Some comments on the behavior of the RELAP5 numerical scheme at very smalltime steps

The behavior of the RELAP5 code at very short time steps is described i.e., Delta t approximate to 0.01 Delta x/c. First. the property of the RELAPS c ode to trace acoustic waves with "almost" second-order accuracy is demonstr ated Quasi-second-order accuracy is usually achieved for acoustic waves at very short time steps but can never be achieved for the propagation of nona coustic temperature and void fraction waves. While this feature may be bene ficial for the simulations of fast transients describing pressure waves, it also has an adverse effect: The lack of numerical diffusion at very short time steps can cause typical second-order numerical oscillations near steep pressure jumps. This behavior explains why an automatic halving of the tim e step, which is used in RELAP5 when numerical difficulties are encountered in some cases leads to the failure of the simulation. Second, the integration of the stiff interphase exchange terms in RELAP5 is studied. For transients with flashing and/or rapid condensation as the mai n phenomena, results strongly depend on the time step used. Poor accuracy i s achieved with "normal" time steps (Delta t approximate to Delta x/v) beca use of the very short characteristic timescale of the interphase mass and h eat transfer sources. In such cases significantly different results are pre dicted,pith very short time steps because of the more accurate integration of the stiff interphase exchange terms.