Zum Verhalten von beschichteten Brennstoffteilchen bei sehr hohen Temperaturen

Coated partiales of different kernel and coating design (U, Th) O$_{2}$-Biso (methane), ThO$_{2}$-Biso (propene), ThO$_{2}$-Biso (mixgas), UC$_{2}$ -Triso (propene) UO$_{2}$-Triso (propene) have been treated at very high temperatures (2300-2500 °C). After this treatment the samples have been analysed microradiographicaly and by metallographic cross sections. After the 24 h heat treatment at 2500 °C the methane-derived pyrocarbon (HTI) particles of THTR-design [(U, Th) O$_{2}$-Biso] do not show any change an the heavy metal distribution. Propene-derived pyrocarbon is under these conditions quasi-permeable to CO, so that the oxide kernel can be reduced to carbide, which then loses heavy metal through the coating. The SiC-interlayer of the Triso coatings decomposes very fast under those conditions to carbon and silicon and therefore the retention of this interlayer is destroyed. The SiC-layer can also be destroyed by the interaction with the diffused heavy metal, if the kernel containes a carbide like the fissile particle design of General Atomic. These results are in a good agreement with thermodynamic considerations made for the various observations

Untersuchungen zur Entwicklung hochtemperaturfester Aktivierungssonden

Es wurden Untersuchungen durchgeführt zur Entwicklung von Aktivierungssonden für Fluenzmessungen in Hochtemperaturreaktorexperimenten, das heißt von Sonden, bei denen auch für Temperaturen oberhalb 1200°c und Expositionszeiten von 200 Tagen die Verluste des Meßnuklids kleiner als 1 % bleiben. Es erwiesensich als aussichtsreich die Sondensysteme, die aus einer hochtemperaturstabilen Legierung des Meßnuklids mit einem Metall hoher Löslichkeit, eingeschlossen in einer Kapsel aus einem Material sehr geringer Löslichkeit, bestehen. Die sich in diesem System ergebende geringe Konzentration des Meßnuklids in der inneren Randschicht der Kapsel führt zu einem entsprechend geringen Materialtransport durch die Kapselwand. Out-of-pile-Versuche mit Sonden dieser Art, bestehend aus (V-1 % Co)-Legierung, eingeschweißt in Al$_{2}$O$_{3}$-Kapseln, zeigten bei 1400°C und bei einer Glühzeit von 104 Tagen einen Kobaltverlust von maximal 6 %$_{0}$. Wenig Aussicht besteht, kobalthaltige Substanzen bzw. Kapselmaterialien für Kobalt zu finden, die allein aufgrund eines hinreichend kleinen Kobaltdampfdrucks bzw. einer sehr kleinen Diffusionskonstanten ein ausreichendes Rückhaltevermögen haben. Zur Frage der chemischen Beständigkeit der Sonden wurden Versuche zur Geschwindigkeit der Korrosion von Al$_{2}O_{3}$ und Niob durch Graphit gemacht. Sie ergaben, daß es möglich ist, die Keramik durch Einschluß in eine Niobkapsel vor der chemischen Zersetzung zu bewahren

Bestimmung von Sauerstoff in Urancarbiden nach einer neuen badfreien Trägergasmethode

A report is given on the determination of oxygen in uranium carbide by a new method of hot extraction in a stream of nitrogen replacing the molten bath of the vacuum fusion method. The nitrogen is not only used as a carrier gas but also as a chemical reactant, which - by the formation of stable nitrides and carbonitrides - removes the oxygen impurities present as U0$_{2}$ and U(C,0) in the uranium carbide. The utility of the new method has been demonstrated by the results of several series of determinations which have been obtained under comparable conditions by vacuum fusion and by the use of argon as carrier gas. The results of thermodynamic calculations on the reaction mechanism of the removal of oxygen from UC-samples by hot extraction without fusion using a stream of nitrogen carrier gas as well as vacuum could be confirmed by chemical, X-ray diffraction, and metallographical analysis of outgassed samples. By avoiding the use of a molten-metal bath the nitrogen carrier gas method presents the following advantages over the other fusion methods: 1. The troublesome influence of graphite precipitations in the liquid metal bath on the process of dissolution and outgassing is avoided; 2. Losses of the gas by gettering on vapourised metal are avoided; 3. Both the possibility to use the graphite crucible several times and the saving of capsules and bath materials reduce the cost of series determinations. The applicability of this method is not limited to the determination of oxygen in uranium compounds. All elements the oxides of which react with carbon in a stream of nitrogen and form stable nitrides or carbonitrides and CO may be analysed by this metho

Hexagonales Urannitrid

An investigation was carried out on the conditions of producing hexagonal $\beta$-uranium nitride. The latter can be produced in its pure form by heating UN or $\alpha$-UN$_{x}$ to 1320 ± 50 °C at a N$_{2}$-pressure of 1 atm. The homogeneity range of thc $\beta$-phase extends from about $U_{3}N_{4}$ to $U_{2}N_{3}$ (measured: minimum UN$_{1,34}$, maximum UN$_{1,45}$). $\alpha$-UN$_{x}$, and $\beta$-UN$_{x}$ are not modifications of the same chemical compound; the $\alpha \rightleftarrows \beta$ transformation observed by various workers is accompanied by changes of the chemical composition. If the nitrogen absorption from the gas phasc is avoided, then $\beta-UN_{x}$, remains stable even below 900 °C. Thc lattice constants were: a$_{0}$ = 3,698 ± 0,002 $\mathring{A}$ and $e_{0}$ = 5,839 ± 0,004 $\mathring{A}$

Zur Bestimmung der Belastungsgrenzen verschiedener nichtmetallischer Materialien mittels Laserbestrahlung unterschiedlicher Leistungsdichte und Pulsdauer

For the selection of materials for plasma facing components (PFC) in future nuclear fusion devices subjected to short-time high heat fluxes it is desireable to determine the threshold values of the occuring damages in laboratory experiments. In order to determine the response of candidate materials to thermal shock-loading different nonmetallic materials were exposed to the beam of a 10 kW NdYAG solid state laser situated in the Institute für Lasertechnik under the Fraunhofer Gesellschaft in Aachen. The impinging power density was variied between 200 and 2000 MW/m$^{2}$ for pulse lengths t$_{p}$ ranging from 0.1 to 10ms. Similar conditions are expected to occur under plasma disruptions in Tokamak fusion devices. The absorbed power density $\Phi_{abs}$ was determined by measuring reflexion losses from the sample surfaces. The main damages on nitride and carbide ceramics were cracking and chemical decomposition. Sublimation was the main structural change observed on all carbon materials though cracking could be detected in pyrolytic grafite and melting in a coatmix material. Two coated materials showed cracking and melting of the coating layer. The threshold values for these structural damages were quantified by using a heat flux parameter $\Phi_{abs} \surd t_{p}$. Parallel to these experiments numerical calculations were carried out. The calculated threshold values are in approximate agreement with the experimental results. Other heat tests on selected carbon materials were carried out to establish a comparison between electron beam and laser beam erosion under nearly equal conditions. Theresults are in a good qualitative accordance. It can be concluded that laser beam testing presents a simple methode to rate materials according to their behavior under thermal shock conditions

Untersuchungen zur Bestimmung der Korrosionskinetik und der Struktur von Korrosionsprodukten an Hochtemperaturlegierungen bei niedrigen Sauerstoffpartioldrücken

On three nickel-base-alloys (NIMONIC 80A, INCONEL 617 and ALLOY 713LC), which are planned as materials for components of the primary cooling circuit of high-temperature reactors, studies for the determination of the oxidation behaviour under reactor relevant conditions were performed. The test conditions were fixed at an oxygen partial, pressure of 10$^{-16}$ bars with a total pressure of 0,2 bars and a temperature of 1080 °C. The use of the X-ray diffraction analysis on the oxidised samples of type NIMONIC 80A showed the corrosion products Cr$_{2}$0$_{3}$, TiO$_{2}$ and Cr$_{2}$Ti$_{2}$O$_{7}$ in the outer oxide coating as well as Al$_{2}$O$_{3}$ in the inner oxide zone. The samples of the alloy type INCONEL 617 showed the oxides Cr$_{2}$O$_{3}$ and Cr$_{2}$Tl$_{2}$O$_{7}$ in the outer oxide coating. Samples of the alloy ALLOY 713LC showed Cr$_{2}$O$_{3}$ and CrNbO$_{4}$ in the outer oxide coating and Al$_{2}$O$_{3}$ in the zone of inner oxidation. Sufficient detection certainty was only ascertained by investigating samples with plane surface. For both alloys first mentioned, it was possible to prove the validity of the paralinear relationship W = k$\cdot$t$^{-1/2}$ - a$\cdot$t for the increase in weight for the explanation of the time law of the oxidation. For the non-destructive determination of the oxide coating thickness on the alloys samples, the X-ray diffraction analysis gave reproducible results of good precision

Der Einfluß der oxidbildenden Spaltprodukte auf das thermochemische Verhalten von UO2_{2}-Kernbrennstoffen

Das thermochemische Verhalten von abgebrannten UO$_{2}$-Kernbrennstoffen wird wesentlich beeinflußt vom Sauerstoffpotential der entstehenden (U,Spaltprodukt)-Oxidphasen. Es ist bekannt, daß die Lanthanide und, in geringem Maße, auch das Zirkonium im UO$_{2}$ gelöst werden und somit die thermochemisehen Eigenschaften des UO$_{2}$ beeinflussen. Während binäre Oxidsysteme von Uran mit Zirkonium bzw. Uran mit einem Lanthanid schon mehrfach untersucht wurden, liegen über die quasiternären (Uran,Zirkonium,Lanthanid)- Oxidsysteme bislang kaum Erfahrungen vor. Zur Untersuchung derartiger Oxidsysteme wurden Oxidpulver erzeugt und unter verschiedenen Atmosphären im Temperaturbereich 1270 K bis 1670 K geglüht. Diese Oxide wurden hinsichtlich ihres Sauerstoffgehaltes und ihrer Gitterstruktur charakterisiert. Die Messung des Sauerstoffpotentials dieser Oxide erfolgte galvanometrisch nach dem EMK-Verfahren. Diese Methode erlaubt es, den Sauerstoffgehalt der Proben gezielt und sehr genau zu variieren, so daß von den Potentialmessungen ein weiter Stöchiometriebereich erfaßt wurde. Die Ergebnisse der Untersuchungen zeigen, daß Zirkonium in Gegenwart von Lanthaniden stärker in UO$_{2}$ löslich ist, als im (U,Zr)-Oxidsystem. Diese Löslichkeit sinkt mit steigendem Oxydationspotential. Das Sauerstoffpotential der (Uran,Zirkonium,Lanthanid)-0xide ist geringer als bei idealer Mischung der Reinoxide zu erwarten wäre

Die Anwendung der Röntgenfluoreszenzanalyse und -feinstrukturanalyse bei der Beobachtung des Oxydationsverhaltens von Legierungen

Samples of three nickel-base alloys, which are candidate materials for the heat transfer systems of high temperature reactors, were annealed for 100 hours at 780, 930 and 1080 °C in an atmosphere of 17 mbar H$_{2}$O and 260 mbar H$_{2}$. The test specimens were examined before and after annealing using X-ray fluorescence and X-ray diffraction analysis. The results obtained were compared with the results of destructive test methods, in particular metallography and microanalysis. The comparison showed that it is possible to obtain qualitative and quantitative data on surface corrosion using the non-destructive X-ray methods. The precision, applicability and limitations of X-ray fluorescence and X-ray diffraction analysis for the investigation of surface corrosion reactions were studied and the observations compared with the results and the interpretations of other authors