INFLUENCE OF THE 3-D PHENOMENA ON THE SAFETY PARAMETERS DURING A ULOF ACCIDENT IN THE MYRRHA REACTOR

In a pool-type liquid metal cooled reactor like MYRRHA, the influence of 3-D thermal-hydraulic phenomena during Loss Of Flow (LOF) transients can have an important impact on safety-relevant parameters. 3-D temperature distributions and local pressure gradients may affect the evolution of the coolant mass flow during the transition from forced to natural convection, with the possible generation of flow instabilities and dissipating flows. Furthermore, the presence of stagnant volumes may influence the characteristic propagation time of perturbations through the system. The 1-D computational system codes which are used to perform safety analyses were originally developed for loop-type reactor designs that foresee the coolant flowing in pipes with energy losses mainly due to wall friction. Therefore this class of codes is not validated to simulate correctly the physics of the phenomena occurring in a pool-type reactor. The objective of this work is to assess the shortcomings of system codes in predicting the response of the MYRRHA reactor to a LOF event and to identify the 3-D safety-relevant phenomena that influence the transient evolution. The adopted strategy is comparing the transient simulation results of the RELAP5 thermal-hydraulic system code with reference CFD estimations. An ANSYS-CFX coarse-mesh CFD model and a RELAP5 1-D model of the MYRRHA primary system were built to perform the transient analysis. The scenario selected for this study is an Unprotected Loss Of Flow (ULOF) caused by the blockage of the two primary pumps: no coast-down is considered (blocked pump rotor condition). The CFD results were post-processed and averaged values of the main safety parameters were compared with the ones predicted by the system code