Confinement matrices for low- and intermediate-level radioactive waste

Abstract

Mining of uranium for nuclear fuel production inevitably leads to the exhaustion of natural ura nium resources and an increase in market price of uranium. As an alternative, it is possible to provide nuclear power plants with reprocessed spent nuclear fuel (SNF), which retains 90% of its energy resource. The main obstacle to this solution is related to the formation in the course of the reprocessing of SNF of a large volume of liquid waste, and the necessity to concentrate, solidify, and dispose of this waste. Radioactive waste is clas sified into three categories: low , intermediate , and high level (LLW, ILW, and HLW); 95, 4.4, and 0.6% of the total waste are LLW, ILW, and HLW, respectively. Despite its small relative volume, the radioactivity of HLW is approximately equal to the combined radioactivity of LLW + ILW (LILW). The main hazard of HLW is related to its extremely high radioactivity, the occurrence of long living radionuclides, heat release, and the necessity to confine HLW for an effectively unlimited time period. The problems of handling LILW are caused by the enormous volume of such waste. The available technology for LILW confinement is considered, and conclusion is drawn that its concentration, vitrification, and disposal in shallow seated repositories is a necessary condition of large scale reprocessing of SNF derived from VVER 1000 reactors. The significantly reduced volume of the vitrified LILW and its very low dissolution rate at low temperatures makes borosilicate glass an ideal confinement matrix for immobilization of LILW. At the same time, the high corrosion rate of the glass matrix at elevated temperatures casts doubt on its efficient use for immobilization of heat releasing HLW. The higher cost of LILW vitrification compared to cementation and bitumen impregnation is compen sated for by reduced expenditure for construction of additional engineering barriers, as well as by substantial decrease in LLW and ILW volume, localization of shallow seated repositories in various geological media, and the use of inexpensive borosilicate glass.

Zirconolite Matrices for the Immobilization of REE–Actinide Wastes

“…Pyrochlores matrices are typically titanates and zirconates, which somewhat distinguishes them from minerals represented by tantalum-niobates with an admixture of titanium. The similarity of compositions makes it possible to use data on radioactive minerals in assessing the long-term resistance of artificial HLW matrices to corrosion and radiation [21][22][23][24][25][26][27]52,53,[55][56][57][58][59][60][61][62]. The natural analogues of other phases are nonradioactive, for example, mineral kosnarite for NaZr 2 (PO 4 ) 3 .…”

“…In this case, the proof of the stability and suitability of artificial phases for the isolation of radionuclides is based on laboratory experiments and calculations. The joint results of studying radioactive minerals, experiments with short-lived isotopes, and irradiating materials with ions allow a more reliable prediction of behavior of matrices in the underground repositories [21,23,25,27,[55][56][57][58][59][60][61][62][63][64].…”

“…In many matrices based on zirconolite, there is an admixture of Ca-REE titanate with a perovskite structure. Perovskite can be an intermediate product [62,146] and as phase of equilibrium associations (Table 6). It is known for rather low hydrothermal stability above 100 • C [200].…”

Radiochemistry

“…Other factors, such as the long term effect of radiation induced corrosion 214 and how to avert mechanical stress 215 due to earth movements, have also received attention. These and further factors [216][217][218] need to be taken into consideration in the formulation of an overall plan for handling and storing 219 nuclear waste.…”

Vitrification of a Simulator of Vat Residues from Liquid Radioactive Waste

“…However, cement matrices are characterized by increased solubility in an aqueous medium, and the volume of final waste forms significantly increases [4]. Some authors argue that the most optimal method for solidifying LRW is inclusion in glass with subsequent burial in near-surface storage facilities [5,6]. The prospect of using glass as a protective matrix is due to the high ability to include elements in its composition, regardless of their charge and atomic size, resistance to radiation damage, and proven production technology [7].…”