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Assignment IDesign of a Marine Outfall

Environmental Hydraulics

DESIGN OF A MARINE OUTFALL WITH A DIFFUSER FOR COOLING WATER

A cooling water flow of 20 m3/s from a nuclear power plant should be discharged into a very large fresh water receiving body (lake). When the cooling water passes the condensers of the power plant, a heat flux of 1600 MW is transferred to this water flow, thus increasing its temperature Δtemp = 19 0C. The cooling water should be discharged to the receiving water in such a way, that the surface water temperature of the receiving water does not increase more than 2 0C in relation to the normal, undisturbed condition. The topographic and geological conditions of the lake are shown in Fig 1. The lake is assumed to be homogeneous (i.e. not stratified) for the sake of the design procedure. It can also be assumed, that the intake is designed in such a way, that recirculation of discharged cooling water to the intake can be avoided. The water movements (circulation) in the lake do not affect the dilution course of events.

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A feasibility study has shown, that the outfall should be constructed as a tunnel in the rock below the bottom of the receiving water all the way to the location of the diffuser. The main tunnel ends with abranch tunnel (both with square cross sections). The branch tunnel is equipped with vertical shafts, through which the cooling water is brought to the horizontal nozzles with circular cross sections, located at a small, vertical distance from the bottom of the receiving water. Each shaft is connected to two nozzles according to Fig 2.

Make an economical optimization of the tunnel and the diffuser system.

DESIGN OF A MARINE OUTFALL WITH A DIFFUSER FOR COOLING WATER (cont’d)

Nuclear Power Plant

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Lake Topography and System Layout

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Buoyant Jet and Dilution

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Schematic of tunnel and nozzle arrangement

Details of Diffuser

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Conditions for optimization

1. Manning’s coefficient M for the tunnel is 322. The excavation cost of the tunnels is 50 SEK/m3

3. The efficiency of the cooling water pumps is 80%4. The cooling water flow rate is constant throughout the year,

equal to 20 m3/s5. Each nozzle is directed horizontally and with a circular opening6. The water depth at the nozzles should be chosen in such a

way, that the excess temperature is 2 0C at most, when the cooling water jet reaches the lake water surface. In this context one should consider the fact, that a homogenization of the temperature distribution takes place in the jet, when it reachesthe surface. The factor 1.4 should be used for this consideration

7. Each nozzle is located at a distance of 2.5 · Dnozzle from the bottom of the lake

8. It is important to consider water level variations in the lake of ± 0.5 m during a year

9. The distance between each pair of nozzles is 50 m10. Construction of a vertical shaft requires a special,

provisional dam, which is placed around each shaft in order to make it possible to empty the area at each shaft from lake water. The dam could be considered to have the same vertical height as the water depth at the location of the shaft,considering water level variations. The total cost for one dam + vertical piping through the shaft + two nozzles is 10000·y + 10000 SEK, where y = water depth

11. The annual cost of the investment (excavation of the tunnels + the dams) when performing an economical optimization is 10 %

12. The marginal cost for pumping the cooling water is 0.02 SEK/kWh

Conditions for optimization

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Optimization ProcedureThe optimizing process should be carried out as a two-step procedure according to the feasibility study. In the first step the result of 10, 12, 14, 16, 18 nozzles respectively is determined. This means, that for each case (number of nozzles) the required main tunnel length is determined for two different initial jet velocities from the nozzles, 2.0 m/s and 3.0 m/srespectively. For each number of nozzles (10, 12,..., 18) one chooses the jet velocity, which gives the shortest main tunnel length. These five different tunnel lengths are then used as an input to the second step. In this final step three different cross sectional areas of the main tunnel, 18, 20, 22 m2, are investigated, i.e., 15 = 5 · 3 different cases exist. The cross sectional area of the branch tunnel should be half of that of the main tunnel. The 15 cases are studied in terms of an economical optimization. This means, that the lowest total cost of annual investment cost + annual operation cost should be determined. The annual operation cost is, for simplicity, assumed to consist only of the marginal energy cost for pumping the cooling water through the main tunnel, the branch tunnel and out into the lake. When evaluating head losses in the branch tunnel, it as allowed to make a reasonable, simplifying assumption, as this head loss is of minor importance.

Dilution Rate in a Circular Jet

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Density Deviation from Maximum Value

σρ −=1000

Calculation Procedure

I. Determine required dilution rate Sm

II. Determine main tunnel length for 10, 12, 14, 16, and 18 nozzles for uo=2.0 and 3.0 m/s. Select the velocity that yields the shortest tunnel length (Sm + FD => yo =>tunnel length)

III. Investigate three tunnel cross-sectional areas: 18, 20, and 22 m3 (energy equation => Ep and pump power P)

IV. Do an economic optimization

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Required power to pump:

ρ

ηpgQE

P =

Energy loss in the tunnel (Manning’s formula):

/L

Qh L

A M R

AR

P

=

=

2

2 2 4 3

Costs Involved

• pumping

• nozzles and vertical shafts

• tunnel

Compare annual costs