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Thermal Properties of Uranium Dioxide


Experimental and Theoretical Study of Thermophysical Properties of Uranium Dioxide (UO2) and Other Materials Related to the Problems of Nuclear Power Plants Safety

Tech Area / Field

  • FIR-FUE/Reactor Fuels and Fuel Engineering/Fission Reactors
  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors

8 Project completed

Registration date

Completion date

Senior Project Manager
Pradas-Poveda J I

Leading Institute
ITEF (ITEP), Russia, Moscow

Supporting institutes

  • Institute of Problems of Chemical Physics, Russia, Moscow reg., Chernogolovka\nVNIIEF, Russia, N. Novgorod reg., Sarov


  • Sandia National Laboratories, USA, NM, Albuquerque\nGSI, Germany, Darmstadt\nLos-Alamos National Laboratory / Detonation Science and Technology Group (MS P952), USA, NM, Los-Alamos

Project summary

Introduction and review.

Uranium dioxide (UO2) is a standard fuel of modern nuclear power plants. It has rather high melting and boiling temperature (Tm=3,100 °K, Tb ~ 3,800 °K). Numerous theoretical estimations of critical point parameters predict the critical temperature in the UO2 as Tcrit ~ 10,000 °K. Behavior of uranium dioxide in the case of hypothetical reactor accident on nuclear power plant determines the planning and elaboration of the required actions as well as the forecast of possible consequences of such an accident.

Despite of its extreme importance the physical information on high temperature thermophysical properties of the UO2 is rather poor. The modern knowledge of the properties is limited as a rule by the data on properties of solid uranium dioxide at low temperature. The data on properties of liquid UO2, especially at temperatures much higher than the melting one, are not sufficient and reliable. The results of Ronchi et al. on liquid UO2 heat capacity measurements up to 8,000 °K and on total vapor pressure over molten UO2 in so-called “in-pipe” experiments are almost the only available experimental information on high-temperature thermodynamic properties of uranium dioxide.

Meanwhile the evaporation in uranium dioxide has a remarkable feature dramatically distinguishing it from most of “simple” materials: this is so-called non-congruency, i.e. the equilibrium coexistence of phases with different stoichiometry (constituent element relation – Uranium and Oxygen). The most important consequence of this feature is a high degree of oxygen enrichment of vapors over the boiling uranium with simultaneous sharp increase of full equilibrium pressure, i.e. the characteristics directly affecting the safety of nuclear reactors.

Therefore, the fundamental task of high-temperature thermophysics and physics of non-ideal plasma in framework of nuclear safety problem is the investigation of Equation of State (EOS) of partially ionized multi-species mixture of great number of chemical components, including Uranium and Oxygen, which could be created by rapid heating of the fuel and constructive materials during possible accident in nuclear reactor.

The scope of the project is the integrated experimental and theoretical study of thermophysical properties of uranium dioxide at high temperatures and pressures at strong inter-particle interaction with revealing specific traits of phase transition characteristic of non-congruent phase equilibrium, including the UO2 behavior in the vicinity of critical points collection.

Technical approach and methodology.

The problems formulated in the project will be solved by employing of two perspective, mutually adding original experimental approaches:

– method of intense shock compression of investigated materials with followed isentropic expansion (release);

– method of intense quasi-isochoric and quasi-isobaric volume heating of materials by means of high- energy deposition from intense heavy ion beam.

Expected results and their application.

The Project will result in:

– Development of the theoretical model for description of non-congruent evaporation of heated UO2 products; performing the calculations of general phase diagram for the high-temperature system Uranium – Oxygen;

– Carrying out the measurements of phase equilibrium parameters of heated products of uranium dioxide, using the shock compression techniques and the technique of quasi-isochoric heating by intense heavy ion beams;

– Experimental determination of maximum level of Oxygen enrichment of gas phase in products of non-congruent evaporation of UO2 system; confirmation (or denial) of the theory predictions;

– Experimental determination of the dependence of parameters of liquid uranium dioxide evaporation process on the temporal profile of energy deposition by intense heavy ion beams;

– Development and testing the experimental method of measurement of electric conductivity of UO2 products heated by heavy ion beams. Comparing the data obtained with available theoretical models taking into account the effects of strong inter-particle coupling;

– Assessment and forecast of peculiarities of possible consequences of accidents on the modern nuclear power plants, basing on the analysis of the obtained new experimental and theoretical data on high-temperature behavior of uranium dioxide.

The scientific significance of the Project is obtaining the unique thermophysical information on behavior of materials in the high energy density region, developing new techniques for realization and diagnostics of such states.

The practical merit of the Project is determined by its ultimate aim, i.e. the development of environmentally benign methods for prevention of accidents caused by peculiar thermophysic properties of uranium vapors created by rapid heating of fuel on modern nuclear power plant. The results obtained in this project can be of interest for industrial applications at design of the plant installations from the point of view of modern procedure for assessment of risk and possible consequences of the accidents.

Meeting ISTC goals and objectives.

The project is entirely correspondent to ISTC aims for it provides scientists and personnel formerly engaged in weapons development an opportunity to conduct research aimed at peaceful activity. Besides it promotes real integration of Russian scientists in the international scientific community and supports basic and applied research and technology development for peaceful purposes. It should be noted the especially civil trend of presented project (the safety of nuclear energy production).

Scope of activities.

The project comprises the following main stages and investigations:

ITEP-Moscow designs and constructs the ion beam focusing system for beam-target experiments, a special camera of beam-target interaction, develop and tests the needed experimental methods. Jointly with IPCP RAS and GSI Darmstadt ITEP provides the experimental study of phase equilibrium parameters in the products of heating of uranium dioxide, using intense heavy ion beams.

IPCP-Chernogolovka together with VNIIEF-Sarov develops the experimental technique of generation of high energy density state of matter by using the high explosives. Jointly with ITEP and VNIIEF, IPCP conducts the experimental measurements of such parameters of heated materials as pressure, brightness temperature, sound velocity, and electric conductivity.

VNIIEF-Sarov together with IPCP-Chernogolovka develops new cumulative explosive generators for increasing the shock compression intensity, employs the method of isentropic release of preliminary intensively shock-compressed solid and porous samples to measure the phase equilibrium parameters of heated UO2 products.

At GSI-Darmstadt the joint experiments are performed on rapid heating of investigated materials by deposition of intense heavy ion beam energy on accelerator facility SIS-18.

Role of foreign collaborators.

At the project execution the researchers from Russian scientific centers will continue the mutual contacts with colleagues from Germany, France and USA, with the researchers from GSI-Darmstadt actively participating in experiments both in ITEP with heavy ion beams and in IPCP RAS on the explosive cumulative generator.

As a result of joint efforts of Russian and foreign collaborators a decisive contribution will be made to the solving the fundamental problem of high-temperature thermophysics, which is of high importance both for basic research and for application to the problem of safety of modern nuclear energy production.

It is proposed to discuss with foreign collaborators:

– the plan of organizing the project works;

– the basis for the theoretical model and calculation methods;
– the list of diagnostic techniques and equipment for conducting the investigations;
– the program and work schedule of common researches;
– carrying out the discussion of results.


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