READY special report 1: Application of TRAB-3D to BWR and PWR transient calculations

TRAB-3D is the newest member in the reactor dynamics code family developed in VTT. It is now possible to analyze both PWRs and BWRs taking into account the real core loading. The core geometry can be either rectangular, as in most western reactors, or hexagonal, as in e.g. VVER reactors. In addition to the core models the codes include models for the cooling circuits and reactor control systems. This paper describes the neutronics/thermal-hydraulics coupling methodology and validation of the three-dimensional TRAB-3D for the application to PWR and BWR transient calculations. Code development in the fields of reactor physics and dynamics, as well as in thermal-hydraulics has been one of the key areas of reactor safety research in Finland since the middle of the seventies. In reactor dynamics, which combines neutron kinetics, heat conduction, heat transfer and hydraulics of the cooling circuit, the first product was a LWR core model (Rajamäki 1980a), which is basically one-dimensional two-group code, but includes a synthesis model for radially nonuniform dynamics. The same core model was applied in the development of the BWR dynamics code TRAB (Rajamäki 1980b and Räty et al. 1991), which is continuously used in the safety analysis of the Finnish TVO power plant. It models the core and the main circulation system inside the reactor vessel, including the steam dome with related systems, steam lines, recirculati-on pumps, incoming and outgoing flows, as well as control and protection systems. Typical applications are the pump trip and the steam line isolation. The code has also been used for the simulation of the RBMK type reactor in accident conditions. The HEXTRAN (Kyrki-Rajamäki 1995) core dynamics model has been developed on the basis of the stationary two-group diffusion code HEXBU-3D (Kaloinen et al. 1981) for hexagonal geometries, with the fuel heat conduction and channel hydraulics description of the TRAB code. TRAB-3D (Kaloinen & Kyrki-Rajamäki 1997) is a similar code used for rectangular lattice reactors. In addition to the core dynamics TRAB-3D also includes the BWR circuit models of TRAB. The validation work of TRAB-3D consists of the successful calculation of OECD benchmarks, the most recent of which has been the PWR main steam line break (MSLB) benchmark (Ivanov et al. 1999), as well as comparisons with measurements of real TVO plant transients (Daavittila et al. 2000). As the cooling circuit model of PWRs serves the SMABRE code (Miettinen 1985) coupled with HEXTRAN or TRAB-3D. SMABRE is a fast running simulation code and it was developed for such thermal-hydraulic accidents as small breaks, originally in order to facilitate parameter variations to support slow RELAP calculations. Its range of application was, however, soon extended to cover such types of accidents as small break LOCA, primary-secondary leak, steam line break, ATWS and most parts of a large break LOCA in VVER plants.