3.2 Thermal Stability of Plutonium Fuels Estimated from Their Vaporization Behavior Data

Fig. 3-3 Principle of the measurement apparatus of vapor pressure at high temperature
Measured specimens are installed in a Knudsen-cell and heated to high temperature. Gaseous phase (vapor) and solid phase (specimen) are produced in equilibrium inside the cell by heating, then the vapor is ejected in ion states through a small hole of about 0.5 mm in diameter in the cap of the Knudsen-cell. The species of ionized vapors are analyzed by a mass spectrometer.

Fig. 3-4 Temperature dependence of the vapor pressure over SrPuO3-PuO2 in the Pt cell
From the experimental result that the vapor pressure of Sr(g) is two orders of magnitude higher than that of PuO(g) at high temperatures, it is speculated that SrPuO3 is firstly decomposed to PuO2 and Sr, and secondly that PuO2 is decomposed to PuO and O. At lower temperatures the amount of decomposed PuO2 is less than the detectable limit. From the measured results, the standard molar enthalpy of the formation of SrPuO3 has been deduced.

Recently, an attempt of the high burnup of fuel has been performed due to the demand for the cheaper cost of electricity generated by nuclear energy. The fission products and transuranium elements (Np, Pu, etc.) are accumulated in the oxide fuel with high burnup. It is very important to study the vaporization behavior in the complex oxides of perovskite type such as SrPuO3, in order to examine the safety of fuels under long time irradiation and high temperature conditions.
Study of the thermodynamic properties of the complex oxides containing Sr, Ba, etc. (alkali earth elements) and U, Np and Pu (actinide elements) has been carried out in collaboration with the University of Tokyo. The thermodynamic quantities of the vapor pressures of SrPuO3 and the standard molar enthalpy of formation of SrPuO3 have been measured using a quadrupole mass-spectrometer equipped with a Knudsen-cell as shown in Fig. 3-3, which is installed in a plutonium glove box maintained at negative pressure with respect to the atmosphere, as shown in Fig. 3-4. In addition, the mechanism of vaporization and decomposition has been clarified by analyzing the vaporized species over SrPuO3. At present the study of the vaporization behavior of BaPuO3 is being performed.

Reference
K. Nakajima et al., Vaporization Behavior of SrPuO3, J. Nucl. Mater., 248, 233 (1997).

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