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

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.