A cavitation reactor, comprising: - a stator, delimiting a chamber, the stator having a first and a second opening (25, 26) for a fluid to be introduced into and ejected from the chamber, and - a rotor comprising a drive shaft, connected to the stator and rotatable relative to the stator about a rotation axis (A-A) which extends in a longitudinal direction (X-X), the rotor comprising a centrifugal stage fixed to the drive shaft and housed in the chamber of the stator, the first opening of the stator having a lead-in portion facing the centrifugal stage in the longitudinal direction (X-X) and shaped to longitudinally guide the flow of fluid, wherein: - the centrifugal stage comprises a first wall proximal to the first opening of the stator and a second wall distal from the first opening of the stator, the first and second walls (41, 42) being arranged transverse to the longitudinal direction (X-X), and being spaced apart in the longitudinal direction (X-X) to define a gap therebetween, wherein each of the first and second walls (41, 42) has an inner surface, the inner surfaces of the first and second walls (41, 42) facing each other and facing the gap, the gap being delimited in the longitudinal direction (X-X) by the inner surfaces of the first and second walls (41, 42), the gap being a recess formed in the centrifugal stage between the first and the second wall (41, 42), - the centrifugal stage comprises a plurality of partitions in the gap which are circumferentially spaced apart about the rotation axis (A-A), the partitions dividing the gap into a plurality of compartments which extend between a central portion of the centrifugal stage, closer to the rotation axis (A-A), and a peripheral portion of the centrifugal stage, farther from the rotation axis (A-A), the compartments being in fluid communication with the chamber of the stator at the peripheral portion of the centrifugal stage, characterized in that: the first wall is closed at the central portion of the centrifugal stage thereby preventing the flow of fluid between the first opening of the stator and the peripheral portion of the centrifugal stage through the compartments of the gap. - a stator, delimiting a chamber, the stator having a first and a second opening (25, 26) for a fluid to be introduced into and ejected from the chamber, and - a rotor comprising a drive shaft, connected to the stator and rotatable relative to the stator about a rotation axis (A-A) which extends in a longitudinal direction (X-X), the rotor comprising a centrifugal stage fixed to the drive shaft and housed in the chamber of the stator, the first opening of the stator having a lead-in portion facing the centrifugal stage in the longitudinal direction (X-X) and shaped to longitudinally guide the flow of fluid, - the centrifugal stage comprises a first wall proximal to the first opening of the stator and a second wall distal from the first opening of the stator, the first and second walls (41, 42) being arranged transverse to the longitudinal direction (X-X), and being spaced apart in the longitudinal direction (X-X) to define a gap therebetween, wherein each of the first and second walls (41, 42) has an inner surface, the inner surfaces of the first and second walls (41, 42) facing each other and facing the gap, the gap being delimited in the longitudinal direction (X-X) by the inner surfaces of the first and second walls (41, 42), the gap being a recess formed in the centrifugal stage between the first and the second wall (41, 42), - the centrifugal stage comprises a plurality of partitions in the gap which are circumferentially spaced apart about the rotation axis (A-A), the partitions dividing the gap into a plurality of compartments which extend between a central portion of the centrifugal stage, closer to the rotation axis (A-A), and a peripheral portion of the centrifugal stage, farther from the rotation axis (A-A), the compartments being in fluid communication with the chamber of the stator at the peripheral portion of the centrifugal stage,