Neutron source strength in FFTF fuel as a function of irradiation

The neutron source strength associated with fuel irradiated in the FFTF reactor is important in determining the reactivity of the core when it is shut down or being refueled. The monitoring of subcritical reactivity for FFTF will be done with the Modified Source Multiplication (MSM) technique which requires an accurate knowledge of the neutron source strength in high exposure fuel elements replaced with fresh fuel elements in a shut down reactor. As a means of obtaining the most accurate source evaluations possible with existing nuclear data for FFTF irradiated fuel, a technique for generating one-group actinide cross sections for use in isotopic buildup and depletion codes was developed. A further objective of the work was to use the cross sections and recently evaluated spontaneous fission and (..cap alpha..,n) reaction neutron yield data to calculate the neutron source strength in typical compositions of FFTF-type fuel as a function of flux level and irradiation time and then to compare the results with those of a previous study to assess the differences in the nuclear data. This paper will highlight the important aspects of this study and present some results.