Entrainment, impingement and BTA evaluation for an intake located on a cooling water reservoir in the southwest

Abstract

The Comanche Peak Steam Electric Station (CPSES) is a two-unit, combined 2300 MWe nuclear power station located about 65 miles southwest of Dallas. Unit 1 was approved for commercial operation in August 1990; Unit 2 in August 1993. The NRC’s 1981 Final Environmental Statement discussed entrainment and impingement, expressing some concern over intake approach velocities which were estimated to exceed 0.5 ft/s; the final NPDES Permit required a 316(b) Demonstration once Unit 2 went into commercial operation (which turned out to be autumn 1993). CPSES is a once-through power station that withdraws its cooling water from Squaw Creek Reservoir (SCR), which was built by the company to provide a reliable source of condenser cooling water for a nuclear power plant. A secondary benefit has been the development of a nice sport fishing resource in an area starved for such water bodies. SCR was created by damming Squaw Creek and several minor tributaries to create a 3272 acre impoundment. It was completed in February 1977, and filled in May 1977. The Texas Parks and Wildlife Department has done initial and supplementary stockings of forage and game fish since then, including threadfin shad, largemouth bass, smallmouth bass, white bass, hybrid striped X white bass, and channel catfish. SCR has an average depth of 46 ft; a maximum depth of 135 ft; and a volume of 150,000 acre-ft. It has 70 miles of shoreline and a “Shoreline Development Index” (SDI) of five (a SDI of one would indicate a round configuration, whereas SCR has numerous natural coves formed by flooded tributary mouths and other natural indentations in the original valley). Units 1 and 2 share a common intake structure located flush on the shore of an excavated recess of SCR. This recess is 50 ft deep at the trash racks. Its bottom is exposed bedrock and sand, and its shoreline was cleared and rip-rapped to prevent slumping of riparian vegetation and soils into the intake area (something more important to licensing nuclear power plants than fossil ones). CPSES has a standard shoreline intake with eight circulating cooling water pumps rated at 275,000 gpm each; twelve traveling screens with 12″ mesh that are rotated every 4 h or when pressure differentials dictate more frequent rotation; a common debris trough; and no fish return system. The CPSES intake daily withdraws and recirculates about 6.4% of SCR’s volume at full operating capacity. An entrainment, impingement and waterbody monitoring program was performed according to NPDES specifications. In contrast to what many pundits may have predicted, total annual impingement (including threadfin shad) is very low for a plant of this size, and entrainment is dominated by forage species with high fecundity and pelagic spawning habits. Impingement and entrainment of gamefish is extremely low, and fishing for basses and catfish remains good. This paper describes how the design specifications developed to ensure reliability and safety also helped minimize adverse environmental impact by locating the intake in a zone of “low biological value” relative to alternative areas. There are lessons to be learned as the EPA seeks to define new measures of adverse impact, such as proposed use of the “Rapid Bioassessment Protocols” in the Tier 2 screening stage of the Best Technology Available (BTA) process. For example, creating an intake area less favorable to species diversity, and therefore inviting fewer species and individuals to the intake area, may have set the stage for the CPSES intake to be subjected to increased scrutiny rather than getting credit for the multi-disciplinary BTA which it does, in-fact, represent! This, and other features of reservoirs constructed for cooling water purposes, argues in favor of site-specificity and possibly distinguishing such reservoirs from natural lakes in other parts of the country.