GENERAL SCIENTIFIC ACTIVITY LINES OF LABORATORY FOR SCIENTIFIC FOUNDATION AND TECHNOLOGIES OF RADIOACTIVE WASTE SOLIDIFICATION

1. Elaboration of RW immobilization procedures

1.1. Elaboration of procedures to enclose RW into phosphate and borosilicate glass

The procedure of vitrifying is best admitted to technologies for conditioning of HLW, weapons-grade plutonium, and various materials containing plutonium and other radionuclides, and available in a form of sludge, slurry, mud.

Laboratory's employees have worked up glass formulas recommended for waste immobilization, depending on a type of the waste to be solidified and quantitative proportions of components to be fixed. They have defined the range of borosilicate and phosphate glass matrixes, and conditions for their sintering. The technology is developed to vitrify plutonium and plutonium-containing materials, which have been accumulated at the sites of MCC (Zheleznogorsk city) and PA Mayak (Ozersk).

The Laboratory carries out the works aimed at synthesis of new materials, which can be suitable not only for immobilization of the waste, but also for fabricating of an active part of radioisotope sources. In the run of synthesis and studying the properties of cesium aluminophosphate glass they have discovered the generation of new compound Cs₂LiAl₂(PO₄)₃ having a structure of pollucite.

1.2. Elaboration of procedures to enclose RW into ferrophosphate cements

At the Laboratory they have developed the method to enclose high sodium content waste (saline waste with RBMK (FMCR) type reactors) into ferrophosphate matrix. They have worked up matrix compositions based on iron oxides and orthophosphoric acid to immobilize ash residues from radioactive and toxic waste incineration, and other type waste too (for example, a soil contaminated with radionuclides).

Here is a schematic model of ferrophosphate cement matrix generating:

• mixing of waste powders with iron oxides and orthophosphoric acid;
• stirring of a mixture to obtain a gomogeneous paste;
• paste casting;
• paste hardening at a room temperature during 3-4 hours;
• ripening during about 30 days.

Waste content in cements can reach 35 % of mass; mechanical compression strength - up to 100 Kg/cm² ; according to TCLP and ANS-16.1 techniques, leaching of radioactive and toxic elements is within permissible limits.

The advantages of waste immobilization in ferrophosphate cements are as follows:

• radiation and chemical stability of a matrix;
• high percent of matrix filling with waste;
• low cost of matrix material (iron oxide);
• the possibility to use metal production waste (scoria) and natural iron-containing minerals (magnetite) as a matrix material;
• the possibility of process realization at a room temperature;
• no less than 2.5 times decrease in the waste volume.

1.3. Elaboration of procedures for RW immobilization in ceramic matrixes

At the Laboratory they have developed the procedures to insert concentrates of REE and TPE, and also the waste generated in plutonium metal converting and ²⁴¹Am -containing waste into monazite ceramic matrix.

It is proposed to convert oxalate sediments (REE + TPE), which have been accumulated at PA Mayak in the run of liquid HLW processing, into orthophosphate powders (REE + TPE) by the following flowchart:

• mixing of concentrated product (REE + TPE) in 2M HNO₃ and 80% orthophosphoric acid solution to reach a stable colloidal solution (REE + TPE)PO₄;
• calcinations of the obtained solution in thin-film rotor-type thickener to reach fine powder (REE + TPE)PO₄;

• hot pressing during 0.5 hour under a pressure of 29.4 MPa at 900-1200 °у;
• cold pressing under 1000 Kg/cm² pressure, followed by sintering at 1300 °у during 2 hours;
• powder enclosing into ferrophosphate cement.

For the purpose to immobilize liquid HLW containing americium it is proposed to use lanthanum orthophosphate having a structure of monazite as a matrix material. Ceramic matrix is to be made by methods of hot pressing, or cold pressing followed by sintering.

1.4. Elaboration of procedures for RW immobilization in sponge

Porous crystalline matrix material made of hollow vitro-crystalline microspheres (cenospheres) recovered from volatile ash from coil incineration is able, like a sponge, to take up solutions, fixing at that components of RW in available volume. RW immobilization in porous vitro-crystalline block allows to 5-20 times volume decrease, depending on salt-content.

A matrix containing RW is suitable for storage or disposal of waste, for its transport, as well as for further immobilization by vitrifying or hot pressing.

For more detailed information on a subject of Sponge (Gubka) Project refer to: http://www.atom.nw.ru/RIE/projects/gubka/index.htm

2. Equipment design of waste immobilization processes

2.1. Application of cold crucible induction melters

The researches aimed at development of technology for high-level waste solidification using cold crucible induction melters are ongoing at the Institute since 1995. Experimental installation destined for induction dissolving in cold crucible is designed at Laboratory and put into operation. This installation allows to melting of oxide materials and salt mixture, using dry or liquid proportional feeding and to realization of melt pouring.

It is indicated that up to 70 % of mass slurry type waste can be enclosed in borosilicate matrix, when using cold crucible induction melter.

The flowchart of waste solidification assumes an application of gas-purification system, multilevel monitoring of melt temperature, and control of electrical and power parameters.

2.2. Technology using MW heating

• the possibility to realize RW solidification in one stage;
• higher rate of material heating in comparison with radiant heating, as this heating takes place within the volume of material and does not depend upon its thermal conductivity;
• all the equipment requiring periodical maintenance and replacement is arranged outside the area of restricted personnel access.

With the purpose to carry out scientific researches and to prepare input data for pilot commercial installation construction at Mining and Chemical Combine (Zheleznogorsk city), the installation providing MW heating of fluxed sludge has been designed and assembled together with V.P. Vologdin VNIITVCh (High-frequency Current Institute).

Process orders of synthesis are completed with the help of this installation for glassy, vitro-crystalline, and mineral-alike compositions. A series of experiments was conducted for filling in crucible-melters. Investigations in physical and chemical properties of solidified product produced have indicated those to be in compliance with criteria for radioactive waste storage and disposal.