolant flow through ion exchange resin beds. At times neutron poisons such as boron are added to the coolant in the form of boric acid to suppress excess reactivity. In this case the resins used need to be saturated with boron to prevent unwarranted boron removal that can give rise to reactivity gain. For the same reason dilution of borated water in the reactor system by inadvertent addition of unborated water must be prevented. Conversely, boron removal to gain reactivity in a controlled manner can be done by passing the coolant through ion exchange resins that have not been saturated in boron. The moderator system is normally vented to an inert cover gas such as helium in heavy water moderated reac­tors. Build-up of deuterium can take place in the moderator cover gas due to radiolytic decomposition of heavy water. This has to be kept within pre­scribed limits to prevent the concentration reaching explosive limits. For this purpose the cover gas has to be purified by passing over a catalytic recombiner. Similarly, in light water reactors hydrogen build-up in the reactor coolant is vented to catalytic recombiners. From time to time activa­tion products that have deposited on the inner surfaces of system piping need to be removed to bring down radiation fields on piping. This is done by dilute chemical decontamination of the system, ensuring that base metal of the piping and other system components including cladding of fuel assemblies in the core are not subjected to any significant corrosion.

Chemistry of the secondary coolant of the reactor also has to be main­tained within proper limits to ensure good health of the secondary system components such as the steam generators and the steam turbine. Appropriate chemicals are added to the system and the condensate is subjected to pol­ishing by ion exchange resins before being pumped back into the feed water system. Deaeration of feed water is done to maintain dissolved oxygen content at very low values to minimize corrosion of secondary system inner surfaces.

It may be noted that chemistry control of reactor systems plays a vital role in minimizing corrosion and thereby helps in trouble-free operation of the NPP over long periods of time. It also helps in minimizing build-up of radiation fields on system piping and components thereby reducing radia­tion exposures of personnel. Proper maintenance of system chemistry is also necessary from a reactivity safety point of view. A well-trained and competent reactor chemistry group is therefore essential for safe and effi­cient long-term operation of the NPP. The technical competence of this group should be continually enhanced by in-house research as also by keeping abreast with the latest developments in this field worldwide. The chemistry group should also maintain close contact with academic and other relevant institutions in the country having expertise in specific areas such as corrosion and seek their assistance whenever necessary.