STATE OF THE ART IN NUCLEAR PLANT CYCLE CHEMISTRY

The basic principles of water chemistry will be described for PWRs and for BWRs. The PWR discussion includes the systems with U-tube steam generators (PWRs), horizontal type steam generators (VVERs) and Pressurized Heavy Water Reactors (PHWRs). For this type of plant the principles for operation of the primary and secondary system will be discussed. The primary side of PWRs is operated under alkaline (pH control agent is either LiOH or KOH) and reducing conditions (H2-injection or ammonia dosing). This basic principle is now valid for more than 20 years and it has only been modified within the established framework of specified values. Such modifications are called coordinated, modified, or elevated Li/B-chemistry. On the secondary side of PWRs, alkaline and oxygen-free conditions are normally applied to control the system integrity. To adjust the alkaline conditions, volatile agents like ammonia, morpholine, and ethanolamine are used whereas the systems are kept oxygen-free by applying hydrazine or other oxygen scavengers. In BWRs, the fundamental principle is to operate under neutral and oxygenated conditions while keeping the cycle as clean as possible. However, based on material integrity problems, the chemistry conditions have been changed in some stations to neutral and reducing conditions (hydrogen injection). Recently, the injection of trace elements like zinc has been introduced as a countermeasure against radiation field build-up or against Intergranular Stress Corrosion Cracking (IGSCC) problems. Current areas of interest are still the development of countermeasures against radiation fields both in PWR and BWR systems. In addition, primary and secondary side stress corrosion cracking problems in PWR systems are of great concern to operating utilities. For the BWR system the development of proper countermeasures against IGSCC is of great importance. Further developments in water chemistry may include, for example, trace element injection in PWR systems to mitigate radiation build-up, primary water stress corrosion cracking, and stress corrosion cracking of steam generator (SG) tubes. In BWR's proper countermeasures against Irradiation Assisted Stress Corrosion Cracking (IASCC) may be needed.