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.

Solubilities of neptunium and plutonium were studied in J-13 groundwater (ionic strength of about 3.7 mmol; total dissolved carbonate of 2.8 mmol) from the proposed Yucca Mountain Nuclear Waste Repository site, Nevada, at three different temperatures (25, 60, and 90 °C) and pH values (6.0, 7.0, and 8.5). Experiments were performed from both over-and undersaturation at defined CO 2 partial pressures. The solubility of 237 Np from oversaturation ranged from a high of (9.40 ( 1.22) × 10 -4 M at pH 6.0 and 60 °C to a low of (5.50 ( 1.97) × 10 -6 M at pH 8.5 and 90 °C. The analytical results of solubility experiments from undersaturation (temperatures of 25 and 90 °C and pH values 6, 7, and 8.5) converged on these values. The 239/240 Pu solubilities ranged from (4.70 ( 1.13) × 10 -8 M at pH 6.0 and 25 °C to (3.62 ( 1.14) × 10 -9 M at pH 8.5 and 90 °C. In general, both neptunium and plutonium solubilities decreased with increasing pH and temperature. Greenishbrown crystalline Np 2 O 5 ‚xH 2 O was identified as the solubility-limiting solid using X-ray diffraction. A mean thermodynamic solubility product for Np 2 O 5 ‚xH 2 O of log K°s p ) 5.2 ( 0.8 for the reaction Np 2 O 5 ‚xH 2 O + 2 H + h 2NpO 2 + + (x+1)H 2 O at 25 °C was calculated. Sparingly soluble Pu(IV) solids, PuO 2 ‚xH 2 O and/or amorphous plutonium(IV) hydroxide/colloids, control the solubility of plutonium in J-13 water.

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Isotopic compositions have been measured mass spectrometrically for xenon fractions released from the carbonaceous chondrite Murray in stepwise heating experiments. The isotopic ratios varied quite considerably; for example, the 136Xe/132Xe ratio in the 1000°C fraction was almost identical to the atmospheric ratio (0.330), whereas the ratio in the 1300°C fraction agreed with that in Sucor (0.305). It appears that these variations can best be explained as being due to the fact that reservoirs of two isotopically distinct gases (solar and planetary) exist in the meteorite and that mixtures of these gases in various proportions are being released at different temperatures. The major difference in the isotopic compositions of solar and planetary xenon can be attributed to a mass‐dependent fractionation process, which must have occurred during the early stages of the history of the solar system. The abundance ratios of the xenon isotopes are also altered by the cosmic ray irradiation and neutron capture processes. According to this interpretation, it is unnecessary to assume the existence of the so‐called carbonaceous chondrite fission component. The decay products of extinct radionuclides 129I and 244Pu did not alter the xenon isotopic ratios significantly in the case of the carbonaceous chondrite Murray. The light isotopes 124Xe and 126Xe in the earth's atmosphere appear to be partially of cosmic ray origin.

The isotopic compositions have been measured mass spectrometrically for xenon fractions released from the carbonaceous chondrite Murchison in stepwise heating experiments. Variation of the isotopic ratios was found to be relatively small: for example, the 136Xe/132Xe ratio in the 600°C fraction was 0.328 (the atmospheric ratio is 0.330), and the ratio in the 1500°C fraction was 0.310 (the Sucor ratio is 0.301), whereas the ratios observed in other temperature fractions had intermediate values. It appears that these variations can best be explained as being due to the fact that reservoirs of two isotopically distinct gases (solar and planetary) exist in the meteorite and that mixtures of these gases in various proportions are being released at different temperatures. The major difference in the isotopic compositions of solar and planetary xenon can be attributed to a mass‐dependent fractionation process, but the solar xenon contains excesses of xenon isotopes at mass numbers 128, 130, 131, and 132 that appear to have been produced by neutron capture processes that took place in the sun during its deuterium‐burning stage. The solar xenon must have been transported from the sun to the meteorites in the form of solar wind. The relative abundances of the light isotopes of xenon in carbonaceous chondrites are appreciably altered by the cosmic ray irradiation process. The decay products of 129I and 244Pu further modify the isotopic compositions of xenon fractions released from the carbonaceous chondrites. According to this interpretation, it is unnecessary to assume the existence of the so‐called Renazzo‐type fission xenon component (CCF or xenon X) in the carbonaceous chondrites.

A carrier-free chemical procedure has been developed to separate and purify nanogram quantities of the long-lived isomer of 236 Np ( 236i Np, tm = 1.2 χ 10 5 a) and 236 Pu from gram quantities of uranium. A target containing 2 g of highly enriched 23 5 U was irradiated for 3960 μ Ah with 20.1 MeV deuterons. A total of 4.45 χ 10 13 atoms of 236, Np and 1.14 χ 10 14 atoms of 236 Pu was recovered.

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