Downcomer region

The expressions for the slip velocity in the riser and downcomer region of the column are... 

Mean density of two phase mixture in the downcomer region 0m = [Pg.436]

In a typical IPWR, light water coolant enters the base of the core, is heated as it passes upward over the fuel and continues upward through a chimney that provides flow separation from the downcomer region. Self-piessutization is achieved by providing a vapour space at the top of the vessel, such that the vapour phase is in equilibrium with the liquid phase at die core mean outlet temperature. An overpressure of inert gas may be added. 

The coolant is directed downward through a venturi and through heat exchangers devoted to decay heat removal. The flow path continues down through the annular downcomer region to the lower plenum and then back to the core. 

In BWRs, the fuel channels which enclose the rods of individual fuel assemblies impede in-core natural circulation. However, if the residual water level falls below the bottom of the BWR downcomer region while fuel is still heating up in the core region, a strong natural convection loop can be established from the core to the stean i separators and dryers with return to the core inlet via the downcomers. This is depicted in Figure 3.3-7. 

The core model is coupled with the model of in-vessel thermal-hydraulics. Various flow configurations inside the vessel are accounted for, induding the formation of a natural drculation loop between the hot (fully uncovered) core and Ae relativdy cold downcomer regions. The in-vessd model provides the core model with bound conditions such as the gas/steam mixture inflow tanperature, system pressure, and fte temperature of suiroxmding structures. 

What cannot be eliminated, however, is the radiative heat transfer upward within the reactor vessel from the surface of the lower plenum debris bed. About one-half to two-thirds of all energy release within the bed would be radiated upward after bottom head dryout. Initially, the primary heat sink for this radiation would be the water trapped in the downcomer region between the core shroud and the vessel wall above the debris bed. It is the heating of this water that creates the only steam source within the reactor vessel after lower plenum dryout... 

After the water in the downcomer region became exhausted, the upward radiative heat transfer from the debris surface would serve to increase the temperature of the upper reactor vessel... 

Bubbling area, AB the total tower cross-section area less the total of downcomer area, downcomer seal area, and any other nonper-forated regions (often referred to as active area, A,). In practice, onperforated regions less than 4 in wide are counted as perforated... 

Ada should be taken as the most restrictive area in the downcomer outlet region. For instance, if an inlet weir is used and the area open for liquid flow between the segmental downcomer and the inlet weir is smaller than the area under the downcomer apron, the smaller area should be used in Eq. (6.59). 

Downcomer mixing. The AIChE model assumes that liquid along the downcomer length is perfectly mixed. A recent study using a tracer injection technique (174) showed that in large trays, the downcomer liquid is unmixed. A computational analysis (174) showed that efficiency reduction due to an unmixed downcomer is relatively small, but will intensify in the presence of liquid stagnant regions on the tray. 

Downcomers (or diplegs, or standpipes) for transferring particulate solids between regions of different pressure levels need to be designed to satisfy the following requirements ... 

Ab bubbling area, column cross-section area less total of downcomer areas, downcomer seal areas and areas of any other non-perforated region, ft ... 

Fig. 18 Schematic of the primary coolant circuit of a BWR having external pumps. The regions are identified as follows (1) core channels, (2) core bypass, (3) upper plenum, (4) mixing plenum, (5) upper downcomer,...

---