Since 1998 SCK•CEN is developing the MYRRHA project as an accelerator driven system (ADS) based on the lead-bismuth eutectic (LBE) as a coolant of the reactor and a material for its spallation target. MYRRHA is a flexible fast-spectrum pool-type research irradiation facility, also serving since the FP5 EURATOM framework as the backbone of the Partitioning & Transmutation (P&T) strategy of the European Commission concerning the ADS development in the third pillar of this strategy. In this paper, we present recent investigations regarding the primary system design of MYRRHA, aiming at reducing the reactor vessel dimensions and at enhancing its safety features. In particular, we show the new designs of the heat exchanger layout, of the in-vessel fuel handling machine and the consequences on the diaphragm.