PSI - Issue 54

Oleksii Ishchenko et al. / Procedia Structural Integrity 54 (2024) 241–249 Yaroslav Dubyk et al./ Structural Integrity Procedia 00 (2023) 000 – 000

243

3

CB

core barrel;

LB LOCA

large break loss of coolant accident;

RPV RVI

reactor pressure vessel; reactor vessel internals;

FA

fuel assembly.

2. CFD analysis Using full-scale CFD modeling of a reactor to solve dynamic problems associated with abrupt parameter changes is extremely difficult in engineering practice. Features of the reactor flow path and the developed heat exchange surface with local flow turbulators require a detailed computational mesh, which can contain more than 10 9 control volumes according to existing design practice by Volkov et al. (2017). For engineering practice, a representative CFD model was developed that takes into account the main features of the reactor flow path (upper and lower perforation of the barrel). In Dubyk et al. (2018), the core was considered as isotropic porous body in a 1-phase formulation, where the boundary conditions (BC's) at the rupture site were estimated based on the empirical theory of nucleation. The assessment of dynamic forces during decompression was performed in a 1-phase formulation, and the resulting maximum forces exceeded similar estimates performed in the RELAP5 system code by approximately 10 times, which was not fully understood at that time. To a first approximation, such results were explained by more detailed spatial discretization compared to RELAP5, which actually allowed for focused shock wave propagation. The second assumption was that a 1-phase setup was used, in which the speed of sound is several times higher than in a 2-phase medium with similar parameters by Lund and Flåtten (2010), which c an be considered a “harder” response to a shock wave for the same change in the density of the medium. In the following work Ishchenko et al. (2021) the representative reactor model was further developed (see Fig. 1). The core was no longer considered as a porous body, but was presented as a domain in which the flow open area in absolute value corresponded to the real geometry. To assess the dynamics of the decompression wave, a special non-equilibrium phase transition model (SPHM) was developed, which also considered the influence of the

pressure field itself in an approximation similar to cavitation. The proposed approach turned out to be very effective even for predicting the critical outflow pressure, which made it possible to eliminate the assessment of BC's at the rupture site. Instead, the completely natural gradient conditions (the “opening/outflow” type), with absolute value at the boundary that corresponds to the state in the containment zone (0.1 MPa) were applied. To stabilize the calculations, temperature conditions are assumed to be equal to saturation at the pressure on the boundary. In both cases, additional terms of thermal energy release and hydraulic resistance were modelled using volumetric sources. In this work, the computational model of the reactor was optimized by grouping the perforation holes of the upper and lower barrel parts while maintaining the flow area. The features of using realistic fuel load data were also considered. For this, a special template was prepared on a 36-point hexagon, which ensured individual energy release properties for each fuel assembly (Fig. 2). The radiation energy release in the baffle metal due to the gamma ray slowdown, which is also a source of phase formation in the core bypass (cooling channels) during the

Fig. 1. General view of the calculation model with BCs specification.