[en] Security in nuclear power plants demands severe limitations of the maximal drop time of rod cluster control assemblies. In February 1995, several assemblies of the Chinese plant in DAYA BAY failed to comply with these requirements. Electricite de France undertook a research program to get a better insight of this problem since the plant has been built by French and also because the French new 4 loops N4 reactor was equipped with the same guide tubes. This paper is limited to a numerical study of the influence of the pressure forces applied to control rods and due to flow circulation through the guide tubes. After a validation test-case, a first calculation has been carried out on a simplified N4 guide tube. The sensitivity of the pressure forces to transverse flow and to modifications of the geometry has been determined. The program has been extended to guide tubes used in 1300 MW reactors and similar computations have been done. To make simulations more representative, a global computation of the flow in the whole upper internals plenum (UIP) will be achieved to provide accurate boundary conditions for local calculations with better resolution. (author)