A reactor for treating a material in a medium. It includes a body (2) in which is defined a reaction area (10) capable of containing a reaction medium, at least one inlet for introducing the reaction medium into the reaction area, one outlet for discharging effluents out of the reaction area. A protective casing (8) positioned inside the body (2) delimits the reaction area (10). It is spaced apart from the body in order to delimit a confinement area (12) isolating the reaction area of the body. The reaction area and the confinement area are sealably isolated from each other.