RELAP5 analysis of hydrodynamic loads in response to PORV actuation

A fluid transient analysis was performed for the Power Operated Relief Valve (PORV) discharge piping to evaluate hydrodynamic loads caused by a sudden opening of the PORV within 0.05 s. The system fluid was immediately accelerated and a large axial load was observed. This result is expected with the criterion for the disturbance time. In this analysis, the disturbance time is the PORV opening time, which is faster than the pressure wave propagation time of 0.171 s. If the disturbance time is at least 10 times larger than the calculated pressure wave propagation time, the pressure wave propagation is insignificant. Given the complexity and highly transient nature of this phenomenon, a detailed calculation of the fluid transient was performed using the RELAP5/MOD3.3 (Patch 03) computer code. The capability of RELAP5 was previously validated for the analysis of large and rapid pressure waves propagating with the speed of sound along the reactor piping systems. The model was finely nodalized to allow the detailed computation of hydrodynamic loads with a maximum time step which is well below the acoustic courant limit. Consequently, it is important to ensure whether the hydrodynamic load will challenge the allowable stresses for the piping and its supports.