Coupled three dimensional neutronic and thermal-hydraulic system codes can improve the accuracy of reactivity induced accidents analysis in cases where the thermal-hydraulic feedback effects and influence of whole reactor coolant system determines nuclear reaction rate. Coupled code is developed using best-estimate thermal-hydraulic system code RELAP5/mod3.2, which provides thermal-hydraulic analysis for all components in the plant and include realistic description of plant protection and control systems. Two codes were used as neutronic part of coupled code, coarse mesh three dimensional neutronic diffusion code QUABOX/CUBBOX–HYCA, and three dimensional neutronic nodal diffusion code PARCS. In the coupled code RELAP5 calculates system thermal hydraulics, average core channel thermal hydraulics and heat conduction while QUABOX, or PARCS, calculates only 3D neutron kinetics. Basic interface module provides the interchange of the data between the two codes. The direct explicit type of coupling is used. RELAP5 takes the leading role in the coupled code and it is responsible for time stepping and organization of the calculation. In addition, version of the coupled code with subchannel COBRA based thermal-hydraulic core model was developed. All three coupled code versions have been verified using OECD TMI-1 MSLB benchmark calculation. The developed method for asymmetric reactor accidents calculation was applied for NPP Krsko steam line break accident analysis at HFP and HZP EOL conditions. The results obtained with coupled code demonstrated that local feedback effects suppress fission power increase during accident, resulting in lower calculated global power compared to point kinetics case.