Fuel assembly

Fuel pellets at a specified vertical reactor core position of a portion of fuel rods in a fuel assembly are substituted with simulated pellets capable of changing the water/uranium ratio and having compatibility (zircaloy, alumina or neutron absorber). Since local distortion is caused in the neutron flux distribution, it can be utilized for compensating the vertical reactor core position of the neutron flux distribution. Further, when fuel pellets are substituted with simulated pellets at several positions in the vertical reactor core direction, the amount of positional collection for the measured neutron flux distribution can be made as an average value for the amount of correction due to distortions at several positions to further improve the accuracy of compensation. (T.M.)