11.116. When it is possible to meet the applicable water quality regu­lations and standards, once-through cooling would be the preferred method for disposing of the waste heat from the turbine condenser of a power plant. The condenser water is then discharged in such a way as to minimize the thermal impact on the receiving water body. In some situations, a slow discharge at or near the surface of the receiving body may be preferred; the warm discharge water then spreads over a large area. At the other extreme, the water may be forced through jets or diffusers located near the bottom of a flowing stream, so that rapid mixing occurs. Several vari­ations between these extremes are possible. If an adjacent water body is an ocean or a large lake, the intake water is usually drawn from a depth, where the temperature is lower, and is discharged near the surface.

11.117. If the conditions do not permit once-through cooling, some form of closed (or partly closed) cooling cycle must be adopted to deal with the thermal discharge. Cooling ponds (or canals) or cooling towers are used for this purpose. In a closed-cycle system, all the condenser water is dis­charged and cooled in the pond or tower and the cooled water is withdrawn for reuse. Makeup water is added as required. In a partially closed system, part of the water would be discharged directly to an adjacent water body whereas the remainder would be cooled and reused. In some circumstances, a variable-cycle cooling system may be satisfactory: once-through (direct- discharge) cooling is used in the winter and early spring and a closed (or partially closed) system at other times.

11.118. In a cooling pond or cooling canal, the condenser water is dis­charged at one end of a large pond (or small lake) or a long canal and is withdrawn at the other end of a flow path. In the course of its passage through the pond or canal, which may take several days, the condenser water is cooled mainly by evaporation and also to some extent by convec­tion and radiation. The cooling efficiency of a pond or canal can be greatly enhanced by pumping the water through nozzles to form sprays. The in­crease in water surface produced by the sprays increases the rate of heat loss. Depending on the local meteorological conditions, e. g., temperature, humidity, and wind, a pond or canal without sprays will have an area of from 4 to 12 x 106 m2 (1000 to 3000 acres) for a 1000-MW(el) nuclear plant; with sprays, a much smaller area (one-tenth or less) would be sufficient.

11.119. Cooling towers are of two main types: wet and dry. In wet towers, the condenser discharge water flows down over a packing or “fill” and is broken up into droplets. A current of air is drawn through the fill, either by a powerful fan in mechanical-draft towers, or by a tall (up to 165 m high) chimney-like structure, with a hyperbolic profile, in natural-draft towers. Most of the cooling results from vaporization of the water, but there is also some cooling by convection and radiation. The lowest tem­perature attainable is approximately the wet-bulb temperature under the existing conditions; it is thus dependent on both the actual air temperature and the relative humidity. The temperature of the water collected at the bottom of the cooling tower is generally 4° to 6°C above the wet-bulb temperature.

11.120. One of the problems of wet cooling towers is the occurrence of “drift,” that is, the entrainment of small droplets of water in the air leaving the tower. Drift eliminators, which cause the air to make abrupt turns, help to reduce the amount of drift but do not prevent it completely. The drift is worse for mechanical-draft than for natural-draft towers, partly because of the higher air velocities in the former. The presence of chemicals used to prevent biological fouling, corrosion, and structural deterioration in the tower may make the drift a hazard to plant and animal life on the ground in the downwind direction where the water droplets tend to settle. Drift elimination has been improved to such an extent in recent years, however, that sea water can be used without causing significant environ­mental damage.

11.121. Continuous evaporation of the water in the tower causes the chemicals, as well as the minerals normally present, to become more and more concentrated. Ultimately, the concentration reaches a point at which scale may deposit on the condenser tubes. In order to prevent this, some of the water is removed either continuously or periodically and discarded as “blowdown” and replaced with fresh water. In a large power plant, the blowdown rate is about 12 to 15 m3 (3000 to 4000 gal) per minute. The blowdown water may have to be treated for the removal of various chem­icals before it can be discharged to a nearby water body.

11.122. The main loss of water from a wet cooling tower is by evapo­ration of the water and this must be replaced continuously. Including the much smaller losses from drift and blowdown, makeup water is approxi­mately 2.5 to 3 percent or so of the water passing through the condenser; hence, on the basis of the data in §11.111, the average makeup rate, even for a completely closed system, would be roughly 60 to 70 m3 (15,000 to 17,000 gal) per minute for a 1000-MW(el) nuclear plant.

11.123. Most of the water lost from a wet cooling tower enters the atmosphere as water vapor; under certain conditions this could result in the formation of fog, leading to reduced visibility, and to the deposition of ice on roads and power lines in the vicinity in the winter. The available evidence indicates, however, that such occurrences are limited to a rela­tively few days of high humidity in cold weather. Natural-draft cooling towers discharge the moist air at much higher elevations than do mechanical- draft towers, and hence they are less likely to produce fog and ice near the ground.

11.124. Dry cooling towers used in steam electric plants are generally described as air-cooled condensers. They operate by using ambient air as coolant to condense the exhaust steam either directly or indirectly. In the direct condensing cycle, the steam leaving the turbine is passed through a system of finned pipes over which air is drawn by mechanical or natural draft. The air removes heat from the steam so that it condenses, and the condensate is returned as feedwater to the steam generator.

11.125. The most efficient dry tower utilizes an indirect condensing cycle known as the Heller system. The steam is condensed by direct contact with jets or sprays of water from previously cooled condensate. Part of the

resulting warmed water is returned as feedwater to the steam generator, while the remainder is cooled by passage through a tower containing finned pipes over which air is drawn. The cooled water leaving the tower is re­circulated to the condenser sprays.

11.126. In dry cooling towers, heat is removed mainly by convection to the ambient air, and there is no loss of water by evaporation. There is consequently no blowdown (or drift) and, except for leakage, no require­ment for makeup water. Hence, steam-electric plants with dry towers can be located in areas where water is scarce. On the other hand, the absence of vaporization means that the lowest temperature attainable theoretically in a dry tower is the actual air temperature, which is higher than the wet- bulb temperature. As a general rule, therefore, the temperatures of the condensate would be higher for a dry tower than for a wet tower; the turbine back-pressure would also be higher. The overall effect would be a decrease in efficiency. In addition, construction and operation costs are expected to be greater for a dry tower than for a wet tower of the same cooling capacity.

11.127. Despite the greater cost of generating electricity with dry cool­ing towers, estimated to be about 20 percent more than for wet towers and about 27 percent more than for once-through cooling, there may be circumstances in which no other form of cooling is practical. So far, rel­atively few dry (Heller) towers have been used (or designed) for power plants, the largest with a capacity of 350 MW(el). However, studies are being made of the possibility of designing dry towers suitable for large nuclear power plants.

11.128. Wet-dry (or parallel-path) cooling towers, which use both wet and dry cooling, are attracting interest. The mechanical-draft tower consists of two regions: the upper part is a dry tower whereas the lower part is a wet tower. The condenser discharge water is first cooled by passage through finned tubes in the upper (dry) region and then flows down over the fill in the lower (wet) region for further cooling. Air is drawn by a fan in parallel paths through both dry and wet regions, and the streams are mixed before discharge to the atmosphere. The air flow between dry and wet regions can be adjusted to give preference to one or the other. In winter, when the air is cold, advantage is taken of dry cooling, with a marked decrease in fogging and icing as well as a reduction in water consumption, but in summer preference might be given to wet cooling.

1. ?

INTRODUCTION

Technological Risk and Public Perception [1]

12.1. We introduce this chapter, which is devoted to those aspects of nuclear reactor engineering concerned with system safety, with a few thoughts on some nontechnical considerations that are relevant. First, there is the matter of terminology. The word safety is generally defined as “freedom from danger or hazard,” while risk means “the chance of injury or loss.” We will examine these terms more carefully later when we discuss the subject of risk analysis. However, mathematically based quantitative def­initions of risk are beyond our scope. A problem is that in a technological society, no activity has zero risk. Thus, there cannot be absolute safety and the two terms are related to one another, with low risk meaning the same as high level of safety. However, psychologically, we tend to be more comfortable with the term safety than with the term risk, whether we are discussing nuclear reactors, automobiles, airplanes, or chemicals.

12.2. Unfortunately, in considering public perception, there is an ele­ment of fear which is often not at all related to the true risk of a techno­
logical activity as developed from statistical data. In general, self-imposed, or voluntary risks, tend to be more acceptable than involuntary risks. For example, the significant risks of highway travel are normally accepted, whereas isolated cases of chemical residues on some fruits with negligible statistical risks have led to national market withdrawals of such fruits. Intangible factors also affect the public perception of risk. For example, many members of the public fear ionizing radiation as such, even at harm­less levels. Another example is the fear by some of using commercial aviation. For a planned technological activity, it is sensible to balance the benefits obtained with an “appropriate” level of risk that can be adjusted during the design process, taking into consideration the costs required. This balance is often known as the “how safe is safe enough?” question. However, public perception (fear) and sometimes associated political pres­sure can play a role in the “appropriate” balance.