The nitrogen gas enters cold legs through ECCS nozzles first and then migrates to hot legs through the vessel downcomer
and hot leg leak simulating lines that connect the downcomer
to the hot legs, as shown in Figure 1.
The coolant is subsequently drawn into the pump hydraulics and forced radially out to the downcomer
, where it flows to the bottom of the RPV cavity, turns 180 degrees and flows up into the core to complete the flow cycle.
Although the riser is aerated, the downcomer
is gas-free because nearly all bubbles are escaped from the free liquid surface of degassing zone if the superficial gas velocity is relatively low .
"We saw milli-amps of current flowing up and down the downcomer
," said Brehm.
However, for liquid jet with a downcomer
even after gas entrainment occurs at the plunging point, there is almost complete recirculation of the entrained gas unless certain momentum is imposed on the gas bubbles to move downward.
The thick line is the response of the model with four holdup terms which includes the vapour holdup while the thin line is the response of a model with two holdups, but that includes the holdup of the liquid in the downcomer
. Obviously, there is a significant difference in the two responses that would influence the tuning of the column control.
The flooding, often caused by downcomer
blockage, is one of the more readily identifiable column fault.
The COMMIX model included the cold leg entrance into the downcomer
, the downcomer
, the lower plenum, and the core.