Radiation-induced deterioration of fission reactor materials is dominated by displacement damage. In fusion reactors, the influence of (n,a) produced helium upon material deterioration is regarded to be of equal importance because of the high nuclear reaction rate caused by the high-energy fusion neutrons. In this review, the mechanisms and the anticipated rates of helium generation in fusion materials are discussed; helium introduction techniques simulating fusion conditions are reviewed in some detail and the atomistic behaviour of helium in metals as well as the nucleation and growth of helium bubbles are briefly surveyed. These phenomena are the main cause for the influence of helium on macroscopic material properties such as tensile strength, creep and fatigue behaviour and swelling. Typical examples of experimental results of material deterioration and first attempts at their theoretical modelling are given in the main part of the review. It is shown that helium effects can be the determining factor for the lifetime of fusion reactor components, particularly at high temperatures. The review concludes with an outlook on future investigations of helium effects and a call for a systematic approach in the development of helium-resistant alloys.
Type-II superconductors with v = 1 exhibit a first-order phase transition at H~~, which is due to an attractive interaction between flux lines. Thisbehaviorshowsupas adiscontinuousincrease of the flux density from zero to a certain value I30. We have measured Bo as a function of temperature T, Ginzburg-Landau parameter~, and impurity parameter e =0. 882 $0/&, and have determined the critical values of these parameters below which a first-order transition occurs at H,~. We further investigated the dependence of the parameters~~and~2 on $ and T, and we show that only the condition~~(T) =1/v 2 determines the transition from type-I to type-II behavior. Comparison of our experimental results with recent theoretical treatments shows only qualitative agreement.C
Neutron Diffraction by Vortex Lattlices in Superconducting i\b and IXb0.73Ta0.2, Neutron diffraction experiments on flux line lattices in superconducting Nb and XbTa are reported. From expcrimcntal data (form factors, lattice parameters, rocking curvcs ete.) following results are obtained. The microscopic flux distribution of a single flux line in NbTa could be determined. For Nb the form factors are compared with a theory valid in the vicinity of Hc2. A correlation between the flux line lattice and the crystal lattice has been found. Finally the attractive interaction between neighbouring flux lines in Nb has been verified and the resulting maximum lattice parameter compared with the corresponding distance in recent nonlocal calculations. Es wird iiber Neutronenbeugungsexperimcnte a n FluBliniengittern in supraleitendem Nb and NbTa berichtet. Aus den experimentellen Daten (Formfaktoren, Gitterparameter, Rocking-Kurven etc.) wurdcn folgende Resultate gewonnen: Die mikroskopische Feldverteilung eincr isolierten FluBlinie in NbTn konnte bestimmt werden. F u r Kb wiirden die gemesscnen Formfaktoren mit einer in dcr %,he von 11,~ gultigen Thcorie verglichen, und es wurde eine Wechsclwirkung zwischen dem FluBfadengitter und dem Kristallgitter gefunden. ScNieBlich wurdc die anziehende Wechselwirkung zwischen benachbarten Flu& linien im Nb bestatigt und dcr daraus resultierende maximale Gitterparamcter mit Werten aus theoretischen Rechnungen verglichen.
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