Determination of the ⁶Li Content in the LiPb of a Neutronics Mock-Up of the European HCLL TBM

A consortium of several European laboratories has performed neutronics experiments with a representative mock-up of the European helium-cooled lithium-lead (HCLL) test blanket module (TBM) irradiated with DT neutrons from intense neutron generators. The aim of these experiments was to provide experimental data for checking nuclear data and calculational tools for the prediction accuracy of important parameters such as the tritium production rate and neutron and gamma-ray flux spectra. The mock-up consisted of bricks of solid LiPb arranged in layers separated by Eurofer sheets. The ⁶Li concentration in the LiPb determines the slow neutron flux distribution in the mock-up, and an accurate knowledge of this value is of paramount importance for the analysis of these neutronics experiments. The analysis of the tritium production rate experiments revealed discrepancies between the real ⁶Li concentration and the one specified by the manufacturer of the LiPb (natural Li composition). Here we report on the investigation of the ⁶Li concentration in the LiPb with several experimental techniques: 1) time-of-arrival neutron spectra measured inside the mock-up irradiated with short pulses of 14-MeV neutrons from a DT neutron generator, 2) transmission measurements on LiPb bricks with moderated neutrons from an AmBe source to check for differences between bricks, and 3) mass spectroscopic methods on small samples taken from selected LiPb bricks. We found that the ⁶Li concentration varies only very little between the bricks. The weight fraction of lithium in the LiPb was 0.61 wt% as quoted by the manufacturer, but the ⁶Li abundance was half of the natural value in lithium, 3.8 at% instead of 7.5 at%.