Ing. Narciso Beyrut
Comision Federal de Electricidad
Larry Campo, Donald Gardner, Craig Knauss and Edward Taylor
Raytheon Nuclear Company
ABSTRACT
The Comision Federal de Electricidad (CFE), in keeping with good ALARA practices through source term reduction and radioactive waste minimization, has decided to implement condensate prefiltration at the Laguna Verde Nuclear Plant (LVNP), a twin unit boiling water reactor, located near the city of Vera Cruz, Mexico. Unit 1 began operation in July 1990 and Unit 2 in April, 1995.
The main objective of implementing condensate prefiltration is to reduce the iron content of the condensate at LVNP to the EPRI guideline of 2 ppb or less. Reducing the amount of iron in condensate is expected to provide the following benefits:
The review of constructability concluded that condensate prefiltration was feasible at LVNP and capital cost payback, based on projected savings, would occur in approximately 14 months after full implementation. LVNP is currently implementing this technology.
Additionally, the review concluded that non-precoat backflushable filters was the optimum media for condensate prefiltration at LVNP, and that individual prefilters for each of the Units' seven condensate polishers was the best prefiltration system configuration. By providing individual prefilters, installation could be performed on a non-outage basis, or during an outage if
desired, and regardless, would not be critical path for plant re-start. In addition, providing individual prefilter vessels in the condensate polisher cubicles allows for ALARA man-REM during installation, operation and maintenance, which is consistent with LVNP plant design.
CFE is currently implementing condensate prefiltration at LVNP Units 1 and 2. Installation of this technology is projected to be completed in December, 1996 for Unit 1 and in October, 1997 for Unit 2.
INTRODUCTION
The Laguna Verde Nuclear Plant (LVNP) is located in the area known as "Laguna Verde" which is south of the town of Palma Sola and some 70 kilometers north of Veracruz City in the state of Veracruz, Mexico. Surface access to the site is by the Tuxpan-Puerto Juarez Highway (Mex. 180), Villa Cardel-Nautla Section. An access road, approximately 2 kilometers long connects the site to the above mentioned highway at kilometer 44.5.
The Plant consists of 2 units, each unit consisting of a boiling water type reactor and a tandem compound quadruple flow turbine. The turbine-generator units each have a nominal capacity of 674,480 kW at 1800 rpm when supplied with saturated steam at 970 psia. Cooling water for the condensers is taken from the Gulf of Mexico. Unit 1 and Unit 2 began operation July, 1990 and April, 1995, respectively. The plant is owned and operated by the Comision Federal de Electricidad (CFE).
The condensate system operates in the heater drain cascade mode up to 50% load (approximately 8,300 gpm) then switches to pumped forward mode up to 100% load (approximately 16,800 gpm). Polishing of the condensate is accomplished by the use of seven deep bed demineralizers operated in parallel. Each polisher has a design maximum flow of about 2,800 gpm and uses 168 cubic feet of mixed bed resin, which is sized to accomplish ion exchange for removal of solubles and filtration for removal of suspended particulate. The plant has discontinued the use of ultrasonic resin cleaning and only uses backwashing to clean the resin beds. The condensate normally averages 25 parts per billion (ppb) insoluble iron, 5 to 6 ppb copper, and 35 ppb total suspended solids. The majority of the iron contribution was from the main condenser.
In 1994, CFE requested that Raytheon Engineers and Constructors (Raytheon) perform a feasibility study for reducing feedwater iron concentration. The feasibility study concluded that individually installed backflushable, non-precoat type condensate polisher prefilters were the best approach for reducing iron concentrations to EPRI recommendations, while: minimizing cost of equipment, installation and operation; meeting available space requirements; and minimizing plant impact during installation and operation.
A cost verses benefit evaluation was performed to ascertain whether the condensate prefilter for LVNP was viable from a financial perspective. The evaluation showed that capital payback could be achieved in approximately 14 months after installation of the prefilters.
Based upon these results, CFE decided to implement condensate prefiltration at Laguna Verde Units 1 and 2. All work activities were performed in a team approach consisting of Raytheon performing first stage program and methodology development and engineering conceptual and preliminary designs. CFE performed final program and methodology development as well as final design engineering and installation.
SYSTEM DESCRIPTION
The LVNP condensate prefiltration approach consists of plumbing a single prefilter in series with, and immediately preceding a single condensate polisher using the 10" inlet pipe. Condensate flow is from the 24" header to the 10" prefilter inlet, up through the prefilter to the 10" prefilter's outlet, to the 10" condensate polisher inlet.
The filter vessel is made of carbon steel (SA-516 GR70) and is sized for a normal flow rate of 1,680 gpm and a maximum flow rate of 2,520 gpm. The vessel has a 46 inch outer diameter, and is 11 feet tall. To accommodate filter pack removal, the lid is bolted to the vessel, and designed without pipes or attachments that would inhibit removal. The filter vessel is supplied with two 10" flanged connections for inlet and outlet piping. The filter vessel's dry empty weight is approximately 5,600 lbs. The lid weight is approximately 1,200 lbs.
The backflushable, pleated membrane prefilter elements are constructed entirely of pleated polypropylene. The elements are designed to be backflushable, compactable, and incinerable. The elements are 99.9% efficient for removal of particle sizes down to 4.0 microns absolute, and approximately 96% efficient for removal of particle sizes down to 0.5 microns after extended run time. Each filter vessel will use 148, 2.5 inch diameter, 70 inches long filter elements.
The prefilter backflush cycle uses a combination of air and untreated condensate water, which minimizes backflush waste volume. Each prefilter is estimated to generate 6,000 gallons of backflush waste to the liquid radwaste system annually. An automated backflush sequence of a prefilter is estimated to take 30 minutes to complete.
BENEFITS
The major benefits expected from reducing the quantity of iron and other insoluble crud in the condensate are:
Each of these benefits are further discussed below.
Employee ManREM Reduction via Reduced Source Terms
With the reduction of iron and other crud, less material is available for activation in the reactor. It is conservatively projected that employee ManREM dose will decrease by 25%, due to implementation of condensate prefiltration. Based on EPRI data of 400 ManREM (US BWR annual plant, average per Ref. 1), this results in a projected savings of 100 ManREM per year.
Liquid Radwaste Generation Reduction
With the addition of prefilters, the condensate polishers are not expected to be backwashed. It is estimated that each condensate polisher without prefiltration at LVNP produces 280,500 gallons of backwash water (1,963,500 gallons for all seven polishers) annually. With prefiltration, the quantity of backwash water produced by a prefilter is estimated to be 6,000 gallons (42,000 gallons for all seven prefilters) annually. This results in a projected savings of over 1.9 million gallons annually.
Solid Radwaste Generation Reduction
With the addition of condensate prefiltration, the seven condensate polishers are projected to be changed out once every three years. On average, the resins in each of the seven condensate polishers at LVNP are changed out once every year. The seven condensate polishers require 168 cubic feet of chemical equivalent cation and anion mixed resin. Approximately 1,176 (168 * 7) cubic feet of resins are expended each year at LVNP without condensate prefiltration. Approximately 392 (168'7/3) cubic feet of resins are expected to be expended each year at LVNP with condensate prefiltration. This results in a projected savings of 784 cubic feet of resin waste per year.
Intangible Operational Benefits
By implementing condensate prefiltration at LVNP, additional intangible operational benefits are expected:
These items are difficult to quantify for cost savings purposes.
In summary the above described benefits are estimated to result in annual cost savings to CFE of$1.4 million dollars.
CONSTRUCTION AND INSTALLATION BENEFITS
The Laguna Verde design approach employs the use of one individual prefilter for each condensate polisher allowing prefilter vessel installation without shutting down the entire condensate system. In addition, filtration system performance can be tailored to desired water quality and condenser flows by operating only the desired number of prefilter vessels.
CONCLUSION
The Comision Federal de Electricidad (CFE), in keeping with good ALARA practices through source term reduction and radioactive waste minimization, is implementing condensate prefiltration at the Laguna Verde Nuclear Plant Units 1 and 2 in order to lower the iron content in the condensate system. The main objective is to reduce the iron content of the condensate at Laguna Verde Nuclear Plant (LVNP) to the EPRI guideline of 2 ppb.
Based on a cost verses benefit assessment performed for the investigation, this approach is viable from a financial perspective, saves on the order of $1.4 million dollars per year, and has a capital cost pay back time of about 14 months.
REFERENCES