Fuel assembly

In the channel box of a fuel assembly in accordance with this invention, the wall thickness is increased at the upper stream along the flow of coolant and gradually reduced toward the down stream, so that the transverse cross sectional area at the inside of the channel box is increased. Thus, since the flow channel area is enlarged in the channel box toward in the downstream, the thermal margin of the fuel assembly is improved to increase the margin for securing fuel integrity. Further, it is possible to improve the reactor core reactivity by increasing the flow rate of the coolant and lowering the void coefficient at the final stage of the cycle, thereby increasing the period of cycle and improving the economy of power generation. (T.M.)