SECURE Project

Deliverable 3.1

EXECUTIVE SUMMARY

Production of W-188 is performed by irradiating enriched W-186 targets in high flux reactors. Traditionally, mainly tungsten oxide targets have been used for this purpose, but issues have occurred in the past where some ampoules have been found shattered after irradiation. The underlying mechanism that leads to such ampoule breakage has to be investigated to propose mitigation measures.

Nuclear heating of the targets leads to increased temperatures during irradiation, and temperature changes may induce phase transformations. The temperature distribution for actually performed irradiations has been determined by combining Monte Carlo simulations of nuclear heating with finite element calculations of heat dissipation.

During a high-fluence neutron irradiation, a significant part (order of 10%) of tungsten is transmuted into rhenium. This change in chemical composition, together with elevated sample temperatures, could lead to macroscopic and microscopic modifications of sample structure and chemical fractionation effects.

Two approaches were used to study such effects: a) Synthesis of surrogate W₁₋ₓReₓO₃ samples with controlled rhenium fraction x and their study by thermal neutron diffraction at ILL. b) Pre- and post-irradiation examinations of actual ¹⁸⁶WO₃ targets that had been irradiated at medium or high neutron fluence have been performed at SCK-CEN.

All obtained results point to a migration of Re out of the W₁₋ₓReₓO₃ samples and a redistribution in the ampoule in the form of volatile ReOᵧ oxides.

Funded by the European Union. Views and opinions expressed are, however, those of the author only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.