Coolant plenums

BRHX submerged heat exchanger, 2 -intermediate heat exchanger, 3 hot coolant plenum, 4 - reactor coolant pump, 5 -"cold" coolant plenum,... 

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,...

Fig. 25 Plots of the concentrations of oxygen (ppb), hydrogen (ppb), and hydrogen peroxide (ppb), and the calculated ECP (mVsHE) at the top inner surface of core shroud as a function of time along the CEP. Note that the concentration of hydrogen is considerably lower than the feedwater hydrogen, because of the mixing of flows from the lower plenum and because hydrogen is stripped from the coolant by boiling in the core.

The primary components of each RS (core, reflectors, and associated supports, restraints, and controls) are contained in the reactor vessel. The nuclear heat is generated in the reactor core. Removal of the heat energy is provided by the Heat Transport System (HTS) with the main circulator providing the driving force to supply helium coolant into an upper core inlet plenum and to draw heated coolant from a bottom core outlet plenum. The primary coolant is distributed to numerous coolant channels running vertically through the core. The outlet plenum directs the flow to the central portion of the coaxial cross duct which channels the helium flow to the steam generator vessel (see Chapter 5). 

Approximately 89 percent of the circulator helium flow passes through the upper plenum and traverses the active core through the coolant channels in the fuel elements. The remaining 11 percent bypasses the core in the coolant channels in the gaps between columns in the core and reflector and the control rod channels. The primary coolant, which passes through fuel... 

To channel the coolant flow, metal plenum elements containing radiationshielding material are placed on top of the upper graphite reflector, one per column. Hexagonal graphite reflector elements are beneath the active core. These lower reflector elements initially continue the coolant hole pattern from the active core. Flow in these channels exits into the core support blocks. 

The standard fuel column plenum elements have vertical channels to direct coolant to the channels in the top reflector. The volume between these vertical channels is filled with boronated graphite pellets to enhance shielding. The reserve shutdown fuel column plenum elements differ in that they contain a hole for reserve shutdown material. 

The plenum elements located on top of the regular hexagonal reflector and reflector control columns also contain boronated graphite shielding and local coolant channels in selected locations. The plenum elements located on top of the permanent side reflector columns contain boronated graphite shielding, but do not have coolant holes. 

As discussed in Section 4.1.2.2, the radionuclide control function which is performed by the upper plenum thermal protection structure is to limit chemical attack on the fuel by limiting fuel oxidation. This structure functions to provide protection to the upper vessel assuring primary coolant boundary reliability and restricting the possibility of air ingress to the core. 

Primary coolant flow normally passes around the 12 support column structures in the lower plenum cavity on its flow path to the hot duct entrance. During SCHE operation, the primary coolant flow enters the SCHE inlet port (located in the center of the lower plenum floor) by means of 12 gaps between the 12 column supports. The 12 gaps are manifolded into six flow paths to the centermost entrance plenxam of the SCHE inlet port as shown in Figure 4.4-3. The width of the gaps is sized to provide the total flow area at any radial location equal to the area of the SCHE inlet port. 

The 12 primary coolant inlet channels with internal dimensions of 152 mm x 660 mm (6 in. x 26 in.) are located on the outside surface of the core barrel to direct the primary coolant to the top inlet plenum. During loss of forced circulation, these channels, in conjunction with the core barrel and the... 

A boss located on the external surface of the core barrel provides a flat surface to which the hot duct is attached. A hole through the core barrel and boss permits the primary coolant to flow from the lower plenum to the hot duct. 

All hexagonal side reflectors and fuel columns have an individual support post with a post seat at the top and bottom of the lower plenum cavity. The support post diameter is 228.6 mm (9 in.). This size is selected to provide additional graphite in the lower pleniim cavity for neutron shielding as well as mixing the primary coolant. The post size is much larger than structurally required to support the column loads. A fracture of lower plenum support structure is a very unlikely event since the mechanical and thermal stresses are far below design limits. 

Primary coolant enters the upper plenum above the reactor core and flows downward through the coolant channels in the plenum element, then to the top reflector elements above the active core. The metal plenum element on top of each core column contains a small flow plenum. The coolant holes in the plenum elements and the coolant channels in the fuel and top reflector columns below are offset horizontally to minimize the neutron streaming effect. 

The primary coolant is collected into six larger channels in the lower portion of the bottom reflector blocks, and then splits and merges with the coolant flow from the neighboring fuel elements in the core support block layer prior to exiting to the lower plenum. This coolant flows to the cross duct, located at one side of the lower plenum. 

When the Shutdovm Cooling System (SCS) is in operation, the thermal/hydraulic configuration is different in the lower plenum area. The primary coolant exiting from the core is still collected in the lower plenum. From there it flows radially inward via narrow vertical channels between the central reflector column supports at the lower plenum elevation to a central chamber. The central chamber collects the primary coolant and directs it downward through an opening in the metal core support structure to the shutdown cooling heat exchanger. 

---