Since the first air-cooled nuclear reactor built under the squash court of the Uni­versity of Chicago in December 1942, an amazing variety of nuclear reactors have been devised and many of them have been built. In all cases a coolant cir­cuit was included; the main components of such circuits and the circuits ap­plied in the most commonly used nuclear power reactors are described in Chapter 2. Of course, all reactor cooling circuits must include the reactor core itself, a means of circulating the coolant through the core, and a means of ex­tracting the heat from the coolant in order to maintain continuous cooling of the reactor and at the same time (in power reactors) generate useful power. In power reactors the means of extracting the heat from the coolant is almost uni­versally a heat exchanger, which produces high-pressure steam that can be used in a steam turbine to generate power. It is convenient to divide the various types of reactor circuits into three groups:

1. Loop-type circuits. The core itself is contained within a reactor vessel, and the primary coolant circulator and the steam generator are coupled to the reactor vessel by suitable pipe systems.

2. Integral-type circuits. The core, primary coolant circulator, and steam generator are contained within a single vessel, feedwater is fed to this vessel, and steam is taken from it to the turbine.

3. Pool-type circuits. The core and the primary coolant circulators are im­mersed in a pool of coolant. This arrangement is feasible only for unpressur­ized coolants such as sodium. The steam generator is usually outside the reactor containment vessel. This type of circuit is intermediate between the loop-type and integral-type circuits.