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International audienceBesides their interesting optical properties, Rare Earths (RE) can be found abundantly as fission products at the end of the nuclearfuel cycle. After reprocessing, they are stored by dissolution in a borosilicate glassy matrix. RE are also used to simulateactinides when studying nuclear waste immobilization. It is very important to understand the environment of RE in such complexglasses. Moreover, composition changes can strongly affect RE solubility in the glass melt and RE-rich phases (such as REsilicateapatite) can crystallize during cooling. As boron oxide seems to play an important role on the solubility of RE bypreventing the crystallization of RE-rich phases, we focused our study on a system containing only B2O3 and RE2O3 (with RE=La or Nd), and more precisely on the RE-metaborate composition RE2O3.3B2O3. As this glass, which has a congruentcrystallization, is thought to have a structure similar to the corresponding crystalline phase REB3O6, we intended to go further inthe understanding of its structure using 11B MAS NMR, EXAFS at the Nd-LIII edge and optical absorption. The crystallizationbehavior of the glasses thermally treated was also studied using DTA, XRD and optical absorption

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