Simulation of VVER-1000 Guillotine Large Break Loss of Coolant Accident Using RELAP5/SCDAPSIM/MOD3.5

The safety performance of nuclear power plants (NPPs) is a very important factor in evaluating nuclear energy sustainability. Safety analysis of passive and active safety systems have a positive influence on reactor transient mitigation. One of the common transients is primary
coolant leg rupture. This study focused on guillotine large break loss of coolant (LB-LOCA) in
one of the reactor vessels, in which cold leg rupture occurred, after establishment of a steady-state
condition for the VVER-1000. The reactor responses and performance of emergence core cooling
systems (ECCSs) were investigated. The main safety margin considered during this simulation was
to check the maximum value of the clad surface temperature, and it was then compared with the
design licensing limit of 1474 K. The calculations of event progression used the engineering-level
RELAP5/SCDAPSIM/MOD3.5 thermal-hydraulic program, which also provide a more detailed
treatment of coolant system thermal hydraulics and core behavior. The obtained results show that
actuation of ECCSs at their actuation set points provided core cooling by injecting water into the
reactor pressure vessel, as expected. The peak cladding temperature did not overpass the licensing
limit during this LB-LOCA transient. The primary pressure above the core decreased rapidly from
15.7 MPa to 1 MPa in less than 10 s, then stabilizes up to the end of transient. The fuel temperature
decreased from 847 K to 378 K during the first 30 s of the transient time. The coolant leakage reduced
from 9945 kg/s to approximately 461 kg/s during the first 190 s in the transient. Overall, the study
shows that, within the design of the VVER-1000, safety systems of the have inherent robustness of
containing guillotine LB-LOCA.