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The emission of uranium and ruthenium oxides from overheated nuclear fuel is a complex phenomenon which depends on the fuel matrix, on temperature, on the oxygen potential of the amosphere in contact and on the emission sequence. All these factors influence the properties of particles before their dispersion in the reactor containment building and in the environment: these properties are, among others, surface composition, particle size, and, as a consequence, weathering rates in the environment. Emission in air increases the solubility of U-oxides mainly. This first paper describes the main physico-chemical characteristics of condensation particles released by overheated U and Ru oxides mixtures, in small-scale laboratory simulations.

The emission of uranium oxides from overheated nuclear fuels proceeds as a function of time. depending mainly on temperature. oxygen potential of the amosphere in contact. and structure and composition of the fuel matrix itself. This Lime dependent emission results in variation of the U-concentration in the released aerosol. The subsequent annealing ("maturation") reactions acting on the aerosol particles are greatly affected bq the emission sequence of the different elements. The maturation phase strongly influences the properties of particles dispersed in thr environment: e.g. the rate of leaching of the particles is particularly sensitive to their "chemical history".

The physico-chemical characteristics of particles emitted by an overheated nuclear fuel may be significantly modified when they are put in contact with an oxidizing atmosphere at temperatures of the order of 700-900 degrees C. This so-called "maturation" stage induces reactions between radioelements and the UOx matrix of the aerosol particles. These reactions, in turn, deeply influence the solubility of radioelements in the environment. This paper presents experiments aimed at describing the behavior of characteristic radioactive elements present in nuclear fuel (Cs, Sr, Tb), which are emitted as a consequence of an accidental thermal excursion. As the sequence of radionuclide emission from the nuclear fuel depends on the volatility of their chemical forms, it has to be expected that the maturation stage itself is a time-dependent process in the sequence of emission events.

The chemical properties of radioactive molybdenum dispersed in the environment as a consequence of a nuclear reactor accident depend closely on the reactions taking place in the plume of the reactor or in the confinement building. An oxidizing atmosphere reacting with the aerosols at temperatures of the order of 700 degrees C significantly enhances the solubility of Mo and, as a consequence, its availability to the environment.

In case of a nuclear reactor accident, radionuclides are emitted in chemical forms which determine their mobility in the environment. Two stages are important in this aspect: (i) the emission stage which depends on the temperature and composition of the nuclear fuel and (ii) the so called 'maturation stage' during which UO2 aerosols react with gases and vapours in the plume of the damaged reactor or in the atmosphere of the confinement structure. The oxidizing nature and the temperature of the reacting gases exert a strong influence on the solubility of fission products carried by UO2 aerosols.