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Geosynthetics: The solution for managing nuclear power generation water supply in an arid environment

October 1st, 2013 / By: / Containment, Feature

1.0 Introduction

The Palo Verde Nuclear Generating Station is located 55 miles west of downtown Phoenix and is the largest power producer of any kind in the United States. Its three units are capable of generating more than 4,000 megawatts of electricity. The facility provides power to New Mexico, California, Texas, and Arizona.

Palo Verde’s design, construction, and operation are based on robust levels of containment for various materials and processes in compliance with federal and state regulations. On-site containment includes not just radioactive material but more than 4.4 billion gallons of water. This amount of storage is designed to provide sufficient cooling water for longer than one year in the unlikely event of a severe accident.

Given its desert location, Palo Verde is the only nuclear plant in the world that does not sit on a large body of water. Instead, it uses treated effluent from several area municipalities, primarily Phoenix, to meet its cooling water needs. It recycles approximately 20 billion gallons of wastewater annually. The site receives approximately 70-90 million gallons per day (MGD) of wastewater and handles the responsibility of tertiary treatment on-site. The use of the wastewater preserves enough ground or surface water for hundreds of thousands of households each year.

2.0 Water containment for two processes

2.1 The water reclamation facility reservoirs
The design of the massive treatment facility that accepts the 70-90 MGD of wastewater includes two lined reservoirs. The first reservoir was constructed in 1978 and is approximately 85 acres in size, with a second reservoir, constructed in 2006, covering 45 acres. The two storage reservoirs contain approximately a 14-day supply of the water required to operate the facility during normal operations.

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2.1 The water reclamation facility reservoirs
The second use of geosynthetic lined containment on the site is to store “blowdown” from the cooling systems and to process waste. Arizona Public Service (APS) strives to maximize the amount of water that can be recycled through the energy production process. However, a certain percentage of blowdown water must be discharged to control the salinity of the water used in the power plant processes. The cooling water is able to be recirculated through the plant cooling system approximately 25 times, or until its salinity is 20 or more times the salinity of the source water, prior to its diversion to the evaporation ponds. Regulations specific to the Palo Verde site do not allow discharging of blowdown to the aquifer. Therefore, APS receives 70-90 million gallons of water per day, but operates a “zero discharge” water management plan. To fulfill the requirements, Palo Verde operates three large evaporation ponds, totaling approximately 650 acres of lined area. The 650-acre footprint utilizes multiple layers of geosynthetics as a means to ensure a BADCAT (best available demonstrated control technology) leak-free system. The reservoirs allow the natural environment to evaporate approximately 60in./yr of water across the 650 acres, which equates to about 1.1 billion gallons per year. Research is ongoing to provide more detail to the amount of water evaporated and the rate of evaporation across different months of the year.

Water containment for both purposes on the site is subject to aquifer protection requirements. The following organizations apply oversight to Palo Verde water containment processes: the U.S. Nuclear Regulatory Commission and Environmental Protection Agency, the Arizona Department of Environmental Quality, the Arizona Department of Water Resources: Dam Safety, and the Flood Control District of Maricopa County.

3.0 Initial construction of the reservoirs

The use of a geosynthetic liner material in a reservoir application is a common design/construction option for the storage of various liquids. The relatively impermeable characteristics of a geomembrane provides a cost-effective option, especially in areas with a lack of low permeability soil, superior containment, and a decreased construction schedule when compared to a compacted soil liner.

Many factors are involved when designing an impoundment with a geomembrane. Key decisions for the Palo Verde facility include:

  • exposure of the geomembrane to the environment (UV rays, wind, etc.).
  • expected lifetime of the product and system.
  • expected performance of the system (allowable leakage rates).
  • subgrade preparation and armament of sideslopes.

3.1 Wastewater reservoirs
The original design for wastewater containment at the treatment facility included the construction of an 85-acre reservoir that can store up to 788 million gallons of cooling water. This was the first containment area constructed at the facility and was put into service in 1982. The design consisted of 3:1 sideslopes and the lining materials included a rubberized asphalt base liner on the floor and Hypalon on the sideslopes.

The impoundment was single-lined and construction utilized state-of-the-art technology for that time. After more than 20 years of operation, the lining began to fail as it reached its operational end-life. The reservoir then was redesigned and reconstructed. Lessons learned from the operation of the first impoundment along with the use of BADCAT provided information that led to a complete redesign for all future impoundments to be constructed at the Palo Verde site.

The newly designed reservoir would need to be taken out of service for relining; however, the plant could not afford to stop service. Therefore, construction of a second wastewater reservoir began in 2005, which allowed the plant to operate while the 85-acre reservoir was under construction. Most importantly, APS not only wanted to be compliant in its new design, it wanted to set the bar for all future water containment. As with any innovative concept, the design of the next impoundment would incorporate lessons learned from the initial construction to minimize similar issues going forward. Major revisions included flattening the side slopes to 4:1 to reduce the forces of wave action.

Further, the subgrade on the sideslopes was enhanced with soil cement as an additional armament against wave action. This was an innovative approach at the time and the project was recognized by the soil cement industry. This also would become the site’s first double-lined system with an extensive LCRS (leak collection recovery system), leading to the design of the future evaporation ponds. The redesign and reconstruction of the evaporation ponds is detailed in section 4.0 of this article.

3.2 Evaporation reservoirs for storage of blowdown water
The initial site design included two large lined ponds for evaporation of the blowdown water. The first pond constructed for evaporation purposes was the 250-acre pond termed Evap Pond 1. Initial construction was completed in 1986.

The reservoir now is in its third generation, in that the first liner system reached the end of its design life and was replaced in 1991, and the third generation was under construction in 2012. Evap Pond 1 initially was constructed just after the 85-acre reservoir and utilized a similar design of 3:1 sideslopes and an asphalt base liner on the floor and Hypalon on the sideslopes.

Evap Pond 2 was the next reservoir constructed. It utilized high-density polyethylene (HDPE) geomembranes, which were gaining more acceptance within the industry. Construction was completed in 1989. The surface area is approximately 220 acres and again was constructed with 3:1 sideslopes, but used a more traditional liner profile, as recognized today.

The liner system consisted of a geotextile cushion installed directly over the subgrade and a 60-mil black HDPE geomembrane was placed over the textile. The switch to an HDPE liner produced satisfactory results for the site; however, operational concerns arose regarding the use of 3:1 slopes and the lack of a detection system associated with a single-lined system.

The pond’s large surface area allowed desert winds to produce wave action across the pond, thus colliding into the sideslopes and eroding subgrade soil from beneath the geomembrane. Evap Pond 2 has since been redesigned and reconstructed with the alternative design addressing these operational issues. The details of the design, construction, and operation are discussed in section 4.0 of this article.

The initial wastewater treatment (WWT) reservoirs and evaporation ponds are still in service today, but new materials have been installed using a revised design that incorporates a sophisticated leak detection zone in a double-lined profile. Given the size and depth of the ponds, the minimum standard for leakage between the primary and secondary membrane layers is low by industry standards.

To monitor this, a state-of-the-art leak detection system was designed that includes remote level sensors and video monitoring of the sumps. Additionally, the site includes the 45-acre wastewater storage reservoir and the 180-acre Evap Pond 3, which were both constructed to provide operational water storage capacity during reconstruction of the existing ponds.

4.0 Phase II redesign and reconstruction of reservoirs and evap ponds

The first pond that required redesign and reconstruction was Evap Pond 1 and the work was completed in 1991. This pond consisted of 3:1 sideslopes, rubberized-asphalt liner on the floor, and Hypalon on the sideslopes. The subgrade beneath the Hypalon material had eroded in areas due to the wave action across the pond’s 250-acre surface area.

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The redesign/reconstruction sought to stabilize the sideslopes and a BADCAT evaluation also provided a revised geomembrane material choice for the pond. To stabilize the sideslopes, Palo Verde required soil cement armoring on the internal sideslopes of the pond. To accomplish this, the Hypalon material was removed from the slopes and the asphalt floor liner remained. The soil cement layer then was constructed on the side slopes, followed by a geotextile cushion layer and a 60-mil HDPE black geomembrane throughout the entire pond.

This was the first project that included soil cement and HDPE geomembrane as the primary components for containment in the Palo Verde site. All additional construction and reconstruction of reservoirs and ponds now include soil cement armament and an HDPE geomembrane. The next project implemented by Palo Verde included the addition of a 45-acre reservoir for the water reclamation facility.

The existing 85-acre reservoir had been in service since 1982 and was beginning to reach the end of its design/service life. Closing the water reclamation facility (WRF) during reconstruction of the 85-acre reservoir and ultimately ending the supply of cooling water to Palo Verde was not an option, so a second reservoir was constructed to allow operation of all facilities during construction and to add additional capacity when both reservoirs are operating.

The 45-acre reservoir was completed in 2006 and included a design of advanced geosynthetic products and a robust LCRS and monitoring infrastructure. Products included a geosynthetic clay liner (GCL), conductive white-surfaced geomembrane, primary and secondary geomembrane layers, and LCRS. Major components of this next generation design for the Palo Verde site have been included in all subsequent projects.

The 45-acre reservoir located at the WRF was the first “new-excavation construction” project since Evap Pond 2, which was initially constructed in 1989. A major component of this new construction included the use of 4:1 sideslopes, in contrast to the 3:1 design that had been used on all other projects. The 4:1 slope design, which included the soil cement requirement from Pond 1’s reconstruction, was used to limit wave action on the surface of the reservoir.

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The profile of the liner system from top to bottom now included:

  • 60-mil, white-surfaced, conductive, HDPE geomembrane (primary).
  • 200-mil HDPE geonet drainage product.
  • 60-mil, white-surfaced, conductive, HDPE geomembrane (secondary).
  • nonwoven geotextile cushion layer.
  • soil cement subgrade.

The addition of a second layer of geomembrane produced a leak detection zone or leak collection and recovery system (LCRS) for the reservoir. The dual-lined or primary and secondary geomembrane layers can be compared with the design and operation of a double-hull ocean vessel, when containment of liquids is discussed. The outer hull or primary geomembrane layer is the initial containment barrier and is subjected to large head pressures, especially in the case of the ponds’ approximate 30-ft depth. The head pressure may expand any imperfections in the primary geomembrane, thus providing a pathway for leakage into the detection zone or space between the two layers.

Detection zone piping then is utilized to pump out the liquid and redistribute it into the evaporation pond. Removing the liquid from the detection zone also removes or eliminates head pressure that could build on the secondary geomembrane. The absence of pressure on the secondary geomembrane increases the overall performance of a liquid impoundment. The incorporation of conductive geomembrane provides for the use of spark testing of the installed geomembrane layers. This test adds an additional layer of quality control to ensure the best possible installation and performance of the system.

Palo Verde increased the performance of the detection zone by adding two additional features into the 45-acre reservoir design. The first enhancement required intricate grading of the subgrade elevations. The design used varying elevations that would separate any flow of liquid within the detection zone to separate monitoring areas, or sumps. This created subsections in the 45-acre floor plan, allowing operations personnel to determine which area of the primary liner may be leaking in the event that “action level flow values” are exceeded.

Pipe trenches were constructed into the subgrade (above the secondary geomembrane) to collect and transport any leakage to the detection sumps. The detection sumps are equipped with visual operation hatches, flow meters, and video monitoring equipment focused on the detection pipe outlets.

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With completion and operation of the 45-acre WRF reservoir, reconstruction of the 85-acre reservoir could begin. Reconstruction was completed in 2007. All design revisions included in the 45-acre reservoir were also implemented into the 85-acre reconstruction project. Although the existing sideslopes of the reservoir were 3:1, the reconstruction included removal of the existing Hypalon geomembrane and excavating the slope back to a 4:1, then installing the soil cement armament required on all pond sideslopes.

With both WRF reservoirs reconstructed and operational, focus was shifted to reconstruction of the evaporation ponds. A new construction/excavation project was necessary to address the eventual reconstruction of Evap Ponds 1 and 2. Evap Pond 2 had been in service for more than 20 years by this time and was reaching its operational end-life. It was not possible to take Evap Pond 2 out of service and continue to have adequate volume for storage of blowdown, so plans were made to design and build Evap Pond 3, which would then be two separate ponds (A and B). Each would create 90 acres of surface area.

Evap Ponds 3A and 3B were completed in 2009 and had designs similar to the 45-acre WRF reservoir, with two significant exceptions:

  • A GCL product was used in place of the geotextile cushion layer that was installed above the soil cement and below the secondary 60-mil HDPE geomembrane. The GCL material layer added increased performance in terms of overall containment of the geosynthetic system.
  • A video monitoring system was installed within the detection sumps to observe the detection system piping outlets. The amount of flow within the detection system is recorded via flow meters. However, the addition of the cameras increased monitoring efforts to a 24-hour “real time” situation.

With the completion of Evap Pond 3 in 2009, the site began to focus on the reconstruction of Evap Ponds 1 and 2, as the geosynthetic materials in those ponds were reaching the end of their respective design lives.

After 22 successful years of service, Evap Pond 2 reconstruction was completed in 2011. The single-lined containment system of a geotextile cushion layer and one layer of 60-mil HDPE geomembrane was completely removed from the pond. Subgrade contours were regraded to create isolated areas of the LCRS on the floor of the pond. This allows for more rapid identification of any areas that showed signs of leakage.

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The existing 3:1 sideslopes were reconstructed to 4:1, as had become the standard, and the soil cement armament was installed. Evap Pond 2 used the same geosynthetic profile from Evap Pond 3 (GCL, 60-mil conductive white HDPE, geonet drainage, 60-mil conductive white HDPE).

One additional design revision was incorporated into Evap Pond 2 construction, however. The berm used to separate Evap Ponds 3A and 3B produced another deterrent to wave action across the full surface area of the pond by splitting the surface area in half, from 180 acres to 90 acres, for each area. For this reason, two berms were created within the 220 acres of surface area in Evap Pond 2, creating Evap Ponds 2A, 2B, and 2C.

Reconstruction and the third generation of Evap Pond 1 was under construction in 2012. Evap Pond 1A was completed in 2013, completing the redesign and reconstruction of all lined reservoirs and ponds at the Palo Verde site. Evap Pond 1 “third generation” used the same design as the reconstruction of Evap Pond 2.

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Accountable installation is required, expected, and specified at the APS facility. An installation for a typical APS project consists of 30–40 employees. The labor components of the crew include a production supervisor and additional “nonworking” supervisors for each individual component of the work. For example, a nonworking supervisor is required to oversee the unloading of geosynthetic materials from delivery trucks. An additional superintendent is required during the deployment of geosynthetic materials. A full-time certified safety officer is provided by the installer to oversee and manage all safety requirements at the site. Six to 10 operators are required to operate the flatbed trucks, forklifts, skid-steer loaders, and water truck.

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Another step in providing the best levels of liquid containment is the APS requirement that all welding personnel are certified welding technicians (CWTs) recognized by the International Association of Geosynthetic Installers (IAGI).

In total, more than 3,120 acres of geosynthetic materials have been installed and are currently used in operation of the Palo Verde facility. The acreage converts to approximately 136,000,000 square feet of material, or 2,360 U.S. football fields. For reference, if all of the material was cut into a 12in.-wide strip, that strip would reach around the 24,900 miles of the earth’s equator.

Innovative state-of-the-art geosynthetic products and safe, accountable, highest-quality installation played major roles in the containment of Palo Verde’s water resources in this arid environment

Chris Eichelberger, director of business development, American Environmental Group Ltd. (AEGL)

Gerald Hersh, project manager, American Environmental Group Ltd. (AEGL)

Shabbir Pittawala, special projects manager, Arizona Public Service (APS)

Acknowledgement
Content for this article was supported by Walter Steinbeck, GSE Environmental’s western territories regional sales manager. We thank Walt for his contributions and relentless service throughout the entire life span of the geosynthetics work on this project.

Reference

Wigginton, C., Annala, M., Mitchell, J., Fongemie, R., and Pittalwala, S. (2008). “Soil-Cement Plays Key Role in Protecting Large Water Impoundments at Nuclear Generation Station,” Portland Cement Association, Skokie, Ill.

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