The selected proton energy in MYRRHA is 600 MeV. This value is the result of an "optimisation" process in which several arguments were considered. Neutron yield
The number of neutrons produced per incident proton increases with energy but not linearly. Target size and design
A higher energy proton beam requires a larger spallation target zone in both axial and radial directions because more target material is needed to absorb the beam energy. When a maximal neutron flux density is required for irradiation purposes, a larger target size is not necessarily an advantage. He and H production in structure materials
A high energy proton beam will tend to produce relatively more neutrons with a high energy above several MeV. These neutrons in particular significantly increase the rate with which trough nuclear reactions, hydrogen and helium gas are produced in the steel of the structure materials. Because the gas cannot readily escape, it causes swelling and a general degradation of the material strength. Accelerator construction costs
It is clear that more beam energy will require a larger accelerator and thus a higher construction cost. In addition, more energy in the beam also means higher energy radiation and thus shielding must be increased accordingly.
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