3-D numerical studies of single control rod withdrawal transients in an LBE cooled critical reactor

• SIMMER-IV code is applied to analyze the MYRRHA critical core. • Single control rod withdrawal (CRWD) transients are calculated by the 3-D thermal hydraulics-neutronics coupled code. • Its transient 3-D effects on the power shape are investigated and presented. • The difference between dynamic and static reactivity due to CRWD is presented. • The potential of the reactor against a severe accident is demonstrated with a higher CRWD worth. The presented studies are carried out within the EU project ANSELMUS. A recent design version of MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications), which is a Lead-Bismuth-Eutectic (LBE) cooled reactor developed at SCK-CEN (Belgian Nuclear Research Centre), is investigated. The SIMMER-IV code is employed for 3-D simulations of single control rod withdrawal (CRWD) transients at the critical operation mode. A new CRWD model for the SIMMER-IV code is developed, so that the CRWD can be simulated for any constant withdrawal speed and from any initial position. The basic case is the complete withdrawal of a control rod (CR) filled with B4C absorber with the natural boron, where the reactivity worth is about 0.9 $, from a fully inserted position within 3 s. Cases with and without scram after 3 s are considered. The spatial kinetics effects on the power distribution are evaluated by comparing relative variations in time of local power densities and of the total one. The dynamic reactivity values during CRWD have been confirmed to be close to those obtained by static calculations. The transient with the scram at 3 s results in nothing severe, but that without the scram leads to local fuel melting. A further example, where the CR is filled with an enriched by B-10 absorber, its reactivity worth being 1.7 $, is calculated and shown as well. The withdrawal leads to a severe accident with fuel pin degradation, but without prompt supercritical power excursion. The numerical scenarios are presented and investigated by means of parametric studies.