FUEL ASSEMBLY AND NUCLEAR REACTOR

PURPOSE:To control a void rate in a wide region including a gap water region to contrive the increase of a spectrum shift effect by providing the lower portion of a water rod or the lower portion of a channel box with an inflammable poison and setting so as to burn its existence quantity out in an operation cycle. CONSTITUTION:A fuel assembly 1 consists of fuel rods 2, a water rod 3 and a channel box 4, an inflammable poison, for instance, structural materials 8 containing Gd2O3 are arranged in the lower portion of the channel box 4 and structural materials 9 containing the inflammable poison are also provided in the lower portion of the water rod 3. The quantity of the inflammable poison is set so as to burn out in an operation cycle. The void rate is increased during the beginning and middle period of the operation cycle to promote the conversion of U into Pu so as to burn the inflammable poison out during the last period thereof. Thereby, the void rate is controlled in a gap water region 6 to increase a spectrum shift effect.