Closure plate for preloading a plate reactor comprising a stack of plates for planar flow of fluid or fluids, said flow being bounded by said plates, wherein said plates have a given width and a given length, wherein the closure plate comprises a sheet with a face to face against a top of said stack, wherein the face has a convexity lengthwise said sheet, said convexity having a convex cross-section. Plate reactor comprising one said stack of plates and said one or two closure plates for preloading. Method of manufacturing a respective closure plate for preloading and a respective plate reactor.