Nuclear fuel cycle and its impact on the environment

In this paper, we consider the present-day situation and outlooks of the development of nuclear power generation in Russia and other countries. It was noted that the implementation of the concept of a closed nuclear-fuel cycle accepted in Russia relies on the solution of the problem of the disposal of spent nuclear fuel (SNF) and radioactive waste (RAW). This paper presents the main results of investigations focused on the development of radiation-safe methods of manufacturing nuclear fuel elements, including mixed uranium–plutonium oxide fuel for fast-neutron reactors; creation of low waste-production technologies of SNF processing and RAW disposal; and the analysis of fundamental features of the behavior and speciation of radionuclides in environmental objects for the development of efficient methods of radioecological monitoring and remediation of radionuclide-contaminated areas.     

Chemical-technological and mineralogical-geochemical aspects of the radioactive waste management

This paper considers various matrices that are able to incorporate components of radioactive wastes (RAW) of different origin. It is noted that attempts to develop the single phase crystalline matrix to immobilize all RAW components failed. The only single phase matrix brought to the industrial application is glass, which is able to accumulate practically all RAW components but in limited concentrations. Prospects are related with some types of ceramics for immobilization of narrow fractions of RAW or individual radionuclides (for instance, minor actinides), as well as some types of low-temperature matrices (iron-phosphate, magnesium–potassium–phosphate, and geopolymers). Approaches to choosing the technology of waste form synthesis are considered. Perspectives of application of both high-temperature (cold-crucible induction melting, self-propagating high-temperature synthesis) methods and modified cementation technologies are demonstrated. It is noted that the final isolation of RAW from the biosphere suggests their disposal in underground repositories. The most difficult technical problem is the disposal of RAW containing long-lived radionuclides. It is shown that the quantitative assessment of repository safety with allowance for their characteristics and all possible processes and phenomena is required to substantiate the safe disposal of long-lived radionuclides.     

Heterogeneity of grain-size fractions of nanodiamonds from the Orgueil C1 meteorite

A model for the composition of meteoritic nanodiamonds is suggested based on analysis of the concentrations and isotopic compositions of C, N, and Xe in the nanodiamond-rich grain-size fractions, which were separated for the first time from the Orgueil CI chondrite. According to the model, meteoritic nanodiamond consists of two populations of grains (denoted C_HL and C_N). The size distributions of grains in populations in the C_HL and C_N populations are different: the C_HL population is finer grained than C_N. The grains of the C_HL population are characterized by a radial gradient in the carbon isotopic composition, and they contain implanted anomalous noble gases (HL component) and the heavy nitrogen isotope ¹⁵N. Following (Clayton et al., 1995), the probable astrophysical source of this population of nanodiamond grains is thought to be the mixing helium and hydrogen shells of a Type-II supernova, and the mechanism that produced these grains was the slow CVD process. The C_N population grains have homogeneous isotopic compositions of carbon (δ¹³C ≡–100‰) and nitrogen (δ¹⁵N ≡–400‰) and contain almost all nitrogen of the nanodiamond-rich fractions. This population of nanodiamond grains was likely formed by a fast unequilibrated process, when shock waves affected organic compounds or gas rich in C- and N-bearing compounds during the early evolution of the protosolar nebula. Calculations within the framework of the model show that the nanodiamond-rich fractions separated from the Orgueil meteorite have the C_N/C_HL ratios varying from 1 in the finest grained fraction to 10 in the coarse-grained one. At these proportions of the populations, weighted mean δ¹³C values of C_HL grains in the fractions lie within the range of 42 to 394‰, and the concentrations of ¹³²Xe-HL and ¹⁵N are (49–563) × 10^–8 cm³/gC and (1.1–6.2) × 10^–5 cm³/gC, respectively.     

In situ high-temperature X-ray diffraction and spectroscopic study of fibroferrite,FeOH(SO<Subscript>4</Subscript>)·5H<Subscript>2</Subscript>O

The thermal dehydration process of fibroferrite, FeOH(SO₄)·5H₂O, a secondary iron-bearing hydrous sulfate, was investigated by in situ high-temperature synchrotron X-ray powder diffraction (HT-XRPD), in situ high-temperature Fourier transform infrared spectroscopy (HT-FTIR) and thermal analysis (TGA-DTA) combined with evolved gas mass spectrometry. The data analysis allowed the determination of the stability fields and the reaction paths for this mineral as well as characterization of its high-temperature products. Five main endothermic peaks are observed in the DTA curve collected from room T up to 800 °C. Mass spectrometry of gases evolved during thermogravimetric analysis confirms that the first four mass loss steps are due to water emission, while the fifth is due to a dehydroxylation process; the final step is due to the decomposition of the remaining sulfate ion. The temperature behavior of the different phases occurring during the heating process was analyzed, and the induced structural changes are discussed. In particular, the crystal structure of a new phase, FeOH(SO₄)·4H₂O, appearing at about 80 °C due to release of one interstitial H₂O molecule, was solved by ab initio real-space and reciprocal-space methods. This study contributes to further understanding of the dehydration mechanism and thermal stability of secondary sulfate minerals.