RGN N° 6 Novembre-Décembre 2012 p. 56–67

Experience with dispersant application: Long-path recirculation cleanup trial at byron unit 1 during spring 2011 and online addition update

¹ Electric Power Research Institute (USA)
² Dominion Engineering Inc (USA)
³ Byron Nuclear Generating Station (USA)
⁴ STP Nuclear Operating Company (USA)
⁵ ChemStaff (USA)
⁶ NWT Corporation (USA)

Abstract

The use of dispersants in pressurized water reactors (PWRs) has been extensively qualified by EPRI over the past 14 years. Building on the demonstrated success for reducing the rate of corrosion product accumulation within recirculating steam generators (SGs), dispersant injection has more recently been qualified for application during the post-outage startup long-path recirculation cleanup period [1] and for application during SG wet layup [2]. These latter two applications are also of value for plants with once-through SGs.

Dispersant application during the long-path recirculation (LPR) cleanup process can reduce the amount of corrosion products available for transport to the SGs by more effectively flushing the condensate and feedwater systems prior to initiating flow to the SGs. The first application occurred at Byron 1 in Spring 2011 under EPRI sponsorship. Extensive qualification and preparation led to a successful outcome. Dispersant was added in two batches while the unit was transitioning from short-path to full LPR cleanup. The target maximum concentration was 800 ppb, while the largest measured dispersant concentration during the application was approximately 650 ppb. There were no difficulties in decreasing the dispersant concentration below the target threshold value prior to reaching Mode 1 entry at the conclusion of the cleaning application. The application was very successful, resulting in the following conclusions:

• Approximately 10-20 lb (5-9 kg) of additional iron mass was removed during the LPR cleanup as a result of dispersant addition.

• It appears that the added cleanup resulting from dispersant addition during the outage caused a significant decrease in the amount of iron transported to the SGs during the early part of the subsequent cycle. Specifically, on the order of 10-15 lb (5-7 kg) less iron was transported than would otherwise have been expected based on prior plant experience.

The qualification work, application details, and an assessment of the results are the first focus of this paper.

Since 2009, Exelon and then STP Nuclear Operating Company have implemented online dispersant technology at six PWR units to reduce corrosion product fouling of recirculating SGs. In most cases, dispersant addition has led to a rapid, appreciable increase in SG steam pressure and a consequent improvement in the operating thermal margin (i.e., margin above the turbine valves wide open (VWO) condition). In addition, all treated units have observed substantial increases in the SG iron removal efficiency, leading to a reduction in the SG deposit fouling rate. The second part of this paper summarizes the online experience to date at the Exelon and STP plants on the effects of dispersant on:

• blowdown iron removal efficiency,

• SG heat-transfer efficiency and

• ion exchange resin performance.