The nature of the vapor produced by pulse surface heating of UO2 and several other materials was investigated using normal-mode laser pulses with peak power densities between 104 and 106 W/cm2. Maximum surface temperatures measured by optical pyrometry ranged from 1900 to 4200 K. At temperatures below ∼2400 K for UO2, the vapor pulse followed the Hertz–Langmiur vacuum vaporization theory. More energetic transients produced gasdynamic blowoffs accompanied by significant supersaturation in the expanding plume. Neutron activation analysis of the collected vapor indicated that the total quantity of liquid UO2 evaporated in the pulse followed the Hertz–Langmuir formula. However, mass spectrometric analysis of the emitted vapor showed large deviations in the quantity and waveform shape of the monomer species. These and other features of the blowoff were satisfactorily explained in terms of nucleation and growth of polymeric species. Although UO2 clusters were undetectable by the mass spectrometer, polymers of MgO and ZrO2 were observed.

Prof. D. R. Olander Chairman of CommitteeSafety analyses of nuclear reactors require knowledge of the evaporation behavior of U0 2 at temperatures well above the melting point of 3140 K. In this study, rapid transient heating of a small spot on a U0 2 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. The maximum surface temperatures investigated range from -3700 K to -4300 K. A computer program was developed to simulate the laser heating process and calculate the surface temperature evolution. The effect of the uncertainties of the high temperature material properties on the calculation was included in a sensitivity study for U0 2 vaporization. The measured surface temperatures were in satisfactory agreements. No dimer signal of any vapor molecule was measured, indicating the absence of condensation in the highly supersaturated vapor leaving the surface.A shock wave structure is developed by laser pulsing on a U0 2 target in an ambient inert gas. This structure was photographed during the laser pulse. By applying the Mack disk formula, the total vapor pressure corresponding to maximum temperature was obtained. The resulting low vapor pressure and low heat of vaporization deduced from this measurement is attributed to excessively high surface temperature measured due to nonequilibrium radiation from the hot vapor.Additional diagnostics of the phenomenum included collection of the vapor blowoff on disks followed by neutron activation to determine the angular distribution of the vaporization process. The extent of droplet production was also investigated by disk collection. Liquid droplets are observed, but the quantity of U0 2 they contained was insignificant compared to the total mass evaporated.