Effective multiplication of a core at start-up

Measurements have been made on operating power reactors, such as PHENDC, SUPER-PHENIX and PFR of the variation of reactivity with bum-up and with power level. 

Concerning the nuclear data in the JEF-2.2 library, a general conclusion was that the inelastic scattering cross-sections for many even mass nuclides were about a factor of 2 low because of the neglect of direct interaction effects in the nuclear models used to calculate these cross-sections. The contribution of inelastic scattering is of the order of 15% of the total fission product reactivity effect. The most important even mass nuclei are Mo-98, - 100, Ru-102, - 104, - 106, Pd-106, - 108, Xe-132, - 134, Nd-146, - 148 and Sm-152. Improved theoretical models have now been developed and validated by direct measurements of the differential cross-sections of Pd isotopes. As a consequence of the analyses of the integral measurements, it has been concluded that the contribution of fission products to the variation of reactivity with bum-up can be predicted to within 5% (Is) for a conventional fast reactor. 

Feasibility studies on the incineration of actinide isotopes and fission products are being made in several countries. In the case of fission products attention has been focused on the incineration of Tc-99 and 1-129. Following neutron capture both nuclides decay rapidly to the stable isotopes, Ru-lOO and Xe-130. One method being considered is the use of special moderating assemblies located in the breeder regions of fast reactors. (This technique has been used successfully in PHENDC for the production of Co-60 fi om Co-59). An irradiation test has been carried out in the Fast Flux Test Facility. The percentage of transmutations in a 10.5 equivalent full power days irradiation were about 0.7% for Tc-99 and 0.4% for 1-129. The 

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