In the framework of pre-licensing phase of MYRRHA, source term assessment is carried out for the MOX fuel and LBE coolant to provide safety relevant pieces of information. The buildup of activation and spallation products in the LBE during operation and after shutdown of the reactor was estimated. The attention was pointed to the sub-critical configuration, in which higher particle fluxes are expected with enhanced production of safety concerning radionuclides. In the spallation target assembly, in which there is the linear accelerator, the worst conditions are expected and we focused on the estimation of nuclide inventory in this region. Among safety concerning volatile nuclides H-3, Hg-195, Hg-197 and Po-210 convey the largest dose.