The stages in tea making are illustrated in Figure 4.2. The electric kettle (re­ferred to by our publisher as the “English samovar”) is first filled with water (a) and then connected to the main electricity supply (b). Ultimately, the water in the kettle boils (c), and the tea is brewed (d).

We may represent the stages illustrated in Figure 4.2 in more scientific terms by plotting graphs (as shown in Figure 4.3) of the following quantities:

1. The amount of water present in the kettle—the inventory.

2. The power input to the element

3. The temperature of the water

4. The surface temperature of the element

5. The temperature of the element windings

As will be seen, the kettle has many of the characteristics of a reactor system. Since it does not have any recirculating coolant, its temperature rises until the boiling point of the liquid is reached, at which point boil-off of the liquid oc­curs. ‘^ben the kettle is partially emptied (simulating a loss-of-coolant acci­dent), the temperature of the surface of the heating element increases. The

sequence of events shown in Figure 4.3 iUustrates the case of rewetting or quenching, which occurs when the kettle is returned to its normal position and the remaining water quenches the hot heating elements with a very audible hiss. Furthermore, modern kettles are equipped with engineered safety systems; for instance, kettles often have a device that ejects the plug by means of a spring actuated by a bimetallic strip if the kettle boils completely dry during the boil-off period. In fact, even newer kettles have a device that detects the emis­sion of vapor and switches the kettle off in a more easily reversible way when the water is boiled; this is another form of an engineered safety system.

The big difference between the kettle and a reactor system is that in the case of the kettle, the safety systems are able to switch off the power input com­pletely and any overheating occurs principally as a result of stored energy within the heating element. In a reactor, however, fission product heating con­tinues at a low rate relative to full power after the reactor has been shut down.

It is also interesting to compare the kettle with the reactor in the context of failure of the engineered safety systems. Plug ejection might occur several times during the lifetime of a kettle; reinstatement of the operation of the kettle can be achieved immediately afterward. However, should the plug fail to be ejected (due to, say, corrosion of the spring), then heating of the element will continue and can lead to its melting. Failure of the engineered safety systems leads to the need for a major repair of the kettle, and this would certainly also be the case in the nuclear reactor.