V. I. Ratnikov

Thermal decomposition of TiH2: A TRXRD study

Thermal decomposition of SHS-produced TiH2 powder in vacuum at temperatures below 570°C was explored by time-resolved XRD. The process got started with transformation of starting TiH2 into solid solution of hydrogen in β-Ti (β-Ti[H]) and then followed by the polymorphic β-Ti[H] → α-Ti[H] transition and subsequent hydrogen elimination to yield α-Ti. Marked changes in the phase composition of starting TiH2 were found to get started around 450°C.

Self-propagating high-temperature synthesis (SHS) of a pyrochlore-based ceramic for immobilization of long-lived high-level waste

A Y2Ti2O7-based ceramic of pyrochlore-type structure as a host material for immobilization of actinide-containing HLW was prepared by SHS compaction. Up to 25 at. % Zr was introduced into the pyrochlore structure instead of Ti to enhance the chemical and radiation resistance. The ceramic matrices exhibit low porosity, good water resistance, and high mechanical strength. The study was performed with simulated HLW.

Self-propagating high-temperature synthesis for immobilization of high-level waste in mineral-like ceramics: 1. Synthesis and study of titanate ceramics based on perovskite and zirconolite

Preparation of ceramics based on synthetic perovskite and zirconolite (analogs of titanate minerals) using self-propagating high-temperature synthesis (SHS) was studied. Ceramics was prepared as matrix material for immobilization of high-level waste (HLW). Using model HLW, the optimal synthetic conditions were determined which allow preparation of compact low-porosity material (in the form of cylindrical blocks) exhibiting high strength and low rate of leaching of Cs, Sr, Y, Ce, and La into double-distilled water. The phase composition and micro structure of the resulting materials were studied. As found, immobilization of Cs is accompanied by significant loss of this element.

Self-propagating high-temperature synthesis of Al2O3/TiC-based ceramic materials

Dense Al2O3/TiC composite ceramic materials are synthesized using the SHS compaction of mixtures based on TiO2 + Al + C. Mineralizing and heating additives are introduced into compounds. The phase composition and microstructure of combustion products are investigated by x-ray phase analysis, electron microscopy, and microprobe techniques. Two Al2O3 modifications are revealed. Special attention is devoted to the presence of residual graphite. The mechanisms of phase formation and formation of the microstructure of combustion products are considered.

Self-propagating high-temperature synthesis for immobilization of high-level waste in mineral-like ceramics: 2. Immobilization of cesium in ceramics based on perovskite and zirconolite

Immobilization of Cs in ceramics based on perovskite and zirconolite by self-propagating high-temperature synthesis (SHS) was studied using model systems. The ceramics was obtained as cylindrical blocks. Cesium can be completely immobilized in ceramics at 1250°C in the form of synthetic mineral pollucite and, in this case, it is uniformly distributed through the bulk of the ceramic sample. The optimal synthetic conditions were determined, and the phase composition of the ceramics was evaluated. The leaching rates of Cs from ceramics in double-distilled water at 20°C were determined.

SHS hydrogenation of titanium: Some structural and kinetic features

Hydrogenation of Ti sponge was performed in conditions of hydrogen deficiency (a) and excess (b). The extent of conversion was low and non-uniform in case (a) and close to unity in conditions (b). The combustion products were characterized by XRD, SEM, time-of-flight mass spectrometry (TOF MS), and time-resolved XRD.

Mineral-like ceramics for immobilization of nuclear wastes by forced SHS compaction

Dense cylindrical samples based on Y2Ti2O7 ceramics with a pyrochlore structure were produced by forced SHS compaction and characterized as a matrix for immobilization of actinide-containing nuclear wastes. Synthesized materials were found to exhibit high hydrolytic resistance and mechanical strength.

Microstructure and composition of SHS products of powder multicomponent oxide mixtures with aluminum and graphite

The microstructure and composition of products of SHS combustion of two types of multicomponent oxide mixtures with aluminum and graphite powders, differing in the presence or absence of a thermite additive, are studied. The combustion product—oxide carbide ceramic based on Al2O3/TiC—serves for immobilization of graphite-containing radioactive wastes. It is proposed to use results of the studies of “undercombusted” samples of such composites for analysis of the initial stages of the process of their front combustion. Procedures of combustion of the composites with the presence or absence of a thermite additive are specified. The problems of distribution in the combustion products of the impurities introduced into the original system, which simulate the most characteristic composition of nuclides in nuclear-fuel degradation products and in radiation-exposed graphite, are studied

SHS of mineral-like ceramics for immobilization of radioactive nuclear wastes

Synthetic analogs of titanate ceramics (perovskite and zirconolite) designed for use as a matrix for immobilization of high-level nuclear fuel reprocessing wastes (HLW). Such ceramics have been prepared by SHS method from a mixture of titanate ceramics and non-radioactive model oxides. Synthesis conditions have been optimized. The synthesized low-porosity cylindrical compacts exhibited a high strength and low rate for leaching Cs, Sr, Y, Ce, and La in bidistilled water. The phase composition and microstructure of synthesized products have been characterized. The immobilization of Cs was found to be accompanied by a marked loss of this element.

SHS immobilization of cesium in mineral-like matrices

The feasibility of SHS immobilization of Cs in perovskite/zirconolite-based mineral-like ceramics has been explored by using a mixture non-radioactive model oxides imitating nuclear wastes. Resultant ceramics were obtained in the form of cylindrical blocks. It has been found that Cs can be completely immobilized (at 1250°C) in ceramic blocks as a synthetic analog of pollucite (CsAlSi2O6). The Cs distribution over synthesized ceramic blocks was found to be uniform. The process conditions were optimized, and the phase composition of synthesized ceramics was determined. The leach rates for Cs in bidistilled water at 20°C have been measured.