BOILING WATER REACTORS

Consider the simplified schematic of one of the most common types of nuclear reactor, a boiling water reactor. (See the accompanying illustration.) Boiling water reactors were first pioneered in the United States, where a number of them continue to produce electricity. Japan has recently installedseveral boiling water reactors of a more advanced design.

A boiling water reactor is, in the end, a type of steam engine, and has much in common with other steam engines.

But while many of the design concepts employed in these reactors are the same as what one finds in the early steam engines, the technology employed in nuclear reactors has, not surprisingly, been greatly modified to make better use of the tremendous steam temperatures and pressures that these plants are capable of producing.

Apart from the fuel used by these reactors and the scale on which they operate, the most obvious difference between Newcomen’s and Watt’s engines and contemporary boiling water reactors is the turbine. The turbineis the analogue of Watt’s sun and planet gear system. It converts the linear or straight-line motion of flowing steam into rotary motion. Turbines are designed to rotate continuously in one direction and at a steady speed and are driven by the high-pressure steam that flows in a continual torrentout of the pressure vessel, the analogue of Newcomen’s boiler. Designers prefer to use turbines because they have found that the back-and-forth motion of pistons, the continual opening and closing of valves, and the sun and planet gear systems are poorly suited for electrical power generation, a process in which all components must operate for prolonged periods of time under conditions of high temperature, highpressure, and other large forces.

For boiling water reactors, the cycle by which electricity is produced works as follows:

1. Upon passing through a valve, the expanding steam pushes against the blades of the turbine, causing the turbine to spin. The turbine is connected to the electrical generator, the device that actually produces electricity, by a shaft. (The entire purpose of any power plant is to drive the generator.)

2. Water is pumped into the reactor vessel, where some of it is converted to high-temperature, high-pressure steam.

3. After exiting from the turbine, the steam enters the condenser, where it is cooled and in the process condenses back to the liquid state. Cooling occurs when the steam comes into contact with a device called a heat exchanger. The heat exchanger is kept cool by water that circulates through it continuously. The water that cools the heat exchanger is physically separate from the water that passes through the pressure vessel. Only heat, not water, is exchanged inside the condenser. One purpose of the condenser is to reduce the back pressure of the steam after it has exited the turbine. In a way that is conceptually similar to the function of Watt’s condenser, the condenser in the boiling water reactor enables the turbine to make maximum use of the steam.

4. Heat is generated by the nuclear fuel, which is located in the reactor vessel.

5. From the condenser the liquid water is pumped back into the pressure vessel, and the cycle repeats.

6. The steam, which leaves the pressure vessel at about 550°F (290°C) and at a pressure of about 71 atmospheres, travels along the steam line until it reaches the turbine.

Boiling water nuclear power plants have a great deal in common with their Age of Steam predecessors. They are machines that use thermal energy to create steam to do mechanical work. The characteristic that most distinguishes these nuclear power plants from other types of heat engines is the fuel that they use to generate thermal energy, but the consequences of using nuclear fuel are profound.