938 research outputs found
Habits of the Common Shelduck
This is where the abstract of this record would appear. This is only demonstration data
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Radiation-induced decomposition of U(6) alteration phases of UO2
U{sup 6+}-phases are common alteration products of spent nuclear fuel under oxidizing conditions, and they may potentially incorporate actinides, such as long-lived {sup 239}Pu and {sup 237}Np, delaying their transport to the biosphere. In order to evaluate the ballistic effects of {alpha}-decay events on the stability of the U{sup 6+}-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr{sup 2+}) for six different structures of U{sup 6+}-phases: uranophane, kasolite, boltwoodite, saleeite, carnotite, and liebigite. The target uranyl-minerals were characterized by powder X-ray diffraction and identification confirmed by SAED (selected area electron diffraction) in TEM (transmission electron microscopy). The TEM observation revealed no initial contamination of uraninite in these U{sup 6+} phases. All of the samples were irradiated with in situ TEM observation using 1.0 MeV Kr{sup 2+} in the IVEM (intermediate-voltage electron microscope) at the IVEM-Tandem Facility of Argonne National Laboratory. The ion flux was 6.3 x 10{sup 11} ions/cm{sup 2}/sec. The specimen temperatures during irradiation were 298 and 673 K, respectively. The Kr{sup 2+}-irradiation decomposed the U{sup 6+}-phases to nanocrystals of UO{sub 2} at doses as low as 0.006 dpa. The cumulative doses for the pure U{sup 6+}-phases, e.g., uranophane, at 0.1 and 1 million years (m.y.) are calculated to be 0.009 and 0.09 dpa using SRIM2003. However, with the incorporation of 1 wt.% {sup 239}Pu, the calculated doses reach 0.27 and {approx}1.00 dpa in ten thousand and one hundred thousand years, respectively. Under oxidizing conditions, multiple cycles of radiation-induced decomposition to UO{sub 2} followed by alteration to U{sup 6+}-phases should be further investigated to determine the fate of trace elements that may have been incorporated in the U{sup 6+}-phases
Quasicontinuum -decay of Zr: benchmarking indirect () cross section measurements for the -process
Nuclear level densities (NLDs) and -ray strength functions
(SFs) have been extracted from particle- coincidences of the
Zr()Zr and Zr()Zr
reactions using the Oslo method. The new Zr SF data, combined
with photonuclear cross sections, cover the whole energy range from ~MeV up to the giant dipole resonance at ~MeV. The wide-range SF data display structures at ~MeV, compatible with a superposition of the spin-flip
resonance and a pygmy resonance. Furthermore, the SF shows a
minimum at ~MeV and an increase at lower -ray
energies. The experimentally constrained NLDs and SFs are shown to
reproduce known () and Maxwellian-averaged cross sections for
Zr using the {\sf TALYS} reaction code, thus serving as a benchmark
for this indirect method of estimating () cross sections for Zr
isotopes.Comment: 10 pages and 9 figure
Relation between the 16O(alpha,gamma)20Ne reaction and its reverse 20Ne(gamma,alpha)16O reaction in stars and in the laboratory
The astrophysical reaction rates of the 16O(a,g)20Ne capture reaction and its
inverse 20Ne(g,a)16O photodisintegration reaction are given by the sum of
several narrow resonances and a small direct capture contribution at low
temperatures. Although the thermal population of low lying excited states in
16O and 20Ne is extremely small, the first excited state in 20Ne plays a
non-negligible role for the photodisintegration rate. Consequences for
experiments with so-called quasi-thermal photon energy distributions are
discussed.Comment: 4 pages, 2 figures, Proceedings Nuclear Physics in Astrophysics-II,
Debrecen, Hungary, 200
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Radiation-Induced Decomposition of U(VI) Phase to Nanocrystals of UO2
U{sup 6+}-phases are common alteration products, under oxidizing conditions, of uraninite and the UO{sub 2} in spent nuclear fuel. These U{sup 6+}-phases are subjected to a radiation field caused by the {alpha}-decay of U, or in the case of spent nuclear fuel, incorporated actinides, such as {sup 239}Pu and {sup 237}Np. In order to evaluate the effects of {alpha}-decay events on the stability of the U{sup 6+}-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr{sup 2+}) of U{sup 6+}-phases. The heavy-particle irradiations are used to simulate the ballistic interactions of the recoil-nucleus of an {alpha}-decay event with the surrounding structure. The Kr{sup 2+}-irradiation decomposed the U{sup 6+}-phases to UO{sub 2} nanocrystals at doses as low as 0.006 displacements per atom (dpa). U{sup 6+}-phases accumulate substantial radiation doses ({approx}1.0 displacement per atom) within 100,000 years if the concentration of incorporated {sup 239}Pu is as high as 1 wt%. Similar nanocrystals of UO{sub 2} were observed in samples from the natural fission reactors at Oklo, Gabon. Multiple cycles of radiation-induced decomposition to UO{sub 2} followed by alteration to U{sup 6+}-phases provide a mechanism for the remobilization of incorporated radionuclides
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