A boiling water reactor for an exothermal reaction, said reactor comprising, • a reactor shell having a reactor shell volume arranged to hold a cooling medium under pressure, • said reactor shell volume comprising at least one cooling medium inlet and at least one cooling medium outlet, • said reactor shell comprising a reactant inlet and a product outlet, • a reaction enclosure embedded within said reactor shell volume, said reaction enclosure comprising a reaction zone with a plurality of reaction tubes for holding catalyst within the tubes, • an inlet manifold extending between said reactant inlet and said reaction zone, and an outlet manifold extending between said reaction zone and said product outlet, • an upper and a lower tube sheet, comprising holes for connection of each end of the reaction tubes to said tube sheets, - wherein said cooling medium is arranged to flow between said cooling medium inlet and said cooling medium outlet, around said reaction tubes, so that said reaction tubes are in thermal contact with said cooling medium, - wherein the boiling water reactor holds catalyst within the tubes and above the tubes on top of the upper tube sheet, and - wherein the boiling water reactor further comprises at least one reactant by-pass insert mounted at the top of said upper tube sheet adapted to by-pass a part of the reactant from the catalyst arranged on top of the upper tube sheet and into at least a number of the tubes. - wherein said at least one reactant by-pass comprises spacers on the outside to ensure a void between the outside of said at least one reactant by-pass insert and the inner side of the top end of the reaction tube. • a reactor shell having a reactor shell volume arranged to hold a cooling medium under pressure, • said reactor shell volume comprising at least one cooling medium inlet and at least one cooling medium outlet, • said reactor shell comprising a reactant inlet and a product outlet, • a reaction enclosure embedded within said reactor shell volume, said reaction enclosure comprising a reaction zone with a plurality of reaction tubes for holding catalyst within the tubes, • an inlet manifold extending between said reactant inlet and said reaction zone, and an outlet manifold extending between said reaction zone and said product outlet, • an upper and a lower tube sheet, comprising holes for connection of each end of the reaction tubes to said tube sheets, - wherein said cooling medium is arranged to flow between said cooling medium inlet and said cooling medium outlet, around said reaction tubes, so that said reaction tubes are in thermal contact with said cooling medium, - wherein the boiling water reactor holds catalyst within the tubes and above the tubes on top of the upper tube sheet, and - wherein the boiling water reactor further comprises at least one reactant by-pass insert mounted at the top of said upper tube sheet adapted to by-pass a part of the reactant from the catalyst arranged on top of the upper tube sheet and into at least a number of the tubes. - wherein said at least one reactant by-pass comprises spacers on the outside to ensure a void between the outside of said at least one reactant by-pass insert and the inner side of the top end of the reaction tube. - wherein said cooling medium is arranged to flow between said cooling medium inlet and said cooling medium outlet, around said reaction tubes, so that said reaction tubes are in thermal contact with said cooling medium, - wherein the boiling water reactor holds catalyst within the tubes and above the tubes on top of the upper tube sheet, and - wherein the boiling water reactor further comprises at least one reactant by-pass insert mounted at the top of said upper tube sheet adapted to by-pass a part of the reactant from the catalyst arranged on top of the upper tube sheet and into at least a number of the tubes. - wherein said at least one reactant by-pass comprises spacers on the outside to ensure a void between the outside of said at least one reactant by-pass insert and the inner side of the top end of the reaction tube.