Elaboration and characterization of britholites loaded with tetravalent actinides
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Elaboration and characterization of britholites loaded with tetravalent actinides Olivier Terra1, Fabienne Audubert2, Nicolas Dacheux1, Christophe Guy2 and Renaud Podor3 Groupe de Radiochimie, IPNO, Bât. 100, Université Paris-Sud, 91406 Orsay, France 2 CEA Cadarache, DEN/DED/SEP/LCC, 13108 Saint Paul Lez Durance, France 3 LCSM, Université H. Poincaré-Nancy I, BP 239, 54506 Vandœuvre lès Nancy, France 1
ABSTRACT Due to a natural occurrence, a good resistance to radiation damage and a low solubility in neutral and basic conditions, Nd-britholite of formula Ca9Nd(PO4)5(SiO4)F2 was already considered as a potential host matrix for the specific immobilization of trivalent actinides. The incorporation of tetravalent actinides like Th, U or Ce in the structure was examined through the elaboration of Ca9Nd1-xAnIVx(PO4)5-x(SiO4)1+xF2 samples. This study was the early beginning of the incorporation of 239Pu and/or 238Pu in order to evaluate the effects of α-decay on the britholite structure. The preparation of the samples was realized through dry chemistry methods using mechanical grinding in order to optimize the conditions of synthesis. The results showed that the incorporation of thorium in britholite structure was successful up to 20 wt.% leading to the formation of solid solutions. For U(IV), the incorporation was incomplete: only 5-8 wt.% of uranium was incorporated instead of 10 wt.% expected. The secondary phase, which was uranium enriched, was identified as the calcium uranate CaU2O5+y. The incorporation of Pu(IV) was studied using Ce(IV) as a surrogate. The results were quite good: major part of cerium was introduced in the structure but was partly reduced in Ce(III) during the heating treatment. INTRODUCTION Apatite with the general formula M10(XO4)6Y2 (where M = Ca2+, Pb2+, Ba2+ … XO4 = PO43-, VO43-, SiO44-…, Y= F-, OH-, I-…) is considered as a potential host matrix for the specific immobilization of actinides [1]. Apatites crystallize in a hexagonal system (S.G.: P63/m). The apatite structure can be described as a nearly compact stacking of XO4 groups (where X = P, Si) forming the skeleton of the apatite. This structure exhibits two types of channels [2]. The first one is occupied by four cations (Ca or REE) in the 9-fold coordination and have a diameter near 2.5 Å. The second is occupied in its periphery by six cations in the 7-fold coordination: the diameter of this channel is between 3 and 4.5 Å, allowing the presence of fluoride ions. The apatite structure seems to be able to anneal the defects generated by self-irradiation even at low temperature. Studies of mineral phases crystallized during the natural nuclear reaction at Oklo showed that metamictization due to radiation damage (fission reaction, alpha particles and recoil nuclei) did not affect significantly silicate apatites (britholites) with one silicate group and two fluorine atoms per unit formula [3,4]. Considering these observations, britholite of formula Ca9Nd(PO4)5(SiO4)F2 was first studied for the immobilization of trivalent actinides:
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