When it is operating at full power, San Onofre, a 2x1120MWe PWR on the south California coast, draws in 1.6 million gallons of water per minute via two 16ft-wide intake pipes. At the point of intake, the flow rate is reduced by attaching a 45ft wide concrete velocity cap with 7 foot vertical openings to the intake pipe. Although this reduces the water flow and allows some fish to swim away, every year around 100 tons of fish per year enter the intake system, of which 20 tons per year are lost to the intake screens and rakes. It is a common experience in power plants, but San Onofre’s solution is far from common.
Once inside the intake forebay, fish are encouraged to move to a quiet area of the intake where the plant’s unique ‘fish elevator’ is situated. Several times a day the fish elevator, which is a stainless steel basket around 15ft long and 5ft wide, is raised via a chain-driven system and is pitched forward, sluicing its contents into a closed pipe which carries them 3000ft offshore and discharges them. This system is said to save 80% of the fish sucked into the plant.
Various stimuli are used to encourage fish to move towards the elevator, most importantly during the period of the plant’s regular ‘fish chases’.
THE FISH CHASE PROCEDURE
The water system at San Onofre, like other coastal generating stations in California, is subject to regular heat treatments to control the growth of fouling organisms, such as mussels and barnacles, that grow on the walls of the circulating water system. Many fish accumulate in the water system between heat treatments, often residing in habitat provided by gate slots and other structures within the system. Without the “fish chase” procedure, all the fish in the circulating water system at the time of the heat treatment would be killed.
The fish chase is a procedure unique to San Onofre which was developed to get as many live fish out of the system as possible before beginning the heat treatment. This is accomplished by slowly manipulating cross-over gates in the vicinity of the screenwell, where most of the fish reside. This operation re-circulates effluent water so that the water is slowly warmed. The gate manipulations also create eddy currents that will dislodge fish that have congregated in areas of
low flow.
The elevated temperatures and new flow patterns are intended to agitate fish enough that they will seek new habitat and will find their way into the fish return elevator for release back to the ocean. The fish chase is monitored by a biologist to assure that the fish are not overly stressed by the procedure. The fish chase procedure saves over 4t of fish per year that would otherwise be killed.
LIGHT AND SOUND
As well as disturbing water flow patterns, and changing the temperature of the water, other management techniques including light and sound patterns were assessed at San Onofre.
Lights were not effective in guiding and saving fish. Laboratory studies had demonstrated that most fish species were attracted to light and could be guided into certain areas of the test tanks by proper placing of lights. But when they were installed, the lights actually decreased the ability of the existing fish protection system to return fish to the ocean. The aim of the lights was to guide fish away from the trash screens and into the fish return elevator. Instead, the lights appeared to have slowed the progress of the fish through the system and resulted in a higher incidence of impingement on the screens.
Mercury lights placed above the fish return elevators did appear to improve survival by providing light under which the fish could school. Without the light, it is believed that the fish were less able to school and had higher contact with the concrete walls. The mercury lights, however, were not effective in guiding fish into the fish return elevator.
High-frequency (120kHz) sound has in some studies been shown to effectively and repeatedly repel members of the Genus Alosa (American shad, alewife and blueback herring) at sites throughout the USA, but other studies that have been carried out have not shown sound to be consistently effective in repelling other species.
Low-frequency sound (less than 50Hz) has had inconsistent results in studies throughout the country.
Behavioral barrier studies at San Onofre found that low frequency sound elicited an avoidance response from some species, but it was not possible to install the sound devices. They could not be installed on the intakes because of technological limitations of the sonic devices and concern about adverse environmental impacts, especially in regard to marine mammals.
Installation within the cooling water system was not justified either, because of the expected ‘bouncing’ of the sound signals off the walls of the system. This would only serve to confuse the fish since they would be unable to determine the actual source of the sound waves.
Other devices, including bubble curtains, electric fields and barrier nets, have been evaluated, but none appear to be effective or technologically feasible. No further refinements are currently planned.
The elevator has been very successful in solving the problem of fish kill at San Onofre, and the annual fish cull at the plant is now less than is taken by a commercial fishing boat in one day.
At the plant, fish numbers tend to be higher in February and March when there are storms, and in July and August during spawning season of some local fish species. However, unlike some coastal plants in other areas very large runs of fish are rare at San Onofre and they have never been large enough to interrupt operation.
Other coastal generating stations in California have had occurrences of sardines, jellyfish, and pelagic red crabs that have been heavy enough to shear shear-pins on travelling screens and trip units. These occurrences are very rare, with only a few incidences in the last 20 years, but this is a problem that has still to be solved.
