A numerical model of the effects of reactor cooling water on fjord ciculation

Typ: Rapport
Serie: RHO 6
Författare: Wilmot Wyne


In the search for possibie sites for new nuclear power plants in Sweden a site on Bråviken, a narrow fjord, is being considered. A numerical hydrodynamic model has been developedt o predict the probable effects of the waste heat discharged into the estuary on the natural estuarine flow.

The model employs the basic equations of motion and conservation of salt and heat with appropriate approximations to make predictions. The primary approximation in the model consists of considering the estuary as a channel in which cross channel effects do not explicitly appear. The along channel motion is thus primary determined by the along channel density gradients.

A number of different cases have been investigated. The basic states in the summer and in the winter have been established in order to evaluate effects of heated water in two different conditions of thermal stratification . For each of these seasons two different intake configurations have been considered: a surface intake and a bottom intake. The outlet is located at thesurface. The above situations have been considered for several wind conditions: no wind, a 5 m/s constant wind bending to transport water in to the estuary, and a 12 m/s time dependent wind which gradually builds up blowing out of the estuary, changes direction and then dies away. A series of experiments with various runoff values at the head of the fjord have been conducted.

With the construction of a bottom intake located at the depth of about 40 meters there will be little noticable effect on the circulation, temperature or salinity fields in the estuary in the summer. However in the winter the bottom intake offers only a partial improvement over a surface intake. During the winter the heated water would cause changes of as much as 50 % in the natural state. The surface intake would cause changes which sometimes are almost twice as big. The problem arises because the10°C heated water creates sizable horizontal density gradients which are sufficient to counteract the weak natural flow .