Volume 12, 2000
|Page(s)||87 - 134|
|Publié en ligne||1 mai 2007|
An assessment of the effect of the Sizewell power stations on fish populations
Effet des centrales de Sizewell sur les peuplements de poissons.
Fawley Aquatic Research Laboratories Ltd, Marine and Freshwater Biology Unit, Fawley, Southampton, Hampshire, SO45 1TW, UK
2 Health, Safety & Environment Division, British Energy, Barnett Way, Barnwood, Gloucester GL4 3RS, UK
Sizewell A and B Nuclear Power Stations are located on the Suffolk coast of East Anglia. The A station is a 650 MWe Magnox plant, completed in 1966 and operated by British Nuclear Fuels; the B station is a 1258 MWe pressurised water reactor (PWR), commissioned in 1995 and operated by British Energy Generation Ltd. Both power stations are direct cooled and rely on abstractions of cooling water (CW) from the North Sea: together they can abstract some 80 m3 s-1. The water is passed around the plant condenser circuits and returned to the sea, along with reject heat and any chlorine residues from antifouling treatment. The abstraction of this water is accompanied, to some extent unavoidably, by entrained fish present either as ichthyoplankton (eggs, larvae and postlarvae of fish), or as fully-formed juvenile or adult fish which have to be removed by mechanical screening systems ('drum' screens) to avoid CW condenser blockage. The entrained ichthyoplankton passes through the entire cooling system and is discharged back to sea along with the heated water. The later life stages of fish and other material that become impinged upon the drum screens are removed from the water. At Sizewell B provision is made to return the more robust species of fish back to the sea alive. In the late 1970S the Central Electricity Generating Board (CEGB), then owner of the whole Sizewell site, announced plans to build the PWR power station which was to become known as Sizewell B. Local fishermen lodged an objection to the scheme on the grounds that the mortality of juvenile fish on the drum screens might be increased to an unacceptable level. Between 1981 and 1982 a joint study of the fish catch was carried out by the CEGB and the Ministry of Agriculture, Fisheries and Food (MAFF). This study showed that the losses on the A station of commercially important species, including plaice (Pleuronectes platessa), sole (Solea solea), dab (Limanda limanda), cod (Gadus morhua), whiting (Merlangius merlangus) and herring (Clupea harengus), amounted to 66 tonnes per year (t y 1, then valued at £28 000 per annum. This estimate included an allowance for the potential yield of fish which were below the statutory minimum landing sizes when captured, assuming that the rates of growth, mortality and exploitation would have been similar to those experienced by other fish within the North Sea fisheries. The catch rate was summarised by observing that it was 'less than that of a single small, inefficient trawler' and therefore of minor significance. It was also concluded that no impact on local fisheries could be defined, as stocks within the North Sea tend to migrate over large distances. Nonetheless, it was agreed between CEGB and MAFF that a number of reasonably practicable opportunities existed for reducing the catch of the B station, such as appropriate location and design of the cooling water intake and the incorporation of the 'trash' return system. It was also agreed that, following commissioning, the predictions on fish catch would be validated and the relative success of the various mitigative measures assessed. This report is the result of that agreement, but also provides the opportunity of summarising and incorporating other Sizewell-related impingement and entrainment studies to give an updated assessment. With the construction of the B station, and plans (now dropped) to build a C station, the CEGB's successor and British Energy Generation Ltd.'s predecessor, Nuclear Electric Ltd., commissioned studies both to satisfy the prior agreement with MAFF and further evaluate the actual or potential impact of the Sizewell power stations on North Sea fish- eries. So, from 1991 onwards: the original survey data were re-analysed to assess any likely changes resulting from trends in North Sea stocks; assessments were made of losses due to ichthyoplankton entrainment at the A station; experimental studies were undertaken to determine mortality rates of ichthyoplankton passing through the CW system; on the commissioning of the B station, catch rates were compared with the A station to determine whether design and positioning improvements in the B station intake were beneficial; survival rates on passage through the fish return system on the B station were measured; comparisons of losses of juvenile fish due to the power stations with those due to other sources, such as the East Coast shrimp fisheries, were undertaken, to provide an alternative context within which to view the findings; an expert system known as PISCES was used to make estimates of impingement rates for other English East Coast power stations, so that the combined effects of these stations acting in concert could be determined; other fish-related studies were undertaken to determine, for example, any possible impact of fish losses on the availability of food for fish-eating birds at the neighbouring Minsmere nature reserve. The results of these studies are frequently presented in this report in terms of Equivalent Adult values (EAV's). The EAV method is a procedure where the nurnbers of fish of any age are standardised to the number that would be expected to be alive at the age when 5O% of the stock would mature. Thus, if a fish matures at 3 years old, many millions of eggs or larvae may represent a single 'equivalent adult', and the EAV will be a tiny fraction of unity, whereas a fish older than 3 years will have an EAV of greater than one. The purpose of the EAV method is to allow fish captured at any stage of their life cycle to be compared on an equal footing with fish of commercial size. It is important to note that the EAV method does not take account of density-dependent factors (such as rates of predation, parasitism, feeding success) that might tend to increase the survival, growth and reproductive rates of individuals left in the population when some of their competitors are removed. The values given should therefore be regarded as overestimates.
Les deux tranches de la centrale nucléaire de Sizewell sont refroidies par un circuit ouvert, consommant 80 m3/s d'eau, prélevée dans la Mer du Nord. Après filtration et traversée des condenseurs, l'eau échauffée est rejetée en mer avec des résiduels de chloration résultant du traitement de protection des circuits de refroidissement contre les salissures biologiques. Ce prélèvement dans le milieu naturel occasionne, inévitablement, un certain entrainement d'œufs, larves et post-larves de poissons, de juvéniles ou de poissons adultes. Une partie de I'ichtyoplancton parvient à traverser l'ensemble du circuit de refroidissement de la centrale jusqu'à son retour en mer, via le rejet des effluents thermiques. Par contre, l'autre partie du plancton et les stades plus âgés (juvéniles et adultes), ainsi que les autres corps flottants, sont retenus par les tambours filtrants installés en stations de pompage pour éviter l'obstruction des condenseurs. Pour répondre aux objections des professionnels de la pêche craignant une mortalité inacceptable de juvéniles à l'occasion du projet de construction de Sizewell B, décidée à la fin des années 1970, des mesures ont été prises par le CEGB (Central Electricity Generating Board), l'ancien propriétaire, pour restituer au milieu environnant les espèces les plus robustes. Pour ce faire, en 1981 & 1982, une étude conjointe des captures de poissons fut entreprise par le CEGB et le Ministère de l'agriculture, des pêches et de l'alimentation (MAFF). Cette étude montra que les prises d'espèces commerciales, notamment de plies (Pleuronectes platessa), de soles (Solea solea), de limandes (Limanda limanda), de morues (Gadus morhua), de merlan (Merlangius merlangus) et d'harengs (Harengus clupea), atteignaient, à la tranche A, 66 t /an, correspondant à environ 280 000 FF/an.
Key words: Entrainment / impingement / fish return / power plant / Equivalent Adult Values / bycatch / North sea
Mots clés : Entrainement / piégeage / retour en mer / centrale électrique / Equivalent Adulte (EVA) / pêches accessoires / mer du Nord
© EDF, 2000
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