Different Simulations of the Phase 2 of the OECD/NRC BWR Turbine Trip Benchmark with the Code DYN3D

The OECD/NRC Boiling Water Reactor (BWR) Turbine Trip (TT) Benchmark based on the turbine trip test 2 (TT2) in the reactor Peach Bottom 2 [1] is approved for vali-dating the coupled system thermal-hydraulic and 3D neutron kinetics codes for BWR's. The phase 2 of the benchmark consists in the calculation of the core re-sponse for given thermal-hydraulic boundary conditions. This part of the benchmark is used for the validation of the DYN3D code [2].

The transient was initiated by the closure of the turbine stop valve. The pressure wave, which is moved to the core is attenuated by the opening of the bypass valve. When the wave reaches the core the void in the core is reduced, which results in an increase of the reactivity and power. The power peak is limited by the Doppler effect and the reactor scram.

The standard calculation with DYN3D is based on the consideration of 764 coolant channels (1 channel per fuel assembly), the consideration of the assembly disconti-nuity factors (ADF), the phase slip model of MOLOCNIKOV. The consideration of assembly discontinuity factors (ADF) is possible not in all three-dimensional codes. DYN3D allows calculations with and without the ADF to study their influence on this transient. Several participants of the benchmark perform calculations with 33 ther-mal-hydraulic channels, which correspond to the thermal-hydraulic map used in the TRAC-BF1/NEM model [1]. The influence of the number of coolant channels is stud-ied also in this paper. The phase slip model of MOLOCHNIKOV [3] is the standard model of DYN3D for void fraction calculation. A comparison was made with the ZUBER-FINDLAY model [4]. The cross sections sets were condensed over radial planes to generate sets for the one-dimensional simulation, which was also per-formed with DYN3D. The results of the different options are compared with the re-sults of the standard calculation.