Regional Basic Professional Training Course in Korea

8.5.3. Decrease in Reactor Coolant Flow Rate
❙ 491 ❙
8-A. Deterministic Accident Analysis (Non‐LOCA)
For a PWR, the initiating events that can result in decrease in reactor coolant flow rate are
as follows:
• Total loss of reactor coolant flow;
• Single reactor coolant pump (RCP) rotor seizure with loss of offsite power;
• Reactor coolant pump shaft break with loss of offsite power.
Among the events listed above, first event is an AOO and the remaining two events are
accidents.
An RCP coastdown due to an interruption of the power supply or failure of the control
system is the most typical case for a reduction of primary coolant flow. Reduction of the
RCS flow leads to an imbalance between heat produced by the fuel and heat removed
from the core, potentially threatening the margin‐to‐DNB. Thermal imbalance also leads
to an overall pressure–temperature transient, typically resulting in a short term
pressurization of both the primary and the secondary circuit.
The possible cause of the RCP rotor seizure or shaft break is a mechanical damage. The
event itself is characterized by a very sharp reduction of the flow in the corresponding
loop, typically within a time shorter than one second. In the RCP rotor seizure case, it is
assumed that the support components are designed to withstand transient mechanical
loads without RCS piping failure.
Figure 8‐4 shows one example of predicted DNBR trends versus time for a Single RCP
Rotor Seizure with Loss of Offsite Power.