Towards an Improved Understanding of PWR Coolant Chemistry

Abstract

The corrosion of plant structural material within Pressurized Water Reactor (PWR) primary circuits can lead to the release of ferrite-based particulates (e.g. MFe2O4, M = Ni, Fe, Zn etc.) and dissolved ions (e.g. Co) colloquially known as CRUD (Corrosion Related Unidentified Deposits). Such species may become radioactive when subjected to a high neutron flux in the core leading to the formation of radioisotopes, of which 58Co and 60Co are the most common. The isotopes may then be subsequently adsorbed onto CRUD particles and deposit on plant surfaces generating out-of-core radiation fields, or be safely removed by ion exchange resins in the coolant purification system. This thesis aimed to provide improved understandings of the interactions of cobalt with regards to ZnFe2O4 and NiFe2O4 CRUD, and cationic and anionic resins used in the purification systems. The interactions of CRUD particles with dissolved cobalt can be interpreted as surface complexes and quantified in thermodynamic surface complexation models, which are founded on acid-base properties and the variable surface charge of CRUD. In this work, the acid-base chemistry of commercially available synthetic CRUD particles and their interactions with cobalt were studied by potentiometric titrations, zetametry, and batch sorption techniques under ambient conditions, with a view to parameterising a surface complexation model. The experimental results suggested a combination of slow kinetics and a variety of unavoidable side reactions affecting particle surfaces are inherent under ambient conditions, and modelling of this data would not be representative of potential sorption reactions in a PWR. Additionally, several synthetic procedures for the production of CRUD particles were also examined and discussed for potential use in further work. Work within this thesis also successfully quantified the kinetics of cobalt removal by cationic and anionic ion exchange resins (IRN77 and IRN78) present in PWRs, taking into account the variable solution chemistry over different pH and temperature conditions which can be encountered. Column experiments were performed on a purpose-built temperature- and flow-controlled test rig, passing solutions typical of PWR coolant through packed ion exchange beds, analysing initial and effluent solution concentrations. Mass transfer coefficients were derived from the results which can be used to assess the efficiency of purification, and can be factored into predictive radioactivity transport models for PWR primary coolant. Possible reaction mechanisms were interpreted with reference to speciation modelling using PHREEQC code.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Fabio Scenini (Supervisor), Francis Livens (Supervisor), Scott Heath (Supervisor) & Gareth Law (Supervisor)