The invention concerns a spacer and a fuel assembly for a nuclear reactor. The spacer has a plurality of cells (10) for holding elongated elements. The cells (10) are defined by a plurality of side portions (12). At least one of said cells (10) has at least one side portion (12) from which a flow-influencing member (20) is formed in that it is folded out from the side portion (12). The side portion (12) from which the flow-influencing member (20) is folded out comprises a first (22) and a second (24) edge surface which are connected to each other and which correspond to the directions of a first (26) and a second (28) edge of said flow-influencing member (20) in a non folded out state of the flow-influencing member (20). The first edge surface (22) has an extension in at least a first direction (14) in which a cooling medium is intended to flow. The second edge surface (24) has an extension in a direction which is perpendicular to said first direction (14).