RETRAN-3D Analysis Of The OECD/NRC Peach Bottom 2 Turbine Trip Benchmark

This paper presents the PSI results on the different Phases of the Peach Bottom BWR Turbine Trip Benchmark using the RETRAN-3D code. In the first part of the paper, the analysis of Phase 1 is presented, in which the system pressure is predicted based on a pre-defined core power distribution. These calculations demonstrate the importance of accurate modelling of the non-equilibrium effects within the steam separator region. In the second part, a selection of the RETRAN-3D results for Phase 2 are given, where the power is predicted using a 3-D core with pre-defined core flow and pressure boundary conditions. A comparison of calculations using the different (Benchmark-specified) boundary conditions illustrates the sensitivity of the power maximum on the various resultant system parameters. In the third part of the paper, the results of the Phase 3 calculation are presented. This phase, which is a combination of the analytical work of Phases 1 and 2, gives good agreement with the measured data. The coupling of the pressure and flow oscillations in the steam line, the mass balance in the core, the (void) reactivity and the core power are all discussed. It is shown that the reactivity effects resulting from the change in the core void can explain the overall behaviour of the transient prior to the reactor scram. The time-dependent, normalized power for different thermal-hydraulic channels in the core is discussed in some detail. Up to the time of reactor scram, the power change was similar in all channels, with differences of the order of only a few percent. The axial shape of the channel powers at the time of maximum (overall) power increased in the core centre (compared with the shape at time zero). These changes occur as a consequence of the relative change in the channel void, which is largest in the region of the onset of boiling, and the influence on the different fuel assemblies of the complex ring pattern of the control rods. (author)