As far as the power generation activity is concerned, the use of a boiling water reactor in a power generation station in this country today is actually not too different from using the conventional fossil fired boiler. The simplified flow path (Fig. 1) shows the usual steam turbine, electric generator, and main condenser. However, in the nuclear case the steam supply is from a nuclear reactor vessel containing nuclear fuel bundles rather than from a conventional boiler burning coal, oil, or natural gas. The structural aspects of the two plants are different, but that is principal­ly due to safety considerations.

In general, then, the boiling water reactor in a nuclear power plant accepts feedwater from a conventional “balance of plant” involving a con-

denser, feed pumps, and regenerative heaters; converts this feedwater into steam within the reactor vessel; and supplies the steam to a conventional steam turbine. Therefore, as far as power generation is concerned, the major difference in the nuclear power process is within the boiler.

Figure 2 shows the flow path within the current bwr design and dem­onstrates how the feedwater is converted into steam within the nuclear re­actor vessel. The feedwater enters the vessel by means of a flow header for equal distribution; it then mixes with recirculated water from the reactor core. The mixed flow passes through jet pumps within the reactor vessel in order to develop enough additional pressure to pass through the reactor core. The flow then enters the core, which consists of many (300 to 600) fuel bundles. The flow passes along fuel rods within the bundles, cooling the fuel rods which are being heated by the fission process. Thus, the flow passing through the core is heated and partly evaporated as steam is formed. This water and steam flow from all of the fuel bundles mixes in the area just above the core and then enters a bank of steam separators. The separators direct the steam toward steam dryers and then out of the vessel to the turbine. The water fraction is returned from the separators to be recirculated with the feedwater flow. Thus, there is a rather simple hy­draulic path within the reactor vessel. The considerations of radioactivity in the reactor system will be discussed later in this paper.

With respect to the nuclear process itself, the reactor core consists of

a well-defined array of fuel assemblies each containing a specified number of fuel rods. These fuel rods are metal tubes sealed at both ends by weld­ing and containing uranium dioxide pellets (Fig. 3). The fuel assemblies are separated by control rods containing boron. Thus by a controlled with­drawal of various control rods from the core area, and depending on the water temperature and steam void content, the reactor reaches the critical stages — that is, the reactor is made to sustain a controlled chain reaction at defined power levels. The generated power is in the form of heat within the fuel rods, which are in turn cooled by the flowing water and steam through the core. These fuel rods are like the electric heating elements on an electric range, except that the fuel rods operate at a much lower tem­perature.

The design concept is simple and does not involve any technologies other than those with which we have considerable experience. Steam sep­arators have been used in many applications, including many operating nuclear reactors; the jet pumps have also been used in many other appli­cations. The hydraulic aspects of coolant flow through redded fuel bun­dles is also a well-known technology, both by experience in various fields of heat exchange and by experimentation of various power situations. The nuclear considerations of the fission process and power distribution among and within the fuel assemblies is also now well known by experience with various operating reactors. In fact, there is not a single element — be it a separator, a fuel assembly, a jet pump, or any other component — that has not been tested fully at the temperatures, pressures, and other important

environmental conditions which exist in a nuclear power plant. Further­more, the boiler is very similar to a fossil-fired one. Instead of heating the water with burning coal, we heat it with hot fuel rods. All of the conven­tional design procedures for boilers are used to design this boiler also. Therefore, nothing in the design of nuclear power plants involves signifi­cant technological unknowns. If anything, we find ourselves designing re­actors at 1,000 psi pressure, compared with fossil units over 3,000 psi, and using vessels at less than 600° F compared with fossil units at 1,000° F. In essence, we know what we are doing in the nuclear industry. This is an important fact and one upon which to base the rest of our considera­tions.