Experience with in-pile fission targets at LOHENGRIN

The LOHENGRIN fission fragment separator uses actinide targets in a neutron flux of about 5×1014neutrons/cm2/s in an in-pile position of the high-flux reactor of ILL Grenoble. For fission yield measurements relatively thin targets (tens of μg/cm2) are used, while for nuclear spectroscopy applications targets up to 1mg/cm2 are employed. This leads to fission rates up to 5×1012/s. The targets are heated by the fission power in vacuum to temperatures of up to 1000∘C. The radiation damage caused by the fission fragments can reach 50 dpa (displacements per atom) per day, an extremely high value comparable to that caused by irradiation with intense heavy ion beams. Therefore the thick targets that were produced with different methods (painting, spray-painting, electrolysis and molecular plating) all suffer from a burnup that is much quicker than explainable by nuclear transmutation. We discuss physical effects responsible for this additional decrease in fission fragment rate and ways to improve the situation.