amount of water required for this type of cooling is
considerable;
ii) once-through cooling with cooling tower, in which the water
is cooled in a tower before it is returned, so that the potential
stress on the body of water is reduced; and
iii) closed-circuit cooling, in which the water heated in the
condenser is cooled in a tower and fed back into the
condenser; the amount of water required for this type of
cooling is small.
In some power plants combinations of the cooling systems
mentioned above are used. For instance some power plants
equipped with closed-circuit cooling systems can also run in oncethrough
cooling mode. In this way, additional operation costs
associated with the more complex closed-circuit cooling system
can be avoided. For a more general overview of power plant cooling
systems, including dry cooling systems, the reader is referred to
Martin [2] and Badr et al. [3].
For power plants using water as cooling medium the reduction
in electricity generation can be due to restrictions regarding the
discharge water temperature. For most of the nuclear power plants
in Germany the maximum dischargewater temperature is between
33 and 35 C while the permissible temperature rise of the cooling
water is usually 10 K. Therefore, this restriction is relevant if the
water temperature at the point of intake reaches approximately 23
and 25 C, respectively. The maximum mixing water temperature
allowed downstream of the discharge point is 28 C for most locations.
If the water temperature at the point of intake is already
high or if insufficient water remains in the river after the withdrawal
to take up the discharged heat, the restriction for maximum
mixing water temperature comes into operation. If the water
temperature at the point of intake even reaches the maximum
mixing water temperature of 28 C the withdrawal water temperature
may be a further restriction. In the latter case, no further
increase in water temperature is allowed and the electricity generation
must be completely shut down.
In 2010, 17 nuclear power plants were in operation in Germany
(see Fig. 1). In some cases, two plants are located more or less at the
same spot (e.g. Biblis A and B in Fig. 1). Table 1 gives basic information
about the separate nuclear power plants according toand [8]. All the plants use water as the cooling medium and are
simulated using the approach presented in Ref.