[en] Historically large physical problems have been divided into smaller problems based on the physics involved. This is no different in reactor safety analysis. The problem of analyzing a nuclear reactor for design basis accidents is performed by a handful of computer codes each solving a portion of the problem. Traditionally each of these codes operates independently from each other using only the global results from one calculation as boundary conditions to another. Industry's drive to up-rate power for reactors has motivated analysts to move from a conservative approach to design basis accident towards a best estimate method. To achieve a best estimate calculation efforts have been aimed at coupling the individual physics models to improve the accuracy of the analysis and reduce margins. The current coupling techniques are sequential in nature. During a calculation time-step data is passed between the two codes. The individual codes solve their portion of the calculation and converge to a solution before the calculation is allowed to proceed to the next time-step. This paper presents a fully implicit method of solving the neutron balance equations, heat conduction equations and the constitutive fluid dynamics equations. The paper discusses the problems involved in coupling different physics phenomena within multi-physics codes and presents a solution to these problems. The paper also outlines the basic concepts behind the nodal balance equations, heat transfer equations and the thermal hydraulic equations, which are coupled to form a fully implicit nonlinear system of equations. (authors)