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Environmental Safety of Extraction Reprocessing

#2034


Improvement in Environmental Safety of Irradiated Material Extraction Reprocessing

Tech Area / Field

  • CHE-RAD/Photo and Radiation Chemistry/Chemistry
  • FIR-FUC/Fuel Cycle/Fission Reactors

Status
3 Approved without Funding

Registration date
30.10.2000

Leading Institute
All-Russian Scientific Research Institute of Non-Organic Materials named after A. Bochvar, Russia, Moscow

Supporting institutes

  • Institute of Physical Chemistry and Electrochemistry, Russia, Moscow\nVNIIKhT (Chemical Technology), Russia, Moscow

Collaborators

  • Florida State University / Department of Chemistry, USA, FL, Tallahassee

Project summary

The problem of environmental safety, inherent in the radiochemical technology of spent nuclear fuel reprocessing, involves many aspects: critical safety, equipment safety (application of equipment, operating under pressure), reagent safety (use of explosive and inflammable chemical agents and substances), radioactive waste storage safety, etc.

Throughout the world the reprocessing of spent fuel rods of nuclear power plants to extract Pu and other valuable components is currently implemented with extraction technology. Tributylphosphate (TBP) is basically used as an extractant with aliphatic hydrocarbons (n-paraffins) as a diluent. Practice of radiochemical production plants shows that, under the effect of ionizing radiation of radionuclides as well as of temperature, which, under specific conditions may substantially exceed that stipulated for the technology, the composition of the extraction solution is subject to substantial changes resulting in poor refinement of U and Pu from fission products, losses of valuable elements with organic solution, extraction process disturbances due to the degrading conditions of organic and aqueous phase separation owing to the formation of stable interphase films (jellies), emulsions, etc. Changes in extraction solution composition, specifically, the build-up of almost insoluble water high molecular hydrocarbon products of diluent decomposition, limits the duration time for use of this extractant in the technologic process and leads to the need for its premature removal.

The operation used to remove spent extractant from the process cycle is the key source of organic waste build up at radiochemical production plants. Uranium, plutonium and fission products, present in these solutions, categorize them as intermediate activity level waste and, hence, they must be isolated from the environment.

Organic waste is one of the most dangerous kinds of waste due to its higher inflammability and explosiveness. Despite investigations that are going on in different nuclear centers, no methods have been developed for the environmentally safe handling of this type of waste. Semi-commercial means of organic waste isolation involve its conservation in special tanks and its disposal in deep porous geologic formations. The radiation effects in aqueous-organic solutions, subjected to long-term storage, can lead to the formation of metal-organic deposits that increase nuclear danger, raise local temperature and release explosive gases; in solutions contained by geologic rocks the radiation effects can give rise to the formation of products influencing the degree of radionuclide migration within beds, etc.

The suggested Project deals with issues of extraction technology safety relating to spent extractant waste that arises as a result of the harmful effect of ionizing radiation and elevated temperature on the process system TBP+diluent+HNO3.

The real improvement of the environmental safety of the processing technology, resulting in substantially reduced waste, may be promoted, on the one hand, by an increased radiation-chemical and thermochemical stability of the extractant (TBP+diluent) and, on the other, through the application of adequately radiation and thermally resistant hydrocarbon diluents of TBP, the radiolysis of which results in a low yield of high molecular hydrocarbon products (almost insoluble in aqueous solutions in standard operations) and in a higher yield of readily soluble low molecular ones.

The realization of the objectives is feasible if investigations in the TBP+hydrocarbon diluent system are implemented in the following basic directions:


- validation of criteria for assessing the quality of irradiated extractant and methods to control these criteria;
- development of a scientifically and experimentally validated method for selecting diluents with specified physico-chemical properties;
- development of a purposeful approach to the selection of protective anti-radiation additives (inhibitors) that provide a longer service life to extraction systems.

The first investigations into the radiolysis of TBP solutions were implemented in the mid 1950s and some success is now evident in the radiation chemistry of this extractant. Available data on the radiolysis and hydrolysis of TBP, diluents and TBP solutions in different diluents are comprehensively covered in the following papers:

1. M.V.Vladimirova, I.A.Kulikov. “Hydrolysis and Radiolysis of TBP”. M., TSNIIATOMINFORM. Issue 1 (89). 1984.

2. A.S.Solovkin, M.V.Vladimirova, I.A.Kulikov. Extraction of Metal Ions with Mono- and Di-n-butylphosphoric Acids, Products of Hydrolysis and Radiolysis of tri-n-butylphosphate. Itogi nauki i techniki. Inorganic Chemistry Series. V.12, M., VINITI, 1985.

3. G.F.Egorov. Radiation Chemistry of Extraction Systems. M., Energoatomizdat. 1989.

4. M.V.Vladimirova, I.A.Kulikov, A.A.Kupriy. Acid Hydrolysis of TBP. M.: Preprint, VNIINM, 6-53 1989.

5. M.V.Vladimirova, I.A.Kulikov, A.A.Kupriy. “Thermooxidation of TBP – Diluent System”. Atomnaya Energiya. 1991. V.71, issue 4, p.333.

However, very serious unresolved theoretical and practical problems are inherent in the radiation chemistry of TBP. The unavailability of complete experimental data on inpidual and complex effects of chemical and physicochemical factors (thermolysis, photolysis, radiolysis) leading to extraction system destruction, and unavailability of developed mechanisms of radiolysis and thermolysis of TBP and diluents, make it currently unfeasible to create irradiation and thermally resistant diluents, to predict the behavior of TBP-based aqueous-organic solutions in a wide range of specified conditions (concentrations of TBP, diluent, HNO3 and other mineral substances and metals, diluent type, dose rate of the particular kind of ionizing radiation, time of irradiation temperature; ratio between the aqueous and organic phase volumes etc), and, finally, to resolve completely the problem of waste minimization and improvement of the environmental safety of extraction systems.

The objective of the Project is to involve complex investigations of radiation-chemical and thermo-chemical processes in TBP-based extraction systems in different hydrocarbon diluents, to acquire more complete experimental and theoretical information on processes, to develop scientific fundamentals with a view to increase the radiation-chemical and thermochemical stability of the above systems and to elaborate validated recommendations and procedures for the selection of diluents for TBP.

The investigations are aimed at minimizing radioactive waste, and improving environmental conditions and fire- and explosion safety of radiochemical production plants.


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