Hydraulic modeling of thermal outfall diffusers for the San Onofre nuclear power plant

Abstract

Various hydraulic model tests were performed in connection with the design and performance of the offshore thermal outfalls for the San Onofre Nuclear Generating Station (jointly owned by the Southern California Edison Company and the San Diego Gas and Electric Company)near San Clemente, California. These include model investigations of: (i) the multiple port discharges for the proposed Units 2 and 3, (ii) the existing submerged single outlet from Unit 1, (iii) the discharge of heated water from the Units 2 and 3 intakes during heat treatment, and (iv) hydraulic characteristics of discharge ports. On the basis of these investigations, the diffusion structure for each of the proposed new units was designed to be 2500 feet long aligned perpendicular to shore. The diffuser for Unit 3 will extend from 3500 to 6000 feet from shore, and that for Unit 2 from 6000 to 8500 feet from shore. Each diffuser will have 63 discharge nozzles aimed offshore (±25° from the pipe axis, 20° above horizontal). The momentum of the discharge produces and offshore drift of the diluted warm-water plume. The maximum temperature rise on the surface caused by the discharge was found to decrease with increasing longshore current speed, and the discharge momentum from the jets was effective in preventing significant re-entrainment in the event of reversing or low currents. Although the California thermal discharge requirements specify a maximum surface temperature increase of 4°F (beyond 1000 feet from the discharge structure), the laboratory target maximum was established at 2.5°F, to account for possible model-prototype differences and unmodeled effects. The hydraulic model studies showed that the proposed outfall design meets the laboratory target value for a variety of possible longshore current conditions; therefore, it is predicted that the prototype outfall operation will meet the California thermal discharge requirements.

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