[en] The study of contact phenomena, such as stress wave propagation, contact time and rebound velocity, as well as the numerical method suitable for this study is presented. A matrix-vector modification technique, or the Sherman-Morrison method, in conjunction with the finite element method and Newmark's integration scheme is used. In using finite element methods for structural analysis, the hexagonal graphite block containing fuel holes and coolant holes is idealized by thousands of elements and nodes. The corresponding matrix-vector equation, derived from the variational principle, includes likewise many thousands of components. As the surface of the structure comes in contact with an other structure, the boundary condition changes are represented by a localized modification to the matrix-vector equation. In considering local plastic deformation, computations are minimized by expressing the relevant modification to the element stiffness matrix in terms of the product of a vector and its transpose. The modification is incorporated into the solution in a manner such that the frequent contact-release phenomenon can be treated with ease. Convergence of the procedure is demonstrated by solving Hertzian problems. Agreement with Hertz' theorem is obtained with a relatively crude mesh representation and time step size. Numerical results of corner and flat face impact of similar and dissimilar blocks are presented. A longer contact time and higher peak stress are found as expected for the corner impact. The rebound velocity appears to be lower for blocks with holes than for blocks without holes. Results for a 7-hole block are compared with experiments. The significance of this comparison is discussed