This whole project would not have been possible without Jérôme Beaucour and Jose Luis Martinez, who accepted me first as a trainee for a neutron guide design internship and then as a Ph.D. student on the related topic of mirror substrate study. The main work partner I had throughout these years was Michael Kreuz who has always been available to discuss scientific studies whether they were from his field of expertise or not, thank you very much. From the same ILL service, I would like to thank Giuliana Manzin and Benjamin Giroud, who advised me whenever I needed it. From the reactor division, I would like to especially thank Marc Samuel and his team, Elvio Mannino, Cédric Laurent and Renaud Silvestre, who assisted me on the T4 irradiations of this study. Also, I cannot forget Xavier Fargues, Frédérique Fréry, Véronique Caillot, and Bruno Desbrière for their help at some point on the hard path to send samples to irradiation in the ILL High Flux Reactor. In the same vein, I wish to thank Ulli Köster, who advised me a lot on in-situ flux monitoring with Zirconium foils and the perturbed flux issue. His explanations on various topics ranging from gamma activation analysis to radioactive material transport were always useful. In terms of radio-protection, I am happy to thank the constant help from Gilles Rignon and his team,

The behaviour of four alkali-borosilicate glasses under homogeneous thermal neutron irradiation has been studied. These materials are used for the manufacturing of neutron guides which are installed in most facilities as devices to transport neutrons from intense sources such as nuclear reactors or spallation sources up to scientific instruments. Several experimental techniques such as Raman, NMR, SANS and STEM have been employed in order to understand the rather different macroscopic behaviour under irradiation of materials that belong to a same glass family. The results have shown that the remarkable glass shrinking observed for neutron doses below 0.5 · 10 18 n/cm 2 critically depends upon the presence of domains where silicate and borate network do not mix.

a b s t r a c tIn this paper we report on a first part of a study on the mechanisms leading to brittle fracture in neutron guides made of glass as structural element. Such devices are widely used to deliver thermal and cold neutron beams to experimental lines in most large neutron research facilities. We present results on macroscopic properties of samples of guide glass substrates which are subjected to neutron irradiation at relatively large fluences. The results show a striking dependence of some of the macroscopic properties such as density, shape or surface curvature upon the specific chemical composition of a given glass. The relevance of the present findings for the installation of either replacement guides at the existing facilities or for the deployment of instruments for ongoing projects such as the European Spallation Source is briefly discussed.

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