The Investigation of the Chemical Degradation of the Acetate Ion under Operating Conditions Relevant to Pressurised Water Reactors

Abstract

Zinc acetate is a commonly used additive in nuclear reactors that is added to the reactor coolant system of pressurised water reactors. Zinc is used in the removal of cobalt and nickel radioisotopes from the metal in the reactor coolant system by substituting with the cobalt inside stainless steel. This cobalt is then moved into the bulk aqueous phase, where it is then removed from solution. Zinc acetate is used due to a high solubility and low cost. The presence of zinc substituted into stainless steel is also beneficial in the prevention of primary water stress corrosion cracking in stainless steel samples. While the fate of the zinc cation at these high temperatures and pressures is well understood, the behaviour of the acetate ion is not understood at high temperatures and pressures. All up-to-date research into the irradiation of acetic acid and acetate ions has been carried out at ambient temperatures. High temperature research has only been carried out in the absence of irradiation. In addition, certain rate constants that would be beneficial to understanding the degradation mechanisms of the acetate ion are currently not known. This project was focused on the understanding of the degradation mechanisms of the acetate ion at both ambient temperature conditions and conditions that are representative of a pressurised water reactor (300 degrees Celcius, 80 bar). A combination of computational kinetic modelling and experimental data was used to simulate the reaction scheme of the degradation of the acetate ion. Reasonable fitting at low absorbed doses was able to be achieved, however a lack of relevant data on the kinetics of recombination reactions meant that very poor fitting was achieved at higher doses. The formation of the malate ion may also not occur through the recombination of a succinate radical ion and a hydroxy radical, as predicted in literature. The rate constants for the formation of the tricarballylate ion was calculated from experimental data. Using a specially designed high-pressure vessel constructed of series 316 stainless steel, high temperature gamma irradiations of sodium acetate solutions were carried out to simulate the harsh conditions inside a nuclear reactor. The radiolytic degradation of the acetate ion decreased with increased temperatures, at odds with the current literature on temperature dependencies of the reactions involved in water radiolysis chemistry and the radiolytic degradation of the acetate ion. This is attributed to a favourable back reaction of acetate ion radicals with hydrogen radicals to reform acetate ions. The effect of interfacial reactions with the stainless steel is predicted to be negligible. The knowledge of the rate of degradation of the acetate ion, and what products are formed from the degradation of the acetate ion better contributes to the understanding of the aqueous chemistry a pressurised water reactor's reactor coolant system undergoes. More importantly, the fraction of gaseous products that are formed is of importance to gauging the impact of nuclear reactors on public health and the environment. This work provides a reaction scheme from which a more comprehensive reaction scheme that better incorporates back reactions could be created.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aliaksandr Baidak (Supervisor)