Experience and optimisation of ethanolamine treatment for a PWR secondary system

The secondary systems of both pressurised water reactor (PWR) units at Koeberg Nuclear Power Station (KNPS) on the West Coast of South Africa have been on ethanolamine (ETA) treatment since 1999 / 2000. Prior to conversion to ETA Chemistry the secondary regime was ammonia / hydrazine all volatile treatment (AVT) which aimed to achieve a pH_25C of 9.65 in the feedwater. ETA was selected as the preferred corrosion mitigation agent as opposed to the alternative secondary treatment technology available at the time to combat flow accelerated corrosion (FAC). The main advantage of ETA over ammonia is that ETA has a lower relative volatility and thus it favours the water phase while ammonia favours the steam phase. Plant chemistry, design and financial aspects were considered in the decision making for KNPS. Implementation of ETA at KNPS initially targeted 2 mg ETA/kg in the feedwater and 4 mg ETA/kg in the heater drains tank. It was found that under these conditions it was necessary to simultaneously inject ammonia to reach the pH specification in the feedwater. While using only pH as a limiting parameter, with ETA on manual control and ammonia on automated control, it was possible for the ETA concentration to drop below the target levels. To prevent this from occurring, during 2002 higher target concentrations for ETA were established of between 3.0 and 3.5 mg ETA/kg in the feedwater and with an upper limit value of 4 mg ETA/kg. In 2003, this upper limiting value and the target concentrations were discarded when it was decided to attain the pH specification without injecting ammonia. When dosing only ETA and hydrazine (in the absence of ammonia dosing) the pH specification was only achievable at greater than 4 mg ETA/kg. This specification was again revised in 2007 and changed to 4 mg ETA/kg as a minimum limit; with no upper limit value. Dosing was still manually controlled but now using feedwater conductivity measurement based on a correlation established with ETA concentration. In 2008, ETA dosing control was automated, which improved the ability to control the concentration in the feedwater. A significant reduction in corrosion product transport (CPT) on both KNPS units is clearly attributable to ETA implementation. This is demonstrated by feedwater iron measurements and also by the mass of material removed by steam generator sludge lancing. This paper discusses 14 years of operating experience under the ETA Chemistry Regime. The rationale that led to the evolution of the chemistry specifications is described. Information is presented on the effects of the various specification changes. ETA influence on SG chemistry, effect on demineralizers and corrosion product monitoring. Operation with ETA has exposed some undesirable effects on ion exchange resins that can lead to deterioration of secondary chemistry. These effects that were at first not fully appreciated are now better understood. At KNPS this operating experience is being used to optimise the secondary chemistry to achieve the maximum advantage of FAC and CPT mitigation while also minimising impurity intrusion. (authors)