CATHARE Multi-1D Modeling of Coolant Mixing in VVER-1000 for RIA Analysis

Abstract

The paper presents validation results for multichannel vessel thermal-hydraulic models in CATHARE used in coupled 3D neutronic/thermal hydraulic calculations. The mixing is modeled with cross flows governed by local pressure drops. The test cases are from the OECD VVER-1000 coolant transient benchmark (V1000CT) and include asymmetric vessel flow transients and main steam line break (MSLB) transients. Plant data from flow mixing experiments are available for comparison. Sufficient mesh refinement with up to 24 sectors in the vessel is considered for acceptable resolution. The results demonstrate the applicability of such validated thermal-hydraulic models to MSLB scenarios involving thermal mixing, azimuthal flow rotation, and primary pump trip. An acceptable trade-off between accuracy and computational efficiency can be obtained.

Thermal-hydraulic analysis of a VVER-1000 core in MSLB conditions

“…Use of a realistic VVER-1000 multi-parameter two-group cross-section library [15] for reactivity accident analysis which has been generated with the APOLLO2 code [16] and validated in the frame of the NURESAFE project [8]; Use of time-dependent assembly-by-assembly MSLB thermal-hydraulic core boundary conditions (inlet temperatures, inlet mass flow rates, and outlet pressures) obtained from a full plant simulation involving a quasi-3D reactor pressure vessel TH model [17]; Coarse-mesh CTF thermal-hydraulic model with one channel per assembly and 30 axial nodes in the heated part; Fuel model with 9 radial rings in the fuel, one for the gas gap and one for the cladding; Temperature-dependent fuel and cladding thermal-physical properties [18]; The spacer grids are not explicitly modeled and are taken into account by the vertical pressure loss coefficients;…”

Best-estimate simulation of a VVER MSLB core transient using the NURESIM platform codes

“…The MSLB core N/TH problem was derived from the OECD VVER 1000 MSLB benchmark (Kolev et al, 2006) using pre calculated MSLB core boundary conditions and the corresponding event sequence. The core thermal hydraulic BC were computed in two options: using a CATHARE2 24 sector multi 1D vessel model with cross flow (Spasov et al, 2010) or a CFD simulation of the lower reactor vessel (Vyskocil, 2015). The validation of the CATHARE input model and the mapping scheme used to convert the coarse mesh data to 163 assembly inlet data are reported in (Spasov et al, 2010.…”

“…The core thermal hydraulic BC were computed in two options: using a CATHARE2 24 sector multi 1D vessel model with cross flow (Spasov et al, 2010) or a CFD simulation of the lower reactor vessel (Vyskocil, 2015). The validation of the CATHARE input model and the mapping scheme used to convert the coarse mesh data to 163 assembly inlet data are reported in (Spasov et al, 2010. The CATHARE2 computed MSLB core BC are specified in a NURESAFE report (Kolev et al, 2014) and shown here in Figs.…”

“…Special attention was paid to the effects of different flow mixing models in the RPV and the core. Time-dependent MSLB core boundary conditions (BC) were obtained in two variants: from CATHARE coarse-mesh RPV simulation (Spasov et al, 2010), and from CFD calculations for the down-comer and the lower plenum (Vyskocil, 2015). The corresponding vessel mixing models were qualified against Kozloduy-6 data from a vessel mixing experiment conducted during the plant commissioning phase (Topalov et al, 2004;Kolev et al, 2007).…”