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Safety analyses of nuclear reactors require knowledge of the evaporation behavior of UO2 at temperatures well above the melting point of 3140 K. In this study, rapid transient heating of a small spot on a UO2 specimen was accomplished by a laser pulse which generates a surface temperature excursion. This in turn vaporizes the target surface and the gas expands into vacuum. The surface temperature transient was monitored by a fast-response automatic optical pyrometer. A computer program was developed to simulate the laser heating process and calculate the surface temperature evolution. A quadrupole mass spectrometer was used to identify and analyze the major vapor species in the vaporizing flow, and to measure the rate of evaporation from the target surface. The information yielded the partial vapor pressure of each species and the composition in the vapor jet. The maximum surface temperatures investigated range from approx. 3700 K to approx. 4300 K.
The kinetics of uranium carbide vaporization in the temperature range 3000 K to 5200 K was studied using a Nd-glass laser with peak power densities from 1.6 x 105 to 4.0 x 105 watts/cm2. The vapor species U, UC2, C1 and C3 were detected and analyzed by a quadrupole mass spectrometer. From the mass spectrometer signals number densities of the various species in the ionizer were obtained as functions of time. The surface of the irradiated uranium carbide was examined by scanning electron microscope and the depth profile of the crater was obtained. In order to aid analysis of the data, the heat conduction and species diffusion equations for the solid (or liquid) were solved numerically by a computer code to obtain the temperature and composition transients during laser heating. A sensitivity analysis was used to study the effect of uncertainties in the input parameters on the computed surface temperatures.
A new method of high-temperature mass spectrometry (TOF MS) was developed, where the specimen surface is heated by a laser pulse of approx. 20 ms. During it, time-resolved measurements of mass spectra and of the temperature are performed. Each experiment covers an entire temperature interval. The method was applied to pyrolytic graphite and uranium dioxide.In graphite study, it was clearly shown that the sublimation occurs in a Langmuir-like mode (free surface vaporisation), despite the very high temperatures and thus pressures. Relative partial pressures of C1, C2, C3, C4 and C5 were measured up to 4100 K. Obtained sublimation enthalpies of the main three vapour species are in a good agreement with literature values. Relative vaporisation coefficients of C1-C5 were estimated by comparison of the present partial pressures at 4000 K with equilibrium ones given in the literature.The vapour pressure curve of UO2 over stoichiometric uranium dioxide was measured between 2800 and 3400 K. Obtained sublimation and vaporisation enthalpies are in agreement with the literature. The vaporisation enthalpy of UO3 was measured for the first time. Relative partial pressure ratios p(UO2)/p(UO), p(UO2)/p(UO3) and p(UO2+)/p(UO+) were measured at around 3300 K and indicate that the vaporisation occurs in a regime close to thermodynamic equilibrium.This method is suitable for the fast and time-resolved mass spectrometric measurements of refractory materials up to very high temperatures, and could now be applied to the study of chemically unstable materials such as hyperstoichiometric urania and some carbides and nitrides.Key words: pyrolytic graphite, HOPG, uranium dioxide, laser vaporisation, TOF MS, vaporisation coefficients, Langmuir evaporation.
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