The accelerator is the driver of the ADS system. It provides the high energy protons that are used in the spallation target to create neutrons which in their turn feed the sub-critical core. In the current reference design of the MYRRHA accelerator, the machine will be able to provide a proton beam with an energy of 600 MeV and a maximum current of 4 mA. Both cyclotron and linear accelerator types may be envisaged for providing this beam. The choice will be dominated by the stringent operational requirements set upon the accelerator by its application as a driver for ADS.
The right beam energy is a compromise between different competing considerations.
The neutron yield: The number of neutrons produced per incident proton increases with energy (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 the highest neutron flux density is requested, a larger target size may be a disadvantage.
He and H production in structure materials: A higher energy proton beam will generate a harder neutron spectrum, i.e. with relatively more high energy neutrons. These neutrons significantly increase the rate with which (through nuclear reactions) hydrogen and helium gas are produced in the steel of the structure materials. Because this gas cannot readily escape, it causes swelling and a general degradation of the material strength.
Accelerator construction costs: More beam energy will require a larger accelerator and a higher construction cost.
Accelerator technology: From a technological point of view it is easier to increase the beam energy than to increase the beam current.
The required beam intensity is given by the amount of neutrons to be produced in the spallation target, which is directly proportional to the beam intensity for a fixed beam energy. The value is set by the core design and the neutron flux requirements of the ADS.
The correct beam shape and profile on target must be defined so as to yield an optimal efficiency while preserving the integrity of the target and of its surroundings. This implies that the proton beam should not be allowed to hit any area with deficient cooling capability. This requirement is obtained by a specific beam scanning system in combination with a beam line design which guarantees the stability of the beam position.
The beam availability must reach a level which is typically an order of magnitude better than the present day state-of-the-art. The number of beam trips longer than 3 seconds has to be reduced to less than 1 per week. This requirement is strongly related to the thermal shocks which a beam interruption causes in an ADS, adversely affecting structural materials of the reactor and possibly causing safety issues. Also the operability of the plant requires an extremely high availability of the proton beam. The identified way of achieving this is called "fault tolerance".
More information: The LINAC for MYRRHA