Fuel assemblies for reactors

Purpose: To attain high burnup degree in nuclear fuel assemblies without increasing local power. Constitution: Taking notice of fundamental characteristics that the power of a fuel rod is dependent on the fuel enrichment degree and the volume of fuels other than the neutron fluxes, the local power is flattened by using fuels of small diameter in of low neutron flux regions and fuels of greater diameter in high neutron flux regions. As a result, requirements for varying the fuel enrichment degree for the high neutron flux regions and the low neutron flux regions independently can be moderated and the burnup degree can significantly be improved. In the case of a pressure tube reactor, 41,000 MW d/t can be attained which corresponds to about 37 % improvement even if U-235 weight is somewhat reduced and fissionable plutonium weight is made identical. Further, in the case of a BWR type reactor, 32,000 MW d/t, that is, about 10 % improvement can be expected. Further in the case of PWR type reactor, the maximum linear power density can be decreased to improve the fuel integrity. (Kawakami, Y.)