Fast-Thermal Coupled System for Integral Measurements of Fission-Product Cross-Sections

In the framework of the co-operation of the West Germans, Belgians and Dutch on fast-breeder research a feasibility study has been initiated on the possibilities for integral measurements of fission-product cross-sections for fast reactor systems. A recent study (Rep. GEAP-4472) suggests that uncertainties may exist regarding, for instance, coolant void coefficients of 10$ for a sodium-cooled and of 20$ for a steam-cooled design. Some of these uncertainties are caused by inaccuracies in cross-sections for fissile and fertile isotopes, and can be reduced considerably by results from critical experiments. The remainder (up to 4$ for sodium and up to 6$ for steam-cooled breeders) is mainly due to inaccuracies in cross-sections of fission products. Because of the high radioactivity, direct measurements in a critical experiment with actual fission products are impossible. The use of mock-ups of fission products still leaves big uncertainties because of the differences in isotopic compositions of natural elements occurring in fission products. It is therefore proposed to convert the existing, heavily shielded, light-water critical experiment KRITO at the Petten site of the Reactor Centrum Nederland into a coupled fast-thermal system, in the fast zone of which by oscillator methods integral cross-sect ions of actual fission product mixtures, elements and nuclides, are to be measured by oscillator methods. To specify the compositions of the fast zone a number of 26 group diffusion calculations have been performed based on the ABBN set. Regarding the influence of fission products on the coolant void effect, which is the major effect to be looked at, it appeared that 90% of the influence is caused by neutron absorption in the range of 2 keV to 50 eV for a sodium-cooled 1000-MW(e) breeder and in the range of 6 keV to 10 eV for a steam-cooled 1000-MW(e) breeder. Compositions were therefore sought for the fast zone giving about the same reactivity versus energy response to a sample of fission products introduced in the centre of the fast zone. It was found that compositions consisting of 90% enriched uranium with graphite in atomic ratios varying from 25 to 50 graphite atoms per 235U atom, and with a slight addition of polyethylene (from zero to one H atom per 235U atom) will produce the wanted spectra and energy-reactivity response for the oscillator. For such compositions the sensitivity of the oscillator will be of the order 1 to 3 x 10-6δk/k per gram of fission product mixtures for a coupled system which derives 50 to 75% of its reactivity from the fast zone. A design has been made which can accommodate the variable fast zone sizes needed for the various spectra. The programme of the measurements is outlined. At the same time a small heavily shielded facility is being designed for chemical separation of fission product elements from reactor wastes and from 239Pu and 235U irradiations in the high flux reactor at Petten. In addition to the fission product cross-sections, integral cross-sections for other materials and even Doppler coefficients could be measured in the proposed system. (author)