A study was made to determine the changes in fast flux intensity and spectra in several irradiation positions of the General Electric Test Reactor caused by varying the material composition in an irradiation cell or in surrounding reactor positions. The material changes are typical of those that could be expected to occur during a long-term irradiation in a test reactor. It has been concluded that material changes within an irradiation cell do perturb the fast neutron spectrum and change the fast flux within that cell. Changes made in surrounding cells do not significantly alter the spectrum in the irradiation cell. Consequently, the fast flux based on the spectral-averaged cross section and the damage indexes used for correlation of data on materials are not altered significantly. Therefore, once a detailed spectral calculation has been made, a new calculation is not required for each change in core loading.