Re and Al 2 O 3 were heated with laser beams from both sides. Acting like planar heat sources, the two`hot plates' eliminate the axial temperature gradient in the sample between the plates. Temperature variation is less than 3% within roughly 30 mm diameter at 2,500 K. Before the melting experiments, the sample was scanned with a laser beam and heated to about 2,000 K to reduce the pressure gradient and to produce a high-pressure solid-phase assemblage. For stable and smooth temperature control, temperatures were increased by adjusting an aperture placed near the beam exit, stepwise, instead of by adjusting power. Each step corresponds to a 50±100 K increase. A 30-mm spot was homogeneously heated by opening the aperture (increasing the step). At the onset of melting, temperature remains constant or drops slightly with the step increment, and then drastically increases (.400 K) within one step. To ensure the reliability of the melting criteria used in this study, we conducted melting experiments at pressures (16±27 GPa) overlapped by the multi-anvil apparatus and the diamond-anvil cell, using the same starting material, and obtained consistent melting temperatures (Fig. 3). We also used the same melting criteria to determine the melting temperature of MgSiO 3 ±perovskite previously studied by other investigators, and our results agree with these recent determinations 13,14 (Fig. 3). The temperature runaway phenomena near the onset of melting observed in simple and complex samples were probably a result of the latent heat of melting, followed by melt migrating away from the heated spot because of the large thermal pressure and, ®nally, the Re foils would have been heated without sample in between. No chemical reaction between Re and sample was observed in the multi-anvil experiments on a scale of 1 mm. The melting temperatures reported here are the last temperatures before melting sets in. Pressures were measured using a ruby-¯uorescence technique after each measurement of melting temperature.

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DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document. I EXECUTIVE SUMMARYThe U.S. Department of Energy, Nevada Operations Office (DOE/NV), Environmental Restoration Division is seeking to evaluate groundwater contamination resulfing from 30 years of underground nuclear tesfing at the Nevada Test Site (NTS). This evaluation requires knowledge about what radioactive materials are in the groundwater and how they are transported through the underground environment. This information coupled with models of groundwater flow (flow paths and flow rates) will enable predictions of the arrival of each radionuclide at a selected receptor site. Risk assessment models will then be used to calculate the expected environmental and human doses.The accuracy of our predictions depends on the validity of our hydrologic and risk assessment models and on the quality of the data for radionuclide concentrations in ground water at each underground nuclear test site. This paper summarizes what we currently know about radioactive material in NTS groundwater and suggests how we can best use our limited knowledge to proceed with initial modeling efforts.The amount of a radionuclide available for transport in groundwater at the site of an underground nuclear test is called the hydrologic source term. The radiologic source ferm is the total amount of residual radionuclides remaining after an undergrokd nuclear test. The hydrologic source-term.is smaller than the radiologic source term because some or most of the radionuclide residual cannot be transported by groundwater. The radiologic source term. has been determined for each of the underground nuclear tests fired at the NTS; however, the hydrologic source term has been estimated from measurements at only a few sites. Laboratory studies have shown that radioactive residues from nuclear tests leach very slowly out of the melt debris formed in the explosion cavity. Many dissolved radionuclides sorb quite strongly on zeolites incorporated in the volcanics of the NTS, and hence do not move with groundwater. Thus for many radionuclides only a small fraction can be expected to move away from the cavity/&imney region where they were deposited at the time of the nuclear explosion; a few field observations confirm this expectation. There are a few radionuclides, however, which are produced in forms which are almost completely mobile in groundwater. Tritium (as tritiated water), 85Kr and radionuclides forming negative ions (e.g. 36Cl, 129I,99Tc and 1ZSb) are in this category. Such materials may serve as conservative tracers of groundwater flow because they do not interact with the rock through which they move.

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