Nuclear waste disposal—pyrochlore (A2B2O7): Nuclear waste form for the immobilization of plutonium and “minor” actinides

Abstract

During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the ''minor'' actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A 2 B 2 O 7 , for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compositions has revealed that many zirconate pyrochlores do not become amorphous, but remain crystalline as a defect fluorite structure-type due to disordering of the A-and B-site cations. The zirconate pyrochlores will remain crystalline even to very high doses, greater than 100 displacements per atom. Systematic experimental studies of actinide-doped and ion beam-irradiated pyrochlore, analyses of natural U-and Th-bearing pyrochlore, and simulations of the energetics of the disordering process now provide a rather detailed understanding of the structural and chemical controls on the response of pyrochlore to radiation. These results provide a solid basis for predicting the behavior and durability of pyrochlore used to immobilize plutonium.

New high-pressure phase and equation of state of Ce2Zr2O8

“…The cation and anion sites of Ce 2 Zr 2 O 7 are split in Ce 2 Zr 2 O 8 by lowering the symmetry from Fd 3m to P2 1 3. Cations form distorted CeO 8 and ZrO 8 polyhedra that are connected by shared edges as in fluorite.…”

“…Information obtained from these studies is also valuable regarding their use as a host lattice for transmutation products of reactors and the safe deposition of plutonium. 3,7,8 The high radiation and chemical stability and low neutron absorption coefficients of Zr make zirconate pyrochlores extremely useful for these purposes, but recent studies have found pressure-driven instabilities. 9 Structural transitions were reported for Sm 2 [10][11][12] Studies on related Y 2 Zr 2 O 7 indicated the formation of a defect-cotunnite structure at 18 GPa.…”

Interstitial Incorporation of Plutonium into a Low-Dimensional Potassium Borate

“…During the last two decades, significant efforts have been made to better understand the incorporation of early actinide elements (namely Th, U, Np, and Pu) into a variety of mineral phases, such as calcite (7)(8)(9)(10)(11)(12)(13)(14)(15), zircon (4,(16)(17), monazite (4,(17)(18), zirconolite (4,(18)(19)(20)(21)(22), perovskite (4,22), garnet (4), pyrochlore (4,(23)(24)(25), brucite (26), and several other systems (27)(28)(29)(30). Particularly for transuranium elements (Np and Pu), it was determined that these can also be incorporated into a series of uranium-or thorium-based minerals such as brannerite (4), ianthinite (31), schoephite, becquerelite, compreignacite, and boltwoodite (32) or even synthetic phases such as Th 2-x/2 An IV x/2 (PO 4 ) 2 (HPO 4 )·H 2 O (An = U, Np, Pu) (33), ThSiO 4 (34), and Ba 3 (UO 2 ) 2 (HPO 4 ) 2 (PO 4 ) 2 (35).…”

Effect of Grain Size on Mechanical Properties of Irradiated Mono- and Polycrystalline MgAl2O4

“…Today about 1400 tons of plutonium exists worldwide; about 300 tons in military installations, 200 tons have been used for fabrication of mixed oxides (MOX) nuclear fuel and about 900 tons is stored in power plants [1]. Taking into account that the fabrication of a nuclear warhead requires less than 10 kg of plutonium and that one 1000 MW reactor produces something like 200 kg of 239 Pu per year it becomes clear that the problem of excess plutonium must be rapidly solved.…”

“…Taking into account that the fabrication of a nuclear warhead requires less than 10 kg of plutonium and that one 1000 MW reactor produces something like 200 kg of 239 Pu per year it becomes clear that the problem of excess plutonium must be rapidly solved. Two strategies are currently envisaged; (i) the use of 239 Pu as a nuclear fuel in new generations of reactors and (ii) direct disposal of spent nuclear fuel (SNF) in geological repositories [1][2][3]. In the first case fissile isotopes have to be embedded in a special material, called inert matrix fuel (IMF), in the second scenario SNF must be transformed into a stable form which can be permanently disposed.…”