# en fr Design of a new neutron delivery system for high flux source Conception dun nouveau système de distribution de neutrons pour source à haut flux

en fr Design of a new neutron delivery system for high flux source Conception dun nouveau système de distribution de neutrons pour source à haut flux - Download this document for free, or read online. Document in PDF available to download.

1 ILL - Institut Laue-Langevin

Abstract : The building of new experimental neutron beam facilities as well as the renewal programmes under development at some of the already existing installations have pinpointed the urgent need to develop neutron guide technology in order to make such neutron transport devices more efficient and durable. In fact, a number of mechanical failures of neutron guides have been reported by several research centres. It is therefore important to understand the behaviour of the glass substrates on top of which the neutron optics mirrors are deposited, and how these materials degrade under radiation conditions. The case of the European Spallation Source ESS, at present under construction at Lund, is a good example. It previews the deployment of neutron guides having more than 100~metres of length for most of the instruments. Also, the future renovation programme of the ILL, called Endurance, foresees the refurbishment of several beam lines. This Ph.D. thesis was the result of a collaboration agreement between the ILL and ESS-Bilbao, aiming to improve the performance and sustainability of future neutron delivery systems. Four different industrially produced alkali-borosilicate glasses were selected for this study: Borofloat, N-ZK7, N-BK7, and S-BSL7. The first three are well known within the neutron instrumentation community, as they have already been used in several installations; whereas the last one is, at present, considered a candidate for making future mirror substrates. All four glasses have a comparable content of boron oxide of about 10~mol.\%. The presence of such a strong neutron absorption element is in fact a mandatory component for the manufacturing of neutron guides, because it provides a radiological shielding for the environment. This benefit is, however, somewhat counterbalanced, since the resulting 10Bn,alpha7Li reactions degrade the glass due to the deposited energy of 2.5~MeV by the $\alpha$ particle and the recoil nuclei. In fact, the brittleness of some of these materials has been ascribed to this reaction.The methodology employed by this study consisted in understanding the general structure of borosilicates and how they behave under irradiation. Such materials have a microscopic structure strongly dependent upon their chemical content and particularly on the ratios between network formers and modifiers. The materials have been characterized by a suite of macroscopic and structural techniques such as hardness, TEM, Raman, SANS, etc, and their behaviour under irradiation was analysed. Macroscopic properties related to their resistance when used as guide structural elements were monitored. Also, changes in the vitreous structure due to radiation were observed by means of several experimental tools. For this purpose, an irradiation apparatus has been designed and manufactured to enable irradiation with thermal neutrons within the ILL reactor while keeping the samples below 100~\degree{}C. The main advantage of this equipment, compared to others previously available, was that it enabled the glass to reach an equivalent neutron dose to that accumulated after several years of use as guides, in just few days.The concurrent use of complementary characterization techniques lead to the discovery that the studied glasses were deeply different in terms of their glass network. This had a strong impact on their macroscopic properties and their behaviour under irradiation. This result was a surprise since, as stated above, some of these materials were well known by the neutron guide manufacturers, and were considered to be almost equivalent because of their similar boron oxide content. The N-BK7 and S-BSL7 materials appear to be fairly homogeneous glasses at different length scales. More specifically, at nanometre scales, silicon and boron oxide units seem to mix and generate larger structures somewhat resembling crystalline Reedmergnerite. In contrast, N-ZK7 and Borofloat are characterized by either silicon or boron rich domains.As one could expect, these drastic differences lead to their behaviour under thermal neutron flux. The results show that N-BK7 and S-BSL7 are structurally the most stable under radiation. Macroscopically, such stability results in the fact that these two materials show very slow swelling as a function of radiation dose. In contrast, the two other glasses are much more reactive. The whole glass structure compacts upon radiation. Specifically, the silica network and the boron units tend to blend, leading to an increase in density up to some saturation, followed by a very slow expansion of the same order as shown by N-BK7 and S-BSL7. Such findings allowed us to explain the drastic differences in the radiation limits for macroscopic surface splintering for these materials when they are used in neutron guides.

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Keywords : Glass Borosilicates glass Nuclear engineering research Neutron sources Irradiation effects

Mots-clés : Verre Effets d-irradiation Borosilicates Source neutrons

Author: Romain Boffy -

Source: https://hal.archives-ouvertes.fr/