A reactor vessel (1) for biomass material, wherein said reactor vessel (1) is a pressurized reactor vessel, said reactor vessel being elongated and comprising: a substantially tubular vessel part (3); two gables (5a, 5b) connected one to each end (19a, 19b) of the tubular vessel part (3), hereby enclosing a vessel internal compartment (7); and a material transporting screw (8) comprising a central shaft (9) provided within the vessel internal compartment (7) extending between the two gables (5a, 5b) along a central axis (A) of the reactor vessel (1), wherein said material transporting screw further comprises screw flight (11) provided around the shaft (9), wherein said material transporting screw is configured for transporting the biomass material through the reactor vessel, wherein both gables (5a, 5b) are inwardly dished.