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Nach dem Ausstieg aus der Atomkraftnutzung wird die Entsorgung der hoch radioaktiven Abfalle in Deutschland vermehrt diskutiert. Im Standortauswahlgesetz ist die Entsorgungsoption „Endlagerung“ festgelegt. Erfahrungsgemas kommen jedoch immer wieder Diskussionen daruber auf, welche Entsorgungsoption die Richtige ist, und zur Umsetzung einer Entsorgungsoption konnen verschiedene Entsorgungspfade beschritten weren. Daher sind neue Ansatze gefragt, um die Sicherheit von Entsorgungspfaden zu beurteilen und vergleichend zu bewerten.

Die Entsorgung hoch radioaktiver Abfalle ist ein anspruchsvolles Vorhaben. Sicherheit muss uber sehr lange Zeiraume gewahrleistet werden. Angesichts der damit verbundenen Ungewissheiten wird immer wieder gefordert, ein End- oder Tiefenlager uber langere Zeitraume kontrollieren zu konnen. Monitoring und Ruckholbarkeit sollen es erlauben, Abweichungen vom erwarteten Verhalten des Lagers fruhzeitig zu erkennen und ggf. Korrekturen am Entsorgungspfad vorzunehmen.

Mit der Entsorgung hoch radioaktiver Abfalle wird vorrangig das Ziel verfolgt, Mensch und Umwelt vor den Gefahren zu schutzen, die von den Abfallen ausgehen. Dazu mussen Risiken uber sehr lange Zeitraume kontrolliert und Vorsorge gegenuber Ungewissheiten getroffen werden. Ungewissheiten sind nicht nur in Bezug auf die Langzeitentwicklung der Lager von Bedeutung, sondern auch in Bezug auf die Standortauswahl, den Bau und Betrieb von End-, Tiefen- und Oberflachenlagern, die voraussichtlich viele Jahrzehnte beanspruchen werden. Wichtige Grundsatze, die dem Ziel der Sicherheit nachgelagert sind, sind Gerechtigkeit und Wirtschaftlichkeit.

International werden heute vor allem drei Entsorgungsoptionen naher in Betracht gezogen oder in nationalen Entsorgungsprogrammen umgesetzt, die Endlagerung, die Tiefenlagerung und die Oberflachenlagerung. Als Entsorgungspfad werden alle Schritte bezeichnet, die erforderlich sind, bis die Abfalle auf Dauer passiv sicher entsorgt worden sind bzw. so behandelt wurden, dass von ihnen kein unzulassiges Risiko mehr fur Mensch und Umwelt ausgeht. Die Risiken der Entsorgung variieren mit dem gewahlten Entsorgungspfad und der gewahlten Entsorgungsoption.

Gesellschaftliche Rahmenbedingungen entscheiden daruber mit, welche Entsorgungsoption und welcher Entsorgungspfad in einer spezifischen Situation die jeweils besten sind. Die Sicherheit einer Entsorgungsoption folgt einem komplexen Muster von Risiken und Ungewissheiten, das sich im Verlauf des Entsorgungspfades wandelt. Monitoring und Ruckholbarkeit bewegen sich im Spannungsfeld von Risiko, Ungewissheit und Akzeptanz. Zwischen diesen Anspruchen muss im Dialog mit der Zivilgesellschaft eine Optimierung angestrebt werden.

Metal organic vapor phase epitaxy (MOVPE) of Mn-rich (Mn,Ga)As on (001) oriented GaAs wafers resulted in atomically ordered (Mn,Ga)As crystallites of two morphological kinds, partially embedded on the wafer surface and fully embedded within the single crystalline matrix. While the former were apparently free of defects (other than unavoidable point defects), the latter contained two domains separated by a grain boundary. Since atomic ordering can be modeled by space group symmetry descent considerations (Barnighausen trees) that start with the space group of the known crystallographic phases of random (Mn,Ga)As alloys with specified chemical compositions, reasonable structure hypotheses have been derived for two atomically ordered Mn0.75Ga0.25As phases that we call the trigonal α' and the monoclinic β' phases. The implications of these structure hypotheses are in agreement with the results of a range of scanning transmission electron microscopy (STEM) and parallel illumination electron diffraction (ED) studies that include quantitative energy dispersive X-ray spectroscopy, X-ray spectroscopic imaging, as well nanobeam diffraction and high angle precession ED. The coexistence of two domains within the fully embedded crystallites is predicted by the corresponding Barnighausen tree and observed experimentally for the fully embedded crystallites.

X-ray diffraction and transmission electron microscopy were used to probe the structure of the misfit compound [(SnSe)1.15]1(VSe2)1 grown using an elementally modulated precursor. The specular X-ray diffraction pattern contained only 00l reflections, which yielded a c lattice parameter of 1.203(1) nm. Cross-section STEM revealed alternating layers of SnSe and VSe2, in agreement with the structure model refined from the X-ray diffraction pattern using Rietveld refinement. Plan-view transmission electron microscopy revealed the in-plane grain structure of the films, yielding grain sizes in agreement with previously reported in plane X-ray diffraction studies and the cross-section STEM images. The plan view images also contained Moiré fringes resulting from grains with different relative tilting on both sides of interfaces as well as Moiré fringes resulting from different relative rotations between domains. An energy-filtered nano-beam electron diffraction pattern obtained from at least one domain in the [(SnSe)1.15]1(VSe2)1 sample investigated in cross section contained a series of resolvable supercell reflections along the c axis that indicated that the supercell c-axis lattice parameter was a multiple of three times that determined using X-ray diffraction. Energy filtered NBED of plan-view samples showed diffraction patterns from select regions with 12-fold symmetry, indicating that the arrangement of the layers is not rotationally random from layer to layer. This suggests that during the self-assembly of the amorphous modulated elemental precursor, the SnSe and VSe2 constituent layers must nucleate off the adjacent interfaces of the growing crystal, yielding layers that are locally rotationally aligned with growing crystal. Different processing conditions during the precursor to crystal self-assembly might enable the domain size and/or the extent of order to be controlled.

We demonstrate experimentally an efficient electron axicon lens using a magnetic vortex. We show that naturally occurring magnetic vortices with circular magnetic moment distributions in a soft-magnetic thin film create conical phase shifts for fast electrons. Such radially symmetric linear phase ramps are equivalent to ideal light optical axicons. We apply this lens to generate efficient nondiffracting electron Bessel beams, which we observe experimentally in through-focus Lorentz images as well as in propagated off-axis electron holograms. This highlights the potential for using magnetic nanostructures as highly efficient and flexible phase plates for crafting desired electron beam shapes.

In der Bundesrepublik Deutschland lauft zurzeit ein Prozess zur Auswahl eines Endlagerstandortes fur bestrahlte Brennelemente und hoch radioaktive Abfalle aus der Wiederaufarbeitung. Da dieser Auswahlprozess und die anschliesende Errichtung und Inbetriebnahme des Endlagers langer dauern wird als bis zum Ablauf der bisher auf 40 Jahre beschrankten Genehmigungen der Zwischenlagerung fur diese radioaktiven Abfalle, muss entweder die Genehmigung der jetzigen Zwischenlagerung verlangert werden oder es ist ein Neubau von Zwischenlagern erforderlich. Bereits fur die bisherige Zwischenlagerung gab es Diskussionen, inwieweit die nach Atomgesetz erforderliche Vorsorge ausreichend gegeben ist. Das betraf vor allem die Standzeit von Behalterdichtungen und Neutronenmoderator, die Uberwachung von Freisetzungen, die Kontrollmoglichkeit des Zustands von Behalter und Behalterinventar sowie die Auslegung gegen schwere Einwirkungen von ausen. Fur eine Verlangerung der Zwischenlagerdauer gewinnen diese Sicherheitsaspekte mit unmittelbarer Auswirkung fur den Strahlenschutz noch verstarkt an Bedeutung. Vor diesem Hintergrund und den bisherigen Erfahrungen werden in diesem Beitrag Sicherheitsaspekte diskutiert, die mit dem Strahlenschutz im Normalbetrieb in Zusammenhang stehen.

The atomic structure of the family of ferecrystals [(SnSe)1.15]m(TaSe2) (m = 1, 3, and 6) was investigated by means of transmission electron microscopy. The tantalum in the TaSe2 layers was observed to have trigonal prismatic coordination similar to that found in the 2H polytype of bulk TaSe2. The structure of the SnSe constituent was found to be similar to that of orthorhombic α-SnSe. In the compounds with m = 1 and m = 3, regions with a local ordering of the layers along a commensurate axis, similar to the ordering in conventional misfit layer compounds, were observed. However, on a longer range the ferecrystals were found to exhibit a turbostratically disordered structure. Stacking defects were occasionally found in the samples in which a layer is interrupted and the surrounding layers are bent around these defects, while maintaining abrupt interfaces instead of interdiffusing. Volume defects were found in one sample of [(SnSe)1.15]1(TaSe2)1 in which a SnSe layer locally substitutes a part of a TaSe2 layer without interrupting the surrounding layers.

Since the initial observation of melting-point reduction with size for gold nanoparticles, the relative stability of crystalline phases has also been demonstrated to be size-dependent in a number of technologically important materials, including CdSe and CdS, Al2O3, [4] and various other metal oxides and chalcogenides. As the surface free energy contribution to the total free energy becomes increasingly important as the size of a system is decreased, a crystalline phase with lower surface free energy may be favored with respect to the thermodynamically stable bulk phase when the crystallite size is smaller than a critical value in one or more dimensions. The role of size as an effective thermodynamic parameter is of fundamental importance, but also provides a mechanism for controlling the crystal structure of a material, and therefore its properties. Although precise control of surface chemistry and nanocrystal size in well-defined material systems is prerequisite to understanding and controlling size-induced phenomena, it nevertheless remains challenging to achieve. Although impressive progress has been made in the preparation of ensembles of inorganic nanocrystals with relatively narrow size distributions, the preparation of nanocrystal ensembles of completely uniform size that can be tuned with atomic precision would constitute a significant synthetic advance, in particular for the application of size-dependent structural and physical properties. Using a combination of experimental and computational techniques, we demonstrate herein that chemically designed nanolaminates, consisting of an intergrowth of chemically and structurally distinct components, comprise a class of materials in which this level of control can be achieved for one crystallographic direction. As a consequence of the ability to precisely control size, the crystal structure of the components can be tuned via a size-induced transformation. In contrast to epitaxial superlattices, which experience structural distortions due to strain induced by epitaxial interfaces, the intergrowths reported on in the present work lack an epitaxial relationship between the components. This structural independence of the constituents allows the effect of size on crystal structure to be delineated from strain effects. It was recently discovered that [(MSe)1+d]m[TSe2]n intergrowths can be prepared by the modulated elemental reactants (MER) synthetic route. Here M= {Pb, Sn, Bi, Ce}, T is an early transition metal, and the integers m and n denote the number of consecutive layers of the respective components in the repeating unit of the intergrowth (Figure 1). The value of d reflects the difference in the inplane atomic packing density for the independent component structures in the intergrowth (hereafter we use the designation [MSe]m[TSe2]n for convenience). We chose to explore the SnSe-MoSe2 system for several reasons. Along with the prospect of developing novel nanocrystalline SnSe materials for optoelectronic and photovoltaic applications, bulk crystalline SnSe undergoes a secondorder (continuous) structural phase transition from a-SnSe (GeS structure type) to b-SnSe (ThI structure type) as the temperature is increased. As the parent compounds SnSe and MoSe2 (Figure 1a and b) are both layered semiconductors, significant charge transfer between the components in the intergrowth is not expected. A stable interface exists

Synthesis and structural characterization of a turbostratically disordered polymorph of (PbSe)1.18(TiSe2)2 is reported. The structure of this compound consists of an intergrowth between one distorted rock salt structured PbSe bilayer and two transition metal dichalcogenide structured Se–Ti–Se trilayers. In addition to the lattice mismatch, there is extensive rotational disorder between these constituents. The electrical resistivity of (PbSe)1.18(TiSe2)2 is a factor of 9 lower at room temperature, and the Seebeck coefficient is almost double that reported for the crystalline misfit layered compound analogue.

The turbostratically disordered misfit layer compound (SnSe) 1.15 VSe 2 was synthesized and structurally characterized. Electrical transport measurements suggest this compound undergoes a charge or spin density wave (CDW or SDW) transition, which has not been observed in previous misfit layer compounds. The (SnSe) 1.15 VSe 2 compound, created through the modulated elemental reactants technique, contains highly oriented intergrowths of SnSe bilayers and VSe 2 structured Se–V–Se trilayers with abrupt interfaces between them perpendicular to the c -axis. X-ray diffraction data and transmission electron microscope images show that each constituent has in-plane crystallinity but that there is a random rotational disorder between the constituent layers. Temperature-dependent electrical resistivity data and Hall measurements are consistent with (SnSe) 1.15 VSe 2 being a metal, however an abrupt increase in the resistivity occurs between 30 and 100 K. The carrier concentration decreases by approximately 1 carrier per vanadium atom during this temperature interval.

A modification of the modulated elemental reactants synthetic technique was developed and used to synthesize eleven members of the [(SnSe)1.15]m(TaSe2)n family of compounds, with m and n equal to integer values between 1 and 6. Each of the intergrowth compounds contained highly oriented intergrowths of SnSe bilayers and TaSe2 monolayers with abrupt interfaces perpendicular to the c-axis. The c-lattice parameter increased 0.579(1) nm per SnSe bilayer and 0.649(1) nm per Se–Ta–Se trilayer (TaSe2) as m and n were varied. ab-plane X-ray diffraction patterns and transmission electron microscope images revealed a square in-plane structure of the SnSe constituent, a hexagonal in-plane structure for the TaSe2 constituent, and rotational disorder between the constituent layers. Temperature dependent electrical resistivity, measured on several specimens, revealed metallic behavior, and a simple model is presented to explain the differences in resistivity as a function of m and n.