Water Chemistry and Corrosion in PWRs

Whatever its advantages, nuclear power generation has to remain competitive with other energy sources by guaranteeing a high level of safety, low dose rates to which workers and the general public may be exposed, an acceptable environmental impact (discharges/effluents) and a low cost price. The corrosion resistance of materials is one of the key factors in these two challenges. The first is to ensure the integrity of structural materials throughout the life of the reactor, limiting degradation and the consequences of ageing under the effect of temperature, radiation and the chemical environment. The safety of pressurized water reactors (PWRs) is based on the principle of a triple barrier around the nuclear fuel: the cladding, the primary circuit wall and the containment vessel. Production costs, meanwhile, are also directly dependent on the good performance of materials, enabling maximum availability and lower maintenance costs associated with in-service checks and interventions to repair any damage.

In its standardized definition (NF EN ISO 8044), corrosion refers to the processes of physico-chemical interaction between a metal and its environment, leading to degradation of the function of the metal, the surrounding environment or the technical system of which they are a part. In other words, corrosion resistance is not an intrinsic property of a material, but essentially depends on the surrounding environment. Controlling corrosion problems therefore requires both a judicious choice of materials and rigorous control of the chemical composition of the environment. This is why we have combined corrosion and water chemistry in a single presentation.

After providing essential basic information on the physico-chemical phenomena involved in the circuits of a nuclear power plant, this article describes the issues involved in the chemistry and corrosion of the main circuits (mainly primary and secondary). The various types of degradation, depending on design and materials, and the associated remedies are described. These include stress cracking of stainless alloys, and erosion corrosion of carbon steels. The behavior of nuclear fuel and the risks of flux anomalies are described. Means of controlling dosimetry and certain environmental releases are also explained. The most important chemical specifications for reliable plant operation are indicated.

Readers will find it useful to consult the specialized articles in the Metallurgy section of this publication, as well as the articles describing pressurized water reactor technology and the "Nuclear Reactor Structure" section in the Nuclear Engineering section.