Shortcomings of 1-D thermohydraulic transient analysis for a pool type reactor (like MYRRHA)

CFX and RELAP5 models produce the same result in normal operation, when the flow is driven by the pump. In natural circulation the total mass flow rate is noticeably varying according to the nodalization geometry: the RELAP5 Multi-channel models, as well as all the CFX models, provide a mass flow value much lower than the RELAP5 1-D model, the RELAP5 2-D Single-channel models. This is due to the lack of thermal stratification in the upper plenum domain and the absence of recirculation flow in the upper part of the reactor primary system. This phenomenon is taken into account in the RELAP5 multi-channel nodalization but peak temperature in the hot plenum is higher than in CFX: thermal conduction between the different temperature LBE streamlines in the above core structure produces a less sharp stratification in CFX. The Power step from Normal Operation results evidence that the RELAP5 2-D models provide a higher value for the characteristic period of the reactor than the CFX simulations do because of the 2-D symmetry of RELAP5 models. The stagnation of some domains of the model reduces the effective total flow area and shorten the actual flowing path.