Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
8.8.1.4. Upset of SG heat transfer
Heat transfer rate between the primary side and the secondary one of SG is drastically
changed. First, the secondary system pressure starts to increase due to closure of Turbine
Stop Valves of the plant. Simultaneously, heat transfer from the SG primary side to the
secondary side increases as the two‐phase mixture generated by the flashing and boil‐off
in the reactor core moves into the SG primary side. Condensation of the steam at the
primary side also increases the heat transfer to the SG secondary side and the SG
pressure. To prevent the increase of the SG pressure, venting of steam in the SG may be
conducted and auxiliary feedwater may be activated to maintain the SG water level.
8.8.1.5. Blowdown
During early blowdown phase, makeup water is supplied by high pressure Emergency
Core Cooling System (ECCS) and makeup water system, however, it is not sufficient to
compensate the coolant discharged out of break. For the case that the core remains in
relatively high pressure, the water in Safety Injection Tank (SIT) may not be injected and
thus core mixture level continuously decreases and core may be uncovered due to boil‐off
and break.
Figure 8.8‐4 Blowdown state following SBLOCA of 2‐loop pant
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