Phase composition and radiation stability of matrices for isolation of REE-actinide waste

Abstract

526Safe treatment with highly radioactive wastes from processing of irradiated fuel is a key problem at the final stage of the nuclear energy cycle. The waste prod ucts are currently immobilized in glassy matrices for loading in underground storage [1]. Glasses are insuf ficiently stable for isolation of long living radionu clides, including actinides. It is suggested to fraction ate such wastes and include long living nuclides in the most stable crystalline matrices in order to increase safety [2]. The available technologies allow us to sepa rate actinides (U, Np, Pu), the REE-actinide compo nent, and products of decay.Zirconates and titanates with the structures of pyrochlore, zirconolite, perovskite, murataite, and others are suggested for isolation of actinide wastes [1-10 and others]. Materials with wastes of simple composition (plutonium and its imitators) were basi cally studied in these investigations; less attention was given to matrices for immobilization of the REEactinide fraction. Its approximate composition is the following (wt %): 80-90 REE + 10-20 actinides, where light lanthanides (La, Ce, Pr, Nd, and Sm) pre vail among REEs, and actinides are mainly repre sented by Am and Cm. The structure of matrices undergoes changes during actinide decay, which may influence their isolation properties.In this paper we report the new data on the struc tural peculiarities of perspective matrices for the REE actinide fraction and their radiation stability during irradiation by accelerated krypton ions.The starting charge was prepared from TiO 2 , ZrO 2 , and a mixture of REE oxides taken in proportions cor responding to the ideal formula (REE) 2 Zr 2 -x Ti x O 7 , where х = 0 (Sample Z) or 2 (Sample T). The propor tions of elements in the REE mixture are the following (wt %): La 4.7 (or in atomic proportions: 0.24La + 0.46Ce + 0.21Pr + 0.75Nd + 0.15Sm + 0.02Eu + 0.03Gd + 0.14Y); they reflect the proportions between them in the REEactinide fraction of wastes from reprocessing of irradi ated nuclear fuel. The size of the Nd 3+ cation is close to the sizes of the Am 3+ and Cm 3+ cations, that is why it was an imitator of these actinides. The mixtures were homogenized carefully and pressed into tablets at 200 atm and 25°С, and after that they were loaded in the oven for 5 h at 1550°С (х = 0) or 1400°С (х = 2). Reaching equilibrium in the zirconate system requires higher temperatures for acceleration of the phase for mation during synthesis. The samples were studied by X ray diffraction (XRD) and on a scanning electron microscope equipped with an energy dispersive spec trometer (SEM/EDS). They were irradiated in the Argonne National Laboratory (United States) on an accelerator combined with a transmission electron microscope [7]. This approach allows us to observe disordering of atoms in the framework and destruction upon irradiation. This method is an express way to determine the doses of amorphization in a wide tem perature range [1-3, 6-8, and others].According to the XRD data, Sample Z consists of dominating pyro...

Matrices for immobilization of the rare earth–actinide waste fraction, synthesized by cold crucible induction melting

“…The amount of this phase in the sample is low, and its stability in water is high [22]; therefore, the presence of this phase should not deteriorate the properties of the matrix. Rare earth titanate exhibits low radiation resistance [34] and undergoes amorphization at a dose of 0.2 displacement per atom, which is close to the values obtained for titanate pyrochlore and murataite. If the effect of the structure amorphization on the radionuclide leaching from the matrix will be negative, the matrix composition can be changed so as to avoid the formation of this phase.…”

Zirconolite Matrices for the Immobilization of REE–Actinide Wastes

“…154 Eu (8.8 years)-154 Gd (stable); 155 Eu (5 years)- 155 Gd (stable). The isotopic composition of natural Nd: 142Nd (27.2%), 143 Nd (12.2%), 144 Nd (23.8%), 145 Nd (8.3%), 146 Nd (17.2%), 148 Nd (5.7%), and 150 Nd (5.6%): 5 stable and 2 very weakly radioactive. 144 Nd: α-decay, T 1/2 = 2.38 × 10 15 years; 150 Nd: double β-decay, T 1/2 = 7 × 10 18 years.…”

“…1 There is a long-lived 147 Sm (T 1/2 = 1.06 × 10 11 years) and a small amount of 151 Sm (T 1/2 = 90 years). 2 Long-lived 144 Nd (T 1/2 = 2.38 × 10 15 years) and 150 Nd (T 1/2 = 7 × 10 18 years) can be considered as stable. 3 It is the share of minor actinides (wt%) in the hypothetical REE-MA fraction and their contribution to heat release of the mixture.…”

“…); 147 Pm (T 1/2 = 2.6 years)-147 Sm (1.06 × 10 11 years, α-decay)-143 Nd (stable). 144 Ce (284 days)- 144 Pr (17 min)-144 Nd (stable). 154 Eu (8.8 years)-154 Gd (stable); 155 Eu (5 years)- 155 Gd (stable).…”

Safe management of actinides in the nuclear fuel cycle: Role of mineralogy

“…There are over 500 synthetic compositions (Chakoumakos, 1984), including actinides (Chakoumakos and Ewing, 1985), with the pyrochlore structure. A number of compositions with thorium and uranium (Laverov et al, 2001(Laverov et al, , 2002, as well as transuranium elements (e.g., Cm and Pu), have been synthesized (Kulkarni et al, 2000;Raison et al, 1999;Weber et al, 1985aWeber et al, , 1985b. Thus, it is not surprising that pyrochlore structure-types have received extensive attention as a potential host phase for actinides (Ewing et al, 2004b).…”