Boiling Water Reactor (BWR) stability is a very complex and relevant issue in nuclear reactorsafety, as well as a challenge for current simulation codes. The presence of reactivity unstabilitiesin the core requires the thermal-hydraulics (TH) codes to be coupled with full three dimensionalneutron kinetics (NK) core models. The spatial description of the reactor core is different for THand NK models. Usually TH code uses fewer channels to represent the core, whereas NK codeuses one explicit node for each fuel assembly. Therefore, a spatial mapping of coarse grid TH andfine 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 inaccurateprediction of safety parameters. The purpose of this work is to study the possibilities of spatialcoupling and develop recommendations for NK-TH mapping for simulating unstability transients.In this work, control rod perturbation (CRP) transient is performed with coupled codesTRACE/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 arecompared to a reference case of 325 thermo-hydraulic channels (THC). A model with a 5 THCgrouped according to assembly position and radial power show fairly accurate results, with arange of discrepancies of the same order as the reference case, and more than 10 times lesscomputational time.