Perhaps a majority of nuclear reactors used for research are of the pool type, often referred to as “swimming pool” reactors. The core is immersed in a pool of

light or heavy water, and heat exchangers are placed outside the reactor vessel to extract the heat. This principle of reactor design can generally be applied only when the primary coolant is an unpressurized liquid, which is the case only for the liquid metal-cooled fast reactor (see Figure 2.15 for an illustration of the cir­cuit). Examples of reactors with this type of coolant circuit are the British proto­type fast reactor (PFR) and the French Phenix reactor. However, it is also possible to design sodium-cooled fast reactors with loop-type circuits. Examples of such reactors are the Japanese sodium-cooled fast reactors JOYO and MONJU.

In the latter designs, the primary circuit pump and the intermediate heat ex­changers are external to the vessel containing the reactor core, as illustrated in Figure 3.8. Thus, in these designs the sodium is pumped from the reactor ves­sel through pipes connecting it to the heat exchanger.

The advantages of the pool-type design are that there are no external pipes, which reduces the risk of pipe ruptures, and there are no connections to the tank containing the coolant pool below the liquid level, as illustrated in Figure 3.9. Moreover, in pool-type reactors, the large quantity of sodium in contact with the core can act as a heat sink in case of circulation failure. In fact, with a well-designed sodium-cooled fast reactor of this type, it is possible to ensure decay heat removal by natural circulation alone, and we shall return to this point in Chapter 4. The pool design, however, has the disadvantage that the main core structures are submerged under many thousands of tons of active sodium and are difficult to get at (to monitor their structural integrity) and to

maintain. Access and maintenance are much easier in the loop-type reactor, but the existence of external pipework introduces the possible hazards of a loss-of- coolant accident.