Comparison of MSLB transient results using the 3D coupled code TRACEv5p05/PARCS and the system thermal hydraulic code RELAP5

This paper presents a comparative analysis of the Main Steam Line Break (MSLB) in a VVER-1000 reactor
simulated with RELAP5 using Point Kinetics and the coupled code TRACE5-P05/PARCS using 3D kinetics. In the
MSLB-scenario, it is assumed that the main steam line break of 580 mm inner diameter is located between the
steam generator (SG) and the steam isolation valve (SIV), outside the containment. In a MSLB, a non-symmetric
overcooling of the primary coolant takes place leading to a positive reactivity insertion. Hence, the main safety
concern is to assess if the core may become critical despite SCRAM and it there is a considerable power increase
(return-to-power). This paper will discuss the capabilities of different computational approaches to simulate the
VVER-1000 plant behaviour during a MSLB; one approach based on 1D thermal hydraulics and Point Kinetics
while the other one based on 3D thermal hydraulics of the reactor pressure vessel (RPV) and 1D thermal hy-
draulics for the remaining plant components based on a 3D neutron kinetics model. The analyses are performed
for Beginning of Cycle (BOC) conditions i.e., with a fresh core loading when the plant is operated at nominal
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power. The neutron kinetic parameters for the RELAP5 Point Kinetics model were generated PARCS for the BOC
assuming a boron concentration of 1630 ppm. The respective 2 energy group homogenized cross section libraries
in PMAXS-format were generated by KIT using the SERPENT2 code.
The investigations were performed in the frame of CAMIVVER-project, which focus was the assessment and
development of reliable neutron physical and system thermal hydraulic models for safety evaluations of VVER-
1000 reactors. The comparative analysis for the MSLB has shown that both applied codes are able to qualitatively
predicts the plant behaviour under MSLB-conditions in similar manner. Differences are caused by the different
approach to represent the core and RPV followed by RELAP5 and TRACE5.05/PARCS as expected.