Qualification of Spatial Coupling of System Reactor Simulators for BWR Stability
Tutor / director / evaluatorKozlowski, Tomasz
Document typeMaster thesis (pre-Bologna period)
Rights accessRestricted access - confidentiality agreement
Boiling Water Reactor (BWR) stability is a very complex and relevant issue in nuclear reactor safety, as well as a challenge for current simulation codes. The presence of reactivity unstabilities in the core requires the thermal-hydraulics (TH) codes to be coupled with full three dimensional neutron kinetics (NK) core models. The spatial description of the reactor core is different for TH and NK models. Usually TH code uses fewer channels to represent the core, whereas NK code uses one explicit node for each fuel assembly. Therefore, a spatial mapping of coarse grid TH and fine grid NK domain is necessary. It is well-known that a simpler TH model will significantly reduce the computational time. However, improper mappings may result in loss of valuable information, thus causing inaccurate prediction of safety parameters. The purpose of this work is to study the possibilities of spatial coupling and develop recommendations for NK-TH mapping for simulating unstability transients. In this work, control rod perturbation (CRP) transient is performed with coupled codes TRACE/PARCS for Ringhals-1 Cycle 14 to determine decay ratio (DR) and natural frequency (NF) using several spatial coupling. The effectiveness and accuracy of predicted DR and NF are compared to a reference case of 325 thermo-hydraulic channels (THC). A model with a 5 THC grouped according to assembly position and radial power show fairly accurate results, with a range of discrepancies of the same order as the reference case, and more than 10 times less computational time.