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Capture of Radionuclides by Depleted Uranium Oxides and Hydroxides

#2694


Investigation of Sorption Capture of Long-lived Radionuclides from Underground Waters by Depleted Uranium Oxides and Hydroxides

Tech Area / Field

  • CHE-THE/Physical and Theoretical Chemistry/Chemistry
  • ENV-RWT/Radioactive Waste Treatment/Environment
  • ENV-WDS/Waste Disposal/Environment

Status
8 Project completed

Registration date
05.02.2003

Completion date
24.02.2009

Senior Project Manager
Tocheny L V

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Supporting institutes

  • A.N.Frumkin Institute of Physical Chemistry and Electrochemistry, Russia, Moscow

Collaborators

  • Oak Ridge National Laboratory, USA, TN, Oak Ridge

Project summary

The objectives of the Project are:

- obtaining of experimental data set on sorption of long-lived radionuclides (237Np and 99Тс) from underground waters by dioxide and hydroxides of depleted uranium under various temperatures, pH, Eh, considering effect of ionizing radiation on the sorbent-liquid phase system;

- development of the model of long-lived radionuclides transport trough sorption layers of depleted uranium dioxide with the following performance of justified by calculation estimations of effectiveness of depleted uranium dioxide use in construction of storage casks with SNF (spent nuclear fuels) while burial in geological (underground) repositories.

Actuality of the problem is caused by accumulation of enormous quantity of residual products of fuel cycle of light-water reactors: spent nuclear fuel and depleted uranium. The Project provides for carrying out a complex of experimental and calculation works, justifying effectiveness of idea of co-dispose of these two types of wastes.

Over 106 tons of depleted uranium in UO2 form, being residual product of the process of enriched as for 235U nuclear fuel obtaining, have been generated worldwide. One of promising ways of use of depleted uranium dioxide (DUO2) can be application of DUO2 as a component of radiation shielding and natural geologic barrier of underground SNF repository, preventing release (while contacting of SNF with underground waters) of radioactive products of SNF and localizing long-lived radionuclides in zone of SNF repositories.

Kinetics of U(IV) hydroxide formation during contact of UO2 with underground waters and the following oxidation of U(IV) to U(VI) with possible formation of readily soluble complex forms of hexavalent uranium (i.e. easily migrating with underground waters) is an important factor, determining operation of geological barrier of DUO2. These processes are totally limited by DUO2 dissolution in underground waters. Therefore, determining the value, on can estimate DUO2 quantity which transforms in time into hydroxide of U(VI) and U(VI) compounds.

It is known that ionizing radiation effects significantly kinetics of uranium oxides dissolution. The process can well determine transformation of U(IV) into U(VI) at initial study of SNF storage, which in its turn should effect sorption capture of radioactive products in geological barrier of DUO2. Thus, obtaining of quantitative parameters (for example, dissolution rate constant) characterizing DUO2 dissolution in underground waters of certain composition both at absence and under ionizing radiation is important stage of conducted researches.

To make a final decision on use of DUO2 as a component of radiation shield and natural geological barrier of underground repository of spent nuclear fuel, it is planned to carry out a set of research works, including:

- study of physico-chemical transformation of depleted uranium dioxide while contacting with underground waters under variation of pH, Eh characteristics and temperature, as well as considering influence of gamma radiation;

- investigation of sorption capture of 237Np and 99Тс from ground-waters by depleted uranium dioxide under variation of pH, Eh characteristics and temperature;

- investigation of sorption capture of 237Np and 99Тс by hydroxides of depleted uranium under variation of pH, Eh characteristics and temperature.

Results being planned to obtain in a process of the Project realization, are new and belong to fundamental researches category. These are, at first, investigations of radionuclides adsorption from aqueous solutions (underground waters) by DUO2, as well as investigation of DUO2 dissolution kinetics in the field of ionizing radiation at contact with underground waters.

Generalization of experimental data obtained will give one a possibility to carry out correct quantitative forecast of possible expansion of long-lived radionuclides due to natural process of radionuclides migration from casks with SNF and also will allow one to elaborate technical solutions of effective use of DUO2 as natural geological barrier in underground SNF repositories.

Organizations – participants of the Project (Russian Federal Nuclear Center - VNIIEF and Institute of Physical Chemistry RAS) have a great experience in works in the field of burial of liquid radioactive wastes, conducting of radiochemical investigations, including sorption processes, as well as in studying of forms of existence of various radionuclides in water and aqueous solutions and effect of ionizing radiation on kinetics of solids dissolution.

All the experiments are planned to be carried out mainly at model specimens of underground waters.

Physico-chemical and radiochemical investigation connected with UO2 transformations and forming of tetravalent uranium hydroxides will be performed with use of standard chemical and radiochemical methods which were traditionally developed and improved in RFNC– VNIIEF and Institute of Physical Chemistry RAS.

It is expected to carry out experiments on adsorption in static (equilibrium) conditions because kinetic parameters of sorption are of no interest for forecasts of radionuclides capture over geological time.

Effect of ionizing radiation on processes of UO2 dissolution will be investigated directly in the process of all system liquid phase (underground water) – solid phase (UO2) irradiation. This will allow one to model conditions realizing in underground SNF repositories almost exactly.

All investigations will be carried out using certified measuring methods and modern recording and measuring equipment.

The Project duration – 24 months. Estimated cost – $200000.

Experience and skill of RFNC – VNIIEF and IPC RAS staff will make it possible to fulfill all the planned investigations on the date indicated.

The Project proposed totally suffice for purposes and goals of ISTC, namely:


- makes it possible to support applied researches and following development of technologies in the field of environment protection;
- provides with alternative employment more than 20 high-skilled specialists of RFNC-VNIIEF engaged in nuclear programs;
- makes for solution of international technical problems in the field of SNF utilization.

Purposes and goals of the Project were previously discussed with specialists of Oak Ridge National Laboratory (ORNL) acting officially as the Project collaborators.

Programs and methods of experimental investigations will be discussed with foreign collaborators.

It is planned in the course of the Project realization for data exchange:


- to present to foreign collaborators comments to technical reports, provided by participants of ISTC projects;
- to organize joint working seminars where collaborators could take part in discussion of results obtained in course of the Project realization.

Besides, collaborators could take part in technical inspection of activity of ISTC collaborators on the Project.


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