The Browns Ferry Nuclear Station (BFNS) intends to implement an extended power uprate (EPU) at all three units beginning in 2018 for Unit 3 and Unit 1, and in 2019 for Unit 2. EPU implementation will increase the total thermal power of each unit by 494 MWth resulting in a total uprate of 20% from the originally licensed thermal power of 3293 MWth.
Each BFNS unit is currently designed with ten bottom tubesheet condensate filter/demineralizers (CF/Ds) in the condensate treatment system that require an application of a powdered resin precoat to perform the function of demineralization. The precoat material is applied as an overlay on top of vertical filter septa. The filter septa have an inner pleated area, and with a precoat overlay, perform the function of demineralization as well as particulate iron removal. In the absence of circulating water leakage into the condenser, the primary function of the CF/Ds is to remove particulate iron that collects in the condenser hotwell. The iron source is from the corrosion of carbon steel piping and components in contact with main steam and heater drain systems.
Each of the 30 CF/D vessels at the three-unit BFNS site must be periodically backwashed and a new precoat applied. When there is minimal condenser leakage, the backwash frequency is a function of the differential pressure (dP) rise rate across the filter septa, with the dP rise rate a function of filter flow rate and particulate iron concentration. Plants typically will initiate a filter backwash based on reaching an administrative run length limit (operational days). Experience has shown operating well beyond a preestablished run length limit in the long-term can shorten septa life leading to costly septa replacements (typically there are over 300 septa per filter vessel). Prior to EPU, the 30 CF/D vessels are backwashed at a rate of about 0.81 backwashes/day, equating to about 296 backwashes/year.
Under rated EPU conditions, main steam, condensate and feedwater system flows will increase by about 14.3%. Experience has also shown under EPU conditions, the iron concentration in the condenser hotwell will likely increase due to an increase in carbon steel corrosion rates from higher steam and heater drain system flows. The combination of higher flow rates and higher iron levels will result in an increase in the dP rise rate across each filter leading to more frequent filter backwashes and precoats.
In the fall of 2015, Structural Integrity (SI) Chemistry and Materials was contracted by BFNS to perform an assessment on the impact of EPU implementation on CF/D system operation and reactor coolant chemistry. SI has performed similar assessments in the past for other Boiling Water Reactors (BWRs) and has noted that while the Nuclear Steam Supply System (NSSS) vendor and/or the contracted design organization assesses the design capability of each plant system under EPU conditions, they do not assess the impact of EPU on achieving station chemistry goals and industry chemistry performance indicators as well as the impact of EPU operation on system waste generation related to maintaining excellent chemistry control.
Using CF/D operational data from each unit, SI modeled CF/D performance under EPU conditions for higher flow rates and postulated increased iron concentrations. The projection predicted the current 3.0 year – 4.5 year CF/D septa life would decrease at all three units by between 40% and 50% if there was a hotwell iron concentration increase of 25% (typical for EPU). Maintaining the current operating strategy and backwash frequency would increase the total operating cost by about $1.1 million annually, since there would be about eight additional vessels with septa replacements per year. SI then projected the added costs for doubling the amount of backwash and precoats that are performed to reduce the number of annual septa replacements with the 25% increase in hotwell iron concentration. The calculated increase in annual operating costs for this case was about $1.2 million. Although the number of additional septa replacements were projected to be reduced from eight to three, the net cost increase is higher due to the increase costs associated with the addition of new precoat material and more significantly, the increase in precoat material radioactive waste disposal costs.
When each CF/D is backwashed, the backwash waste stream of over 5000 gallons is collected in a Backwash Receiving Tank (BWRT), with the contents of the BWRT being subsequently transferred to the Radwaste Plant for further processing in the Condensate Phase Separators (CPS). In the past at BFNS, processing of CF/D backwash waste in the CPS has caused bottlenecks that have led to delays in startup and power ascension and issues with water storage in the Radwaste Plant. Doubling the amount of CF/D backwashes after EPU would only further challenge the liquid radwaste processing system.
SI’s proposed solution to this significant challenge from EPU implementation was the installation of integrated flow distributors (IFDs) in each of the CF/Ds. IFD technology, which was previously presented in Volume 40 of News and Views (2016), are devices that improve the internal flow distribution in bottom tubesheet F/Ds. The improvement in flow distribution in a vessel with an IFD installed compared to the conventional F/D without an IFD is portrayed in Figure 1.
The components of an IFD include a modified lower baffle plate, a flow distribution tube, and an upper series of perforated plates as shown in Figure 2.
IFDs distribute flow within bottom tubesheet F/Ds more evenly, promoting flow uniformity along the entire septum length, resulting in more uniform precoats, improved resin utilization, and minimal precoat erosion. The improvement in precoat uniformity is evident in Figure 3 which shows the precoat condition before and after IFD installation at another BWR.
IFD designs are performed by SI’s partner, the Organo Corporation, from Japan. The designs for each type of bottom tubesheet vessel are confirmed via computational fluid dynamics modeling. IFD installation does not require any welding inside the F/D vessel.
SI projected with IFD installation at BFNS and under EPU operation, the projected annual cost increase of $1.1-$1.2 million noted above would essentially be negated, as the projected annual savings was between $990,000-$1.05 million. The projected payback with installation of IFDs in all 30 CF/Ds was about 2.5 years.
BFNS initiated the purchase of IFDs in 2017. IFDs require the removal of some filter septa, so installation was coordinated with septa replacement. The first IFD was installed in the 1B CF/D vessel; the second IFD was installed in the 3A CF/D vessel. The dP trends for the first precoat run for these two vessels are shown in Figure 4 and Figure 5. The dP trends for the first run after the previous time new septa were installed in each of these two vessels (2012 and 2013) are also shown in each plot. Lower dP values with IFDs installed are clearly evident in the plots. The previous septa replacement for vessel 1B conducted in 2012 had a longer run length for the first run; the station has since adjusted operating practices and limits run lengths much shorter in order to extend septa life.
The ranges and averages for calculated dP rise rates from the first three runs with IFDs compared to the first three runs following the 2012 and 2013 septa replacements are shown in Figure 6 and Figure 7. The dP rise rates are about a factor of two lower for the first three runs with the IFDs compared to historical data for the same filter vessel. The data show the IFD installations have allowed the CF/D vessels at Browns Ferry to be operated with lower differential pressures which will further extend septa life.
BFNS also has bottom tube sheet F/Ds in the Reactor Water Clean-up (RWCU) system. For RWCU applications, precoat runs are terminated based on ion exchange performance, not dP. Plants typically will use conductivity, silica, and/or isotopic Cobalt-60 for terminating precoat runs. As part of the 2015 assessment project, SI concluded that IFD installation in the RWCU F/Ds at BFNS would result in improved chemistry performance and longer run lengths. The station will be installing IFDs along with new septa in all six RWCU F/Ds starting in the spring of 2018. IFDs have been installed in bottom tubesheet F/Ds at five other BWR sites, including four sites with multiple units. RWCU F/D performance data with an IFD from one of these sites was previously presented in Volume 40 of News and Views (2016).