GENERAL RESEARCH LINES OF LABORATORY FOR APPLIED MINERALOGY AND RADIOGEOCHEMISTRY

1. Development and synthesis of actinoid ceramics

The laboratory over many years conducts researches in the area of management of radioactive waste generated at fractioning of high-level waste (HLW) in the run of SNF processing, and also with composite waste originating at weapons plutonium fabrication. The researches cover various type crystalline matrixes suitable for withdrawal of transuranium and other long-lived radionuclides. Theoretical and experimental studies have indicated zirconium (Zr, An)SiO₄ (where An = U, Pu, Np, Am, Cm) to be the most stable matrix for immobilization of actinoids, and particularly, excess weapons-grade plutonium.

Zirconium based ceramics, (Zr,Pu)SiO4 (+ 15 % of weight tetragonal ZrO2), doped with 5-6 % of mass 238Pu. The picture is made in reflected light just at completion of ceramic synthesis

Zirconium dioxide (Zr,An,:)O₂ is another representative of actinoid-containing matrix.

Ceramics on the basis of cubic zirconium dioxide, Zr0.79Gd0.14Pu0.07O1.99 , doped with 9.9 % of mass 238Pu. The picture is made in reflected light just at completion of ceramic synthesis

Crystalline ceramics based on garnet and perofskite, (Y, Gd, Ca, Fe,:)₃(Al, Ga, Fe,:)₅O₁₂ and (Y, Gd, Ca,:)(Al, Ga, Fe,:)O₃ can be considered as a matrix to withdraw a waste of complicated chemical composition, containing actinoids.

1.2. Elaboration of ceramic nuclear fuel and matrixes for actinoid transmutation
The laboratory proposes an application of zirconium dioxide based ceramics to produce ceramic plutonium fuel and as a form of waste immobilization. The following ceramic patterns containing ²³⁹Pu, ²³⁸Pu and ²⁴³Am are obtained by means of experimental synthesis and studied:

It is indicated by means of microprobe and X-ray methods that Pu-containing ceramic matrixes can be synthesized with success by economically acceptable methods (agglomeration or melting in the air) without origination of independent actinoid phases.

1.3. Study of radiation damages in ceramic matrixes

The study of stability of ceramic patterns made on the basis of cubical zirconium dioxide (Zr, Gd, Pu)O₂, impregnated by gadolinium, containing 9.9 % of mass ²³⁸Pu, and pyrochlorine based ceramics (Ca, Gd, Hf, U, Pu)₂Ti₂O₇, containing 8.7 % of mass ²³⁸Pu has indicated that pyrochlorine ceramic tends to lose its crystallinity comparatively rapidly under self-irradiation effect. This process is accompanied by sizeable decrease in chemical stability of the present ceramics. In similar conditions zirconium dioxide based ceramic keeps its crystallinity and chemical stability.

1.4. Synthesis of actinoid doped mono-crystals and study of their radiation damages

In the laboratory there are developed and mastered such technologies of ceramic matrix synthesizing, which allow to obtaining zirconium mono-crystals with high actinoid content. Analyzing of the properties of these mono-crystals, their radiation damages, actinoid leaching from them gives opportunity to form more precise notion of processes taking place in ceramic matrixes consisting of aggregates of microscopic crystals.

Zirconium mono-crystal doped with 238Pu A - picture in reflected light; B - crystal luminescence in darkness

Progressive degradation of zirconium mono-crystal containing approximately 2.4% of mass 238Pu, caused by radiation damages

2. Geological investigations in the area of high-level waste disposal

Comprehansive survey works are carried out in Krasnoyarsk Region to select a site suitable for construction of deep repository of high-level waste originating in the run of weapons plutonium fabricating at Mining and Chemical Combine.

The result of multistage works is as follows:

• Selection of Nizhnekanskiy Granitoid Massif as promising geological environment for repository construction,
• definition of the most prospective "Itatskiy" and "Kamenny" sites within the Massif,
• provisional study of promising sites, using the system of earth-based geophysical survey methods, hydrogeochemical investigations, geological survey, boring, evaluation of tectonic dislocations, etc.

Geological structure of the territory: 1 and 2 - are "Itatskiy" and "Kamenny" sites to be subjected to detailed geophysical surveys, 3 - alternative Eniseyskiy site

The studies include:

• mineralogical and petrographical study of rocks;
• characterization of secondary changes in rocks and in minerals forming them;
• definition of filtration characteristics (porosity, permeability) of representative rock patterns;
• determination of thermal and radiation stability of rocks;
• evaluation of strength characteristics of rocks;
• analysis of equilibrium within the systems of natural radionuclides in a rock.

In addition, together with Laboratory of Radiochemical Grounds for Radioactive Waste Isolation in Geological Formations the Laboratory studies retentive (sorption) properties of rocks, with respect to actinoids and other long-lived radionuclides serving as HLW components. Inter-phase distribution of radionuclides between solution simulating groundwater and rock patterns is studied in wide range of various conditions.

3. Study of Chernobyl "lavas" and hot particles

Radioactive products generated as a result of Chernobyl accident have been studied in Laboratory since 1990.

Major part of nuclear fuel under the accident, interacting with structural materials of reactor fused reactor basement and spread over sub-reactor compartments. Streams solidified have formed so-called Chernobyl "lavas". These "lavas" differ in compositions, and some of them have been subjected to secondary changes.

With the help of optical and electronic microscopy, microprobe and X-ray analyses there are obtained data on chemical and phase compositions of "lavas", and also the data on radionuclide content in separate mineral grains. The following independent phases are recognized in amorphous glassy matrix of Chernobyl "lavas":

• zircon (Zr, U) SiO₄ with high (up to 12 % of mass) uranium content;
• baddeleyite (Zr, U)O₂;
• uranium oxide U₄O₉;
• amorphous phase "Zr- U- O";
• drops of melted stainless steel "Fe - Cr - Ni".

Patterns of black and brown Chernobyl "lavas" (fuel-containing mixtures)

High-uranium zircon constituted by Chernobyl "lavas"

When studying secondary surface changes in "lavas", so-called "yellow spots", the following secondary uranium minerals have been defined:

• apeeantinite, UO₃·2H₂O;
• Na₄(UO₂)(CO₃)₃;
• studtite, UO₄·4H₂O;
• retherfordite, UO₂(CO₃),

as vell as carbonates free of uranium:

• Na₂CO₃·H₂O;
• Na₃H(CO₃)₂·2H₂O.

In 1990-1992 laboratory experts sampled a lot of soil patterns within the territory of ChNPP 30-km area. Multistage treatment of these soil samples allowed separating of fuel-containing "hot" particles. Dimensions of particles separated varied from 0.01 to 0.9 mm. Electron-microscope investigations, qualitative microprobe analysis, gamma-spectrometry have indicated that more than 90% of hot particles represent fragments of nuclear fuel (UO_X) and products resulted from interaction between nuclear fuel and structural materials: zirconium alloy and stainless steel.

Typical hot particles - fragments of non-changed nuclear fuel. Imagery is reached with the help of electron microscope in the mode of inverse scattering.

The results of investigations have allowed in a lot of cases to clarify the scenario of Chernobyl accident.