STABILITY, ACCURACY, AND CONVERGENCE OF THE NUMERICAL-METHODS IN RELAP5 MOD3/

Both theoretical and numerical results on the relationships between th e magnitude of the interphase drag coefficients, the mesh size, and th e stability of the semi-implicit method used in RELAP5 are presented. It is shown that the numerical solutions are both stable and convergen t on meshes with a characteristic ratio (ratio of mesh size-to-hydraul ic diameter) that is not too small, that the code is capable of simula ting physical instabilities on coarse meshes, and that unphysical inst abilities will occur only at small mesh size even for problems that ad mit physical instabilities. Good transition from pre-critical heat flu x (CHF) to post-CHF, however, is necessary to improve the accuracy of certain calculations.