Plutonium Solubility and Self-Irradiation Effects in Borosilicate Glass




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НазваниеPlutonium Solubility and Self-Irradiation Effects in Borosilicate Glass
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Plutonium Solubility and Self-Irradiation Effects in Borosilicate Glass

X. Deschanels, S. Peuget, J.N. Cachia, T. Charpentier2

CEA Valrhô Marcoule, BP 17171, 30207 Bagnols-sur-Cèze, France
Tel 33 4 66 79 60 87, Fax 33 4 66 79 77 08

E-mail: xavier.deschanels@cea.fr or sylvain.peuget@cea.fr

2 Laboratoire de Structure et dynamique par Résonance Magnétique

DSM/DRECAM/SCM – CEA Saclay

CEA-CNRS URA 331

91191 Gif-sur-Yvette cedex


Abstract


This article discusses the solubility of plutonium or other elements (actinides and surrogates) in borosilicate glass. The effect of the temperature and redox conditions during glass processing was studied. The results show that trivalent elements (La, Gd, Nd, etc.) exhibit greater solubility than tetravalent elements (Pu, Th, Hf). Fabricating the plutonium-doped glass samples under reductive conditions reduced the Pu to oxidation state (III) and increased its solubility to 4 wt% PuO2 without reaching the solubility limit. A structural approach based on the results of glass characterization by EXAFS and NMR spectroscopy suggests that the structural role of the trivalent and tetravalent elements corresponds to that of intermediate network modifiers and intermediate network formers, respectively. The second focus of this article is the effect of actinide decay on the long-term behavior of the glass. Borosilicate glass samples were doped with different curium content (0.05 wt%, 0.5 wt%, 1.5 wt% and 4.1 wt% of CmO2). The macroscopic properties (density, microhardness and initial dissolution rate) of the glasses were characterized up to 3 × 1018 α·g 1. No significant effect on the initial alteration rate was detected. The glass swelled slightly, saturating at about 0.5% after receiving a dose of about 2 × 1018 α·g 1.

Further studies are still necessary to confirm the satisfactory long-term behavior of the borosilicate glass matrix at higher doses, and to determined the solubility limit of plutonium in reducing conditions.

Keywords: Borosilicate glass, actinides, self-irradiation, plutonium solubility, macroscopic properties, dissolution rate, radiation damage

PACS codes: 28.41.Kw; 61.43.Fs; 81.05.Kf

1.Introduction


A possible plutonium management solution investigated in other countries, consists in conditioning it in a glass or crystalline matrix [1]. France has opted to reuse plutonium in MOX fuel. However, the borosilicate glass used to condition high-level waste—consisting mainly of fission product solutions—has several advantages for conditioning plutonium as well as the minor actinides (Np, Am, Cm). Its structure is capable of incorporating a large number of elements due to the disordered nature of the glass network. The borosilicate glass used to condition high-level waste contains only small quantities of plutonium and minor actinides—well below 1 wt%. Two main aspects must be assessed for plutonium conditioning purposes.

The first concerns plutonium loading in the glass and determining the solubility limit. The resulting glass must be chemically and microstructurally homogeneous. The presence of heterogeneities (crystalline phases, demixing) could raise problems at the phase interfaces, for example due to differential swelling or stress corrosion. Relatively little work has been published concerning Pu solubility in borosilicate glass [2-13]. Plutonium can generally be loaded in glass only in relatively small amounts, on the order of a few weight percent. Up to 11 wt% plutonium oxide has successfully been incorporated in an aluminosilicate glass rich in rare earth elements, known as Löffler (lanthanum borosilicate: LaBS) glass [11-12]. A single reference [6] reports higher plutonium loading in an oxide glass; this result was obtained by reducing the plutonium to its trivalent state. Under these conditions the plutonium content in the glass ranged from about 10 to 25 wt%. Not only the glass composition but also the plutonium oxidation state have has a major effect on the loading limit. The influence of the oxidation state on uranium solubility has also been noted by Schreiber [14]: the U(VI) solubility is close to 40 wt% UO2 (uranyl chain structure) while that of reduced U(IV) not exceed 9 wt%. A structural approach based on XAFS and NMR analyses were also performed to evaluate the position of the cation in the glass network in function of its oxidation state.

The second factor in the assessment is the glass behavior under self-irradiation. Considerable work has been done in recent decades to guarantee the long-term behavior of the glass used for conditioning high-level waste. Concerning changes in the macroscopic properties of glass under self-irradiation—the main subject of these articles— references [15-23] discuss the results obtained for integrated dose not exceeding 1019 α·g-1, and the bibliographic survey articles [24-26] summarize these findings. The principal source of radioactivity in the glass is α and  decay from the fission products, together with alpha decay from the actinides.  decay and the emission of alpha particles result in significant heating of the glass as well as electronic excitation effects without major consequences on the integrity and macroscopic properties of the glass. However, alpha decay and to a lesser extent  decay cause atomic displacements. A recoil nucleus produces about 1500 atomic displacements with each alpha disintegration, creating considerable disorder in the glass network. A helium atom is also produced. These two phenomena could affect the glass integrity and significantly modify its physical properties (hardness, fracture toughness, swelling). Over the long term (>1000 years) they represent the predominant contribution that must be taken into account.

This article reports the results obtained with regard to the two constraints mentioned above: plutonium solubility and self-irradiation.
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