SCK•CEN opted for liquid metal as coolant. Lead-bismuth eutectic (LBE) was selected due to its low melting temperature (124.5 °C), allowing the primary systems to function at rather low temperatures.
Inside the compact core geometry only an effective target diameter of about 88 mm is possible. With the given properties of the proton beam, this value leads to a beam current density of 65 µA/cm2. A windowless target design, i.e. without a physical separation between the accelerator beam line vacuum and the liquid target material, is proposed.
The sub-criticality level of around 0.95 has been considered as an appropriate level for a first of a kind medium-scale ADS.
MOX fast reactor fuel technology has been chosen due to the large experience in Europe and in particular in Belgium. A maximum plutonium enrichment of 35 % was considered based on the available manufacturing and qualification experience by Belgonucleaire in the past.
To profit from the thermal inertia provided by a large coolant volume, we opted for a pool-type system in which the components of the primary loop (pumps, heat exchangers, fuel handling tools, experimental rigs, etc.) are inserted from the top in penetrations in the cover. The loading of fuel assemblies is foreseen to be from underneath, which is not the classical approach of the sodium fast reactors. The reasons behind the approach are firstly to keep a large flexibility for the experimental devices loading from the top and secondly, from the safety point of view, the fact that all structures including the spallation module are in place before starting the core loading.
The pool vessel, which contains the MYRRHA core the internals, is located in an air-controlled containment environment. Furthermore, several factors lead to the decision to design both operation and maintenance (O&M) and In-Service Inspection & Repair (ISI&R) of MYRRHA with fully-remote handling systems.