Hydrazine substitutes for use as oxygen scavengers in the secondary circuits of pressurized water reactors

Abstract

Proper chemical treatment of the secondary water is vital for the safety and performance of pressurized water reactors (PWR) and has conventionally incorporated hydrazine as an oxygen scavenger. Oxygen scavengers consume dissolved oxygen and thereby mitigate corrosion and other degradation mechanisms of structural materials found in the PWR secondary circuits. However, since hydrazine has been classified as possibly carcinogenic, the use of hydrazine within the EU may become restricted in the near future. This has stressed the need for an appropriate substitute to hydrazine.

The aim of this thesis is to evaluate the feasibility of the two hydrazine substitutes erythorbic acid and diethylhydroxylamine for use as oxygen scavengers in PWR secondary circuits. For this purpose, the hydrazine substitutes were evaluated with respect to their effectiveness to consume oxygen and their ability to protect carbon steel from corrosion in an experimental setting resembling the environment of Loviisa WWER-440 secondary circuits. The experiments relied on electrochemical measurement techniques including chronopotentiometry, linear polarization resistance and electrochemical impedance spectroscopy.

The results show that erythorbic acid consumed oxygen as quickly and effectively, and in some measurements even more efficiently than hydrazine, whereas diethylhydroxylamine reduced the oxygen content at rates of approximately one third of hydrazine. In addition, the results indicated that all three oxygen scavengers temporarily increase the corrosion rate of carbon steel to comparable extents when administered at high concentration ratios (values greater than 8).

The results suggest that erythorbic acid and diethylhydroxylamine can offer safe approaches for chemical treatment of PWR secondary waters, provided that any secondary effects of the substances can be accounted for. These secondary effects include the generation of carbon dioxide and various decomposition products that may acidify the secondary waters and increase the electrolytic conductivity.