Reactor Cooling Systems
The cooling system transferring the heat from the reactor to the heat sink can be configured in a wide variety of designs ranging from fresh-water cooling, through evaporative systems, to dry cooling, including some of their combinations. Reactors are either cooled by using saltwater from coastal (estuarine), freshwater cooling pond, lake water or wet cooling towers, depending on the availability of water resources at the particular site.
1 watt = 3.412 BTU per hr 1 MWt = 3,412,000 BTU per hr 1 Ton (heat load) = 12,000 BTU per hr 1 Cooling Tower Ton = 15,000 BTU per hr BTU = GPM of Water x 500 x Temperature Difference [degrees F] 1 BTU = .293 watt
The heat dissipation system selected, wet or dry, would be dependent on site characteristics. Both wet and dry cooling systems would use water as the heat exchange medium. Wet systems would use water towers and the evaporation process to carry off heat. Dry systems, designed for cold climates, would use water in closed nonevaporative cooling towers to carry off heat to the atmosphere by conduction through radiator-like vanes. In moderate climates, fans would be added to the dry cooling towers to move air over the vanes. There would be some water loss through evaporation in a dry system, but significantly less than with a wet tower. Dry cooling towers would be used for the reactors at all dry sites.
For both once-through and closed-cycle cooling systems, the water intake and discharge structures are of various configurations to accommodate the source water body and to minimize impact to the aquatic ecosystem. The intake structures are generally located along the shoreline of the body of water and are equipped with fish protection devices. The discharge structures are generally of the jet or diffuser outfall type and are designed to promote rapid mixing of the effluent stream with the receiving body of water. Biocides and other chemicals used for corrosion control and for other water treatment purposes are mixed with the condenser cooling water and discharged from the system.
With lake the cooling water from the lake is pumped through a large number of heat exchanger tubes. The cooling water is heated during this process, and is then returned to the lake. The predominant water use at a nuclear reactor is for removing excess heat generated in the reactor. The quantity of water used is a function of several factors, including the capacity rating of the plant and the increase in cooling water temperature from the intake to the discharge. The larger the plant, the greater the quantity of waste heat to be dissipated, and the greater the quantity of cooling water required.
In a once-through cooling system, circulating water for condenser cooling is drawn from an adjacent body of water, such as a lake or river, passed through the condenser tubes, and returned at a higher temperature to the adjacent body of water. The waste heat is dissipated to the atmosphere mainly by evaporation from the water body and, to a much smaller extent, by conduction, convection, and thermal radiation loss.
Recirculating cooling systems consist of either natural draft or mechanical draft cooling towers, cooling ponds, cooling lakes, or cooling canals. Because the predominant cooling mechanism associated with closed-cycle systems is evaporation, most of the water used for cooling is consumed and is not returned to a water source.
In closed-cycle systems, the cooling water is recirculated after the waste heat is removed by dissipation to the atmosphere, usually by circulating the water through large cooling towers constructed for that purpose. Cooling towers are needed when a body of water large enough to provide the cooling, or groundwater is not available in sufficient quantity and there are no other suitable surface water sources available. Closed-cycle cooling towers represents a type of cooling tower that includes both dry cooling towers and hybrid wet/dry cooling towers. Increased cooling tower performance can be achieved by adding surface area or by boosting the flow rate. The former is considerably more expensive than the latter since flow rate can be increased by employing a bigger fan motor allowing increased fan speed.
Wet cooling towers use the same condenser system as in lake cooling, however, the cooling water comes from a large basin at the bottom of the cooling tower. Wet cooling towers use freshwater and achieve 80% of their cooling by evaporation of the cooling water. This evaporation represents a loss of millions of liters of water per year, and dry cooling may be a more attractive option for cooling.
In a natural draft cooling tower the heated cooling water from the condenser is sprayed down the inside of the cooling tower whilst air, under the effects of natural convection flows up through the cooling tower. The air draft evaporates some of the cooling water, lowering the temperature of the remaining cooling water. The draft can also be produced by fans in an induced draft cooling tower. A plume of pure water vapor can often be seen exiting the top of wet cooling towers particularly when the atmosphere has high humidity.
A mechanical draft cooling tower cools circulating water. In the cooling tower, circulating water is directed to the top of the tower and then flows downward through the tower while induced draft fans draw ambient air upward. Heat is transferred to the ambient air primarily through the evaporation of a significant portion of the cooling water. At certain times during the year, a visible plume rises from the tower due to this evaporation cooling process.
A dry cooling tower uses significantly less water. There are two main types of dry cooling technology, the direct system and the indirect system. An air-cooled system operates like a very large automobile radiator. These systems use a flow of air to cool water flowing inside finned tubes. It is essentially a closed loop system where air is passed over large heat exchange surfaces. While air cooling is a reliable and proven technology, it has some technical and economic drawbacks in comparison to a wet mechanical cooling system, which requires the use of significant amounts of water. The principal drawbacks of air cooling are increased noise levels, higher capital costs and larger physical dimensions.
Indirect dry cooling towers use the same condenser system as in lake cooling, however the cooling water is recirculated through banks of finned tubes over which cooling air flows. The air flow is induced in a natural draft cooling tower by convection. The natural draft cooling tower for dry cooling is larger than for an equivalent wet system, since heat transfer rates are much less.
The direct dry cooling mechanical draft cooling tower consists of a concrete structure supporting the mechanical draft fans and exhaust plenum. If fans are used instead of the natural draft tower, a large number of fans would be required to achieve the same heat rejection. This is because the temperature difference between the air and the cooling water is relatively small.
Indirect dry cooling systems tend to have a large capital cost (due to the large cost of the natural draft cooling tower) but low operating cost. In comparison direct dry cooling has a low capital cost but high operating cost (due to large power consumption of the fans). In general, direct dry cooling is favored at sites with low fuel cost (the fan power is less costly), while indirect dry cooling is more suitable at sites with high fuel costs.
The cooling system needs to be located as near as possible to the reactor. It is possible to locate a fan forced cooling system closer to the reactor than a natural draft system. The tube banks may be located directly adjacent to the reactor hall, minimising piping distance, and the fans are located under the tubebanks.
The sources of routine radioactive gaseous emissions to the atmosphere are the air ejector which removes noncondensable gases from the coolant, and gaseous and vapor leakages, which, after monitoring and filtering, are discharged to the atmosphere via the building ventilation systems. The off-gas treatment system collects noncondensable gases and vapors that are exhausted at the condenser via the air ejectors. These off-gases are processed through a series of delay systems and filters to remove airborne radioactive particulates and halogens, thereby minimizing the quantities of the radionuclides that might be released. Building ventilation system exhausts are another source of gaseous radioactive wastes.
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