Fuel rod spacer

A plurality of cylindrical cells each having a fuel rod inserted therethrough and kept at a predetermined position are arranged in longitudinal and lateral directions such that they are in adjacent with each other on the side walls and joined integrally at the inside of side frames constituted as a rectangular frame. Further, protrusions are formed in the circumferential direction of the inner walls of the cylindrical cells, so that the flowing direction of coolant liquid films moving axially along the inner walls of the cylindrical cells is directed to the direction of the outer surface of the fuel rod. This enables sufficient contact of the coolant with the fuel rod, thereby effectively lowering the temperature at the outer surface of the fuel rod. That is, it is possible to improve the heat removing performance of the fuel rod and improve the critical power of the fuel assembly. (T.M.)