Basic Components of PWR (part3)

The coolant

Light water used as steam in PWR, where it is maintain at high pressure, approximately 15Mpa. At this pressure the water will not boil, at least not to any great extent. Since the water does not boil in the reactor. This is done in steam generators, which are heat exchangers with pressurized water on the hot side, high pressure, heated coolant water from the reactor enters at the bottom and passes upward and then downward through several thousand tubes each in the shape of an inverted U, the outer surfaces of these tubes are in contact with lower pressure and cooler feed water returning from the turbine condenser. Heat transferred from the hot water inside the tubes causes the feed water to boil and produce steam. The lower section of a steam generator where this boiling occurs is called, the evaporator section.

The wet steam produced in the evaporator passes upward into a portion of the steam generator known as the, steam drum section.

Here the steam is dried in various moisture separators before exiting to the turbines, steam generators are also manufactured with straight tubes rather than U tubes.

Because water is essentially incompressible, even small changes in coolant volume could lead to large changes in pressure which could have deleterious effect on the system. To prevent this from happening, one coolant loop of “PWR” is equipped with a pressure maintaining surge tank known as a pressurizer, which used to maintain the coolant pressure during steady state operation limits pressure changes caused by thermal expansion and contraction during normal load transients and prevents coolant pressure from exceeding limits.

The Moderator

The moderator used to moderate, that is, to slow down the neutrons from fission to thermal energies, nuclei with low mass number are most effective for this purpose, so that the moderator is always a low mass number material light water used as a moderator in PWR.

Control Rods

Control rod is used for reactor startup, to follow load changes and to control small transient changes in reactivity. The control elements of a rod cluster control (RCC) assembly consists of cylindrical neutron absorber rods having approximately the same dimensions as a fuel rod and connected at the top by spider –like bracket to form rod clusters.

Two types of rod cluster are employed:

1.    Fully length, the fully length type in corporate rods of silver-indium-cadmium absorber material extending the fully length of the core. Stainless steel tubes encapsulate the absorber material isolating it from the reactor coolant. Fully length rod cluster controls provide operational reactivity control and can shut the reactor down at all times, even with the most reactive rod stuck out of the core.

2.    Part length, the part length rod cluster controls, although identical in external appearance and design, incorporate rods with absorber material only in the bottom quarter of the tube. The remainder of the tube is filled with aluminum oxide. The absorber region of the part length rod is positioned at various elevations in the core to shape the axial power distribution. A control axial xenon redistribution and accompanying power level changes. Each rod cluster control is coupled to its drive shift, which is actuated by a separate drive machines mounted on the reactor vessel head, reactivity of the core is changed by raising or lowering the cluster in the core.