Coolant/moderator expansion

Another effect of a density decrease which can be very important in overmode rated liquid- or gas-cooled heterogeneous reactors is the decrease of poison in the core due to the expansion of the coolant-moderator. 

In the other design, PWRs have two closed loops of water circulating in the plant plus a third, external loop to remove the waste heat. Water is pumped through the reactor core in the primary coolant loop to moderate the neutrons and to remove the heat from the core as in the BWR. However, the reactor vessel is pressurized so that the water does not boil. Steam is necessary to run the turbines, so the primary loop transfers the heat to a secondary loop. The water in the secondary loop is allowed to boil, producing steam that is isolated from both the core and the outside. The water in the primary loop usually contains boron (as boric acid H3BO3 0.025 M) to control the reactivity of the reactor. The steam in the secondary loop is allowed to expand and cool through a set of turbines as in the BWR the cold steam condenses and is returned to the primary heat exchanger. A third loop of water is used to maintain the low-temperature end of the expansion near room temperature and remove the waste heat. 

The PWS consists of six primary coolant loops that circulate moderator to cool the reactor. Each loop consists of a main coolant pump, two parallel heat exchangers, expansion joints, and valves. All branch lines connected to the primary water system up to the first normally closed isolation valve require seismic qualification to assure pressure boundary integrity. This qualification has been demonstrated by various dynamic analyses or by the previously mentioned interim verification program procedure. 

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