The coupled code TRAB-3D-SMABRE for 3D transient and accident analyses

VTT's three-dimensional TRAB-3D core dynamics code and SMABRE thermal hydraulic system code have been coupled together using an internal coupling scheme in order to increase flexibility in the thermal hydraulics modelling of the core calculation. VTT's reactor dynamics codes have performed well in all the situations that they have originally been designed for. The most important limitation of the present code models is their inability to handle coolant flow reversal in the core channel, a phenomenon that can be encountered in e.g. BWR ATWS cases or VVER power excursions. The new coupling of the two codes is realized on the level of each node of each channel in the core, with each fuel bundle described with its own channel. TRAB-3D performs only the neutronics calculation, SMABRE takes care of the hydraulics calculation of the whole cooling circuit including the reactor core, while heat transfer calculation can be carried out optionally by either code. The codes have earlier been coupled using a parallel coupling scheme. Several modifications were necessary in SMABRE, concerning modelling of hydraulics, heat transfer, geometry and the matrix solution. The accuracy of the steady state calculation in the coupled code has been improved to a level suitable for both PWR and BWR calculations, as compared against the SIMULATE and reference TRAB-3D codes. Presently BWR dynamics calculations are being tested with single disturbances, such as control rod movements, pump coastdown etc. Besides allowing modelling of reversed flow in the core, the internally coupled code will make future modelling of in-core cross-flows or even 3D flow in a PWR (such as EPR) open core geometry possible, e.g. by using the porous medium approach.