PSI - Issue 54

Oleksii Ishchenko et al. / Procedia Structural Integrity 54 (2024) 241–249 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

242

2

1. Introduction The assumption of a double- ended pipe break in the main coolant loop piping of a nuclear power reactor followed by blowdown (a rapid loss of coolant through the break) has been considered as the worst possible loss-of-coolant accident (LOCA) and is called a design basis accident or DBA. Research in this area has been conducted since the 80s of the 20th century Bader et al. (1985) and one of the most difficult issues is the calculation of thermohydraulic parameters during a LB LOCA in the reactor, which determines the dynamic loads. Many researchers Wang et al. (2017), Hermansky (2011), Pistora et al. (2019) to calculate pressure wave propagation process used system safety analysis codes like RELAP5, CATHARE, ATHLET and TRACE. It is stated that CFD is not applicable for LB LOCA analysis due to its two-phase nature, in other words application of CFD requires great attention to two-phase modelling, which is essential for LB LOCA. The way forward in this direction was proposed in the authors’ paper Ishchenko (2021) and the current research is an extension of the proposed approach. The dynamic impact caused by pipeline ruptures can lead to significant loads (acoustic loads, drag force, annulus pressurization, etc) on the reactor internals and possible violation of the FA and RVI integrity. The abovementioned horizontal and vertical loads produce significant forces on the RVI, especially CB. In case of ‘ cold leg break ’ an unbalanced pressure distribution appears in the front of a broken leg, in case of ‘ hot leg break ’ due to significant acceleration of the flow, dynamic forces propagate from FA and Block of Guide Tubes to the CB. The main focus of the current article is the first case, i.e. ‘cold leg break’, since according to preliminary analysis it is the worst case for the Core Barrel from structural integrity point of view. To study this event both analytical (lumped mass) by Zeman and Hlaváč (2018) , Ishchenko (2021), Zhang et al. (2021) or FEM models Gal et al. (2019), Pistora et al. (2019), Hermansky (2011) can be used. From one hand application of analytical models provides benefits in the speed of calculations, from the other FEM give possibility to obtain a more realistic CB deformation due to the implementation of elastic-plastic models Lee (2021). Thus, another idea of the present research is to improve our modelling by developing the FEM model of RVI with elasto-pastic material behavior. Such complicated analysis is necessary to evaluate the residual deformation of CB, i.e. to check the safety requirement about RVI disassembling after the DBA. Talking about safety criteria, the first one which should be met – it is a structural integrity of RVI during the DBA. From authors’ point of view the structural integrity assessment should include fracture analysis based on brittle and ductile mechanism of rupture, which can be effectively merged using FAD. Such an approach ensures that CB will not rapidly fail due to significant dynamic loadings.

Nomenclature L

length of cylindrical shell; middle radius of cylindrical shell; thickness of cylindrical shell;

R h

m  

membrane stress in circumferential direction; ( ) m b   + membrane and bending stress in circumferential direction; xm  membrane stress in axial direction; ( ) x m b  + membrane and bending stress in axial direction; y  yield strength; ult  ultimate strength; ref  reference stress, which is the stress in the defect zone; I K stress intensity factor (SIF); IC K critical stress intensity factor; E Young's modulus; k spring stiffness; а depth of crack; c half-length of crack; n safety factor; DBA design basic accident; FEM finite element method / finite element model;