Fission product yields: Minor actinides up to 150 MeV

A multinational team with the appropriate expertise participated in a work programme aimed at the development of systematics and nuclear models to assist in the evaluation of energy dependent fission yields for incident neutron energies of up to 150 MeV. New concepts for both systematics and theoretical models were developed, as described in later sections of this report. Various predictions of the fission product mass distributions were compared in a benchmark exercise that gave remarkably good results below 50 MeV. Reasons for the discrepancies at higher energies and some failures of the model predictions are discussed in Sections 5 and 6, pointing the way towards future investigations and fission yield evaluations. The problem of the disposal of nuclear waste is an important feature of the utilization of nuclear power. Thus, during the course of the 1980s and 1990s international efforts concentrated on new concepts of waste removal through transmutation. Several proposals were made, using classical thermal or fast reactors as incinerators, or accelerator driven systems. These highly technical proposals are being studied with regard to their feasibility, neutron economics and environmental safety, and nuclear data (including fission yields) are required for assessments and for the design and operational requirements of such facilities. While adequate nuclear data exist for thermal and fast reactor incineration, this satisfactory situation is not the case for accelerator driven systems. An international working group has studied the overall problem and recommended the assembly of the required nuclear data at intermediate incident neutron energies of up to at least 150 MeV. The staff at the IAEA have also assessed this situation in conjunction with appropriate external expertise and decided to contribute to this effort by the initiation of a coordinated research project (CRP) on fission yields up to 150 MeV. Evaluated fission yields as a function of incident neutron energy have to be presented in a way that any fission yield from any target nuclide can be obtained for any desired energy up to 150 MeV. Previous evaluations have focused on thermal, fast and 'high' (14-15 MeV) neutrons, as individual data sets. New concepts are required for the presentation of energy dependent fission yields, such as: (a) Functions for the description of energy dependent yields; (b) Yield sets at energy intervals with interpolation formulae; (c) Computer programs using systematics and/or theoretical models, with the desired fission yields as output. Any evaluation method will also require new procedures, and previous methods of analysis will not be feasible because experimental data are so scarce. Fission yield measurements are impossible for certain targets, while measurements at higher incident neutron energies are extremely difficult for other targets. Therefore, sufficient coverage of the desired neutron energy range by experimental data cannot be expected. Thus nuclear models and systematics need to be developed from the available experimental data to allow the calculation of fission yields and assist in future evaluations of energy dependent fission yields. The primary objectives of the CRP were to study all the problems involved in the development of nuclear models and systematics and to derive method for fission yield evaluation as a function of incident neutron energy. Hopefully such efforts will result in a new evaluation of the energy dependent neutron induced fission yields up to 150 MeV. However, this CRP entered an entirely new field of research, as usable models and systematics do not exist over such an energy range and the outcome of the work was deemed to be unpredictable. Therefore, the goals of the CRP were subsequently limited to the development of appropriate nuclear models and systematics for the prediction of fission yields as tools for the evaluation of energy dependent fission yields up to 150 MeV. (author)