GENERAL SCIENTIFIC ACTIVITY LINES OF THE SECTOR FOR PROCESSES OF RADIONUCLIDE RECOVERY FROM IRRADIATED MATERIALS

1. Development of HLW fractioning processes

1.1. Study of processes serving as concomitants of extraction processing of irradiated nuclear material

• processes of radiation and chemical conversions of organic components in extraction system of TBP and light (hydrocarbons) or heavy (chlorine-derived hydrocarbons) dilutant;
• generation of inter-phase precipitates on the basis of stabilized and hardly segregated emulsions;
• effects of series radionuclides retention by organic phase;
• generation of the third liquid phase (a phase of SAS or micellar phase) at inter-cycle regeneration of extractive agent;
• generation of thermally unstable products of radiation-chemical extractive agent conversions (so called "red oil").

There were simulated conditions for red oil generating and then in its structure there were identified nitro-containing products and long-lived free nitroxyl radicals. The properties of these products determine their function in heat-producing extractive agent oxidation reactions, which can turn into conditions of thermal explosion.

1.2. UNEX-process: pioneering successful experience in simultaneous recovery of cesium, strontium, rare-earth and actinoid elements from acidic HLW

With the purpose to extract all long-lived radionuclides (Cs, Sr, REE, and AE) simultaneously from acidic HLW it has been suggested by Radium Institute together with Idaho National Laboratory to use synergetic extraction mixture on the basis of cobalt dicarbollide chlorinated (CDH), carbamoyl phosphine oxide (CMPO), and polyethylene glycol (PEG) in polar dilutant. At optimum composition of a mixture, when [CDH]:[CMPO]:[PEG] ~ 5:1:1, all the long-lived radionuclides are to be recovered simultaneously.

Specific types of PEG, CMPO, and delutant have been selected to provide an optimum combination of good extractive and hydrodynamic properties with high chemical and radiation stability, as well as explosion and fire safety for extractive mixture. Conditions for extraction and re-extraction (simultaneous or separated) of target elements, as well as for extractive mixture inter-cycle regeneration are already specified. The composition of recyclable carbonate re-extractive agent is patterned, that allows sizeable growth in effectiveness of the process.

On the bases of the data obtained there is developed single-extragent process flowchart for HLW refinement from long-lived radionuclides that is UNEX-process.

Dynamic tests of UNEX-process are successfully carried out on real HLW generated at INEEL (USA) and MCC (Russia) factories, containing about 4 M nitrates. It has been recovered 99.6% Cs, 99.99% Sr, 99.96% actinoids and lanthanoids that allows in one cycle to reach a raffinate, which is a representative of low-active waste.

Methods of management of all secondary waste of UNEX-process are worked up.

2. Search and investigation of new extractive agents

Promising method to modify extractive agent with the purpose to increase its selectivity consists in phosphoryl group addition to a stiff platform of calyx-arene. Donor centers of extractive agent in some type structures can be optimally located around metal cation that causes substantial growth of extraction efficiency and selectivity. In reality all phosphorylated calyx-arenes, which have been studied at Radium Institute (ISTC Project No 2068) efficiently extract americium or palladium and technetium from acidic (up to 1 M HNO₃) mediums. There are determined phosphorylated calyx-[4]-arenes, which, contrary to ordinary extractive agents, make provision for simultaneous recovery of REE, TPE, technetium, and palladium from acidic HLW.

It has been indicated that it is possible in principle to recover uranium and transuranium elements at SNF processing by single-cycle monoextragent scheme, using solutions of bifunctional neutral phosphorous-containing extractive agents (BNPHOS) in fluoride-containing polar delutants (fluoropols). BNPHOS allow efficient recovery of uranium and transuranium elements from acidic solutions, and polar fluoropols prevent from generating of the third phase, which serves as the main obstacle to BNPHOS usage in radiochemical technologies.

The study of fluorinated compositions as potential delutants for extractive agents has indicated their especial usefulness for high0polar extractive agents, such as chlorinated cobalt dicarbollide, carbomoyl phosphine oxides, calyx-arenes. There has been studied the possibility of nitroaromatic fluoro compositions and fluoridated sulfones usage as dilutants for fluorinated ethers and esters. On evidence of investigations nitrobenzotrifluoride is recommended to be used as dilutantat at PA "Mayak".

3. Fluid extraction study and various-purpose application

The processes of supercritical extraction suggest the substances brought to conditions above critical values of temperature and pressure to be used as dilutants.

Carbon dioxide is a substance to be most often used for supercritical fluid extraction. It is non-toxic, obtainable, chip, and its critical parameters are not too high (temperature - above 37 °у and pressure - above 72 st. atm.)

The interest to supercritical fluid extraction of metals is called by sharp decrease (20 - 200 times) in volumes of secondary liquid radioactive waste.

The environment of supercritical or liquid carbon dioxide is suitable for decontamination of safety clothing and boots, personal supports, small instruments, etc. It has been indicated that the application of hexafluoride acetylacetone and pyridine solution in CK-CO₂ allows to removal of actinoid elements from the surface of textiles and metals at a ratio of 100-1000. The volume of secondary liquid radioactive waste, when using уп₂, is 20-200 times lower in comparison with traditional methods at the same decontamination efficiency.

A mixture of β-diketones and TBP in supercritical CO₂ or in liquid уп₂ can be used to dissolve uranium oxides. It is proved that the solubility of mixed complexes of uranium, plutonium, and neptunium (above 50 g/L for metal) with hexafluoride acetylacetone and TBP is high.

It is shown that the extraction by carbon dioxide at SNF processing is possible in principle. The main advantage of this dry low-temperature process consists in sizeable decrease in a volume of liquid radioactive waste.

By the example of strontium and yttrium separating with supercritical уп₂ containing various complexing agents they have indicated that it is possible to create generators of isotopes, using supercritical carbon dioxide.

The content of strontium-90 in yttrium-90 fraction recovered has proved to be below detection limit of methods available, that is why the ratio of yttrium refining from strontium can only be estimated as >10⁴ .

Combined method of paper conservation and damping of mycelial fungus growth has been worked up and tested. The method includes the treatment of paper with super- or sub-critical CO₂ containing solution of magnesium methylate and thymol. It has been proved that such a treatment does not change mechanical properties of paper, increases its pH, and suppresses mycelial fungus growth.

This method suggests relatively chip manufacturing of movable installation for the treatment of documents and books in museums and libraries.

For more details on supercritical fluid extraction you can refer to: http://extract.ru/index.php4?id=100.

The system of laboratory equipment for supercritical fluid extraction at V.G. Khlopin Radium Institute

4. Works in the field of biotechnology

The works in the field of biotechnology were started in 1992 together with molecular biotechnology sub-faculty of St-Petersburg Technological Institute. The first works were aimed at the study and elaboration of methods for biological degradation of solid cellulose-containing radioactive waste with the help of mycelium microfungi as biological destructors. It was shown that the volume of cellulose-containing radioactive waste can be 4-5 times reduced and turned into unusable remains stable and safe for long-term storing.

Investigations in biosorption treatment of aqueous solutions contaminated with radiation have indicated that high enough purification from ¹³⁷уs, ⁹⁰Sr and other radionuclides can be reached, when using biological sorbents based on destroyed wood fragments containing biomass of mycelium microfungi grown up. It has also been shown that aqueous solutions can be treated from organic substances too by the way of destruction of those at their recovery by sorption on biological sorbents.

They also elaborate methods for biological degradation of polymeric films based on paintwork materials with the purpose to work up biological technology for decontamination of surfaced covered with varnishes and paints, contaminated with radionuclides. Specially selected communities of microbe cultures are used as destructors for varnish-and-paint surfaces. The work is ongoing under the Agreement with USA DOE, concluded in 2002. This work also assumes participation of experts from V.L. Komarov Botanical Institute under RAS (Russia, St-Petersburg) and Central Scientific and Research Institute for Materials (Russia, St-Petersburg).

5. Elaboration of procedures for processing of irradiated structural materials of thermonuclear reactor with the purpose of their further re fabricating

Vanadium-chromium-titanium (VCT) alloy, which is mainly based (up to 90 %) on expensive vanadium, is suggested to application as structural material of DEMO TNR (its primary, blanket, and divertor). Vanadium mass in single reactor will reach 40 tones.

Alloy re-fabricating at completion of reactor operation will expect its radiochemical processing. Alloy treatment from radioactive irradiation products will allow return of alloy materials to production. Main isotopes to be removed are: ⁶⁰Co, ¹⁵²Eu, ¹¹⁰Ag, ⁹³Mo, ⁵⁵Fe. Maximum treatment (10⁵)is required to remove cobalt. Such treatment allows to reaching exposure dose rate below 10 μSv/h, that permits alloy re-fabrication to be realized without protection from ionizing radiation. Extraction is selected as a method for processing of the alloy.

They dissolve the alloy in nitric acid, and solution obtained goes to extraction. The extraction is realized with di-2-ethyl-hexylphosphoric acid in hydrocarbon dilutant (30% Di-2-EGPA + 70 % h.-c. d.) (ref to the diagram).

6. Design of models of radiochemical processes and systems, using chemometric methods

It has been proposed to simulate the periodicity of changing in properties of elements with a sum of various frequency trigonometric functions (Fourier's polynomial), that is used to discover periodicity, when analyzing temporal series. Classification of properties with the help of factorial analysis has determined the difference in a structure of peculiar characteristics of elements, elementary substances, and binary compounds. When the periodicity is interpreted as a sum of elementary trigonometric functions, it allows application of digital filtering in investigations of chemical periodicity. It is proved that periodical function having the lowest frequency can be considered as characteristic of a length of the periodical system and thus can define its upper bound. On evidence of estimation of the available experimental data this bound is located near the 120-th element.

Conventional classification of f-elements represents their sharing into tetrads. We have proved that classification of f-elements, when taking into account all their properties, is rather more complicated. We have designed regression model of relation between properties of elements, electron configuration, and redox potential. This approach allows creation of models for prognosis and optimization of processes aimed at radionuclide recovery and separation.