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The structure of the oxide layer formed on Alloy 800 at 600°C in superheated steam markedly indicates the role of the grain boundaries as easy diffusion paths of Cr and Mn to the alloytoxide interface. Increasing the number of grain boundaries by lo-%% cold work leads to increasing Cr-and Mn-content in the scale and to decreasing oxide growth rates. Variation of the grain size by different annealing treatments leadssince the Cr-content in the scale is decreasing with the grain sizeto a linear relation of growth rate and grain size.The effect of cold work was also demonstrated on the protectiveness of the oxide scale towards carbon uptake and carburization of Alloy 800. After preoxidation of differently deformed specimens at 9OO"C, these were exposed to a CO-C02-H20-H2 mixture at 700°C for long time. The gas mixture was tagged with "C so that the C-ingress into the oxide scale and into the alloy could be sensitively monitored by autoradiography and (upon stepwise polishing) radioactivity measurements of the carbon penetration. The carbon uptake is effectively reduced with cold working; in contrast a non-deformed, electropolished and preoxidized specimen shows relatively high C-content after exposure.The investigations prove the highly favorable effect of mechanical pretreatment on the formation of the oxide scale on an austenitic Fe-Ni-Cr alloy. Cold work and other methods of surface deformation (grinding, polishing, sand blasting, shot peening) generate easy difision paths for fast Cr-diffusion to the surface and sufficient supply of Cr to form a protective oxide layer.Der Aufbau der Oxidschicht, die sich auf Alloy 800 bei 600°C in iiberbitztem Dampf bildet, zeigt deutlich die wichtige Rolle der Korngrenzen als schnelle Diffusionswege des Cr and Mn zur Grenzflache LegierunglOxid. Eine 1040% Kaltverformung fiihrt aufgrund der zunehmenden Zahl an Korngrenzen zu erhohtem Cr-und Mn-Gehalt der Oxidschichten und zu abnehmenden Oxidationsgeschwindigkeiten. Bei Veriinderungen der KorngraSe durch verschiedene Gluhbehandlungen erhalt manbei mit der Korngro6e abnehmendern Cr-Gehalteine lineare Beziehung zwischen Oxidationsgeschwindigkeit und KorngroBe.Die Auswirkung der Kaltverformung wurde ebenfalls fib die Schutzwirkung der Oxidschicht gegen das Eindringen von Kohlenstoff und die Aufkohlung von Alloy 800 gezeigt. Nach Voroxidation verschieden verformter Proben bei 900°C wurden diese liingere Zeit einer CO-COz-Hz-H20-Atmosphtire bei 700 O C ausgesetzt. Das Gasgemisch war mit I4C indiziert, SO da5 die C-Aufnahme in die Oxidschicht und in die Legierung durch Autoradiographie und Radioaktivitats-Messungen (unter schrittweisem Abpolieren) sehr empfindlich nachgewiesen werden koMte. Die C-Aufnalune wird mit zunehmendem Verformungsgrad wirksam vermindertim Gegensatz hierzu zeigt eine unverformte, elektropolierte und dann voroxidierte Probe relativ hohe C-Gehalte nach der Auslagerung.Diese Untersuchungen belegen die giinstige Auswirkung einer mechanischen Vorbehandlung auf die Bildung einer schiitzenden Oxidschicht auf einer aust...

With respect to the behavior of Nuclear Pressurized Water Reactor fuel cladding during accidents the oxidation kinetics of Zircaloy‐4 tubing in steam and the related changes in the mechanical properties have been investigated. Short tube sections were exposed to steam between 600 and 1600°C under isothermal and temperature transient conditions. Tube capsules were creep‐rupture tested in steam under isothermal/isobaric (600–1300°C, 7–150 bar) and various temperature/pressure transient conditions. The oxidation kinetics of Zircaloy‐4 is described in case of shortterm reaction by simple rate laws. The long‐term behavior is proved to be highly influenced by the breakaway transition and finally by total consumption of the tube wall. On the basis of the isothermal behavior the short‐term temperature transient case can be understood and also modelled. Deviations from predictable behavior are the consequences of Zr‐ and ZrO2‐phase transformations. The protective character of preexistent scales is lost above 1100°C for the same reason. The consequences of oxidation on the mechanical properties are increased strength and decreased ductility, whereas the response to creep deformation is an overall increase in oxidation due to oxide cracking.

This paper reviews the state-of-the-art experimental work performed in several countries with respect to the acceptance criteria established for the emergency core cooling (ECC) in a loss-of-coolant accident (LOCA) of light water reactors (LWRs). It covers in detail oxidation, embrittlement, plastic deformation, and coolability of deformed rod bundles. The main test results are discussed on the basis of research work performed at the Karlsruhe Nuclear Research Center (KfK) within the framework of the Nuclear Safety Project (PNS). Reference is made to test data obtained in other countries. The paper concludes that the major mechanisms and consequences of oxidation, deformation, and emergency core cooling are sufficiently investigated in order to provide a reliable data base for safety assessments and licensing of LWRs. All test data prove that the ECC criteria are conservative and that the coolability of a LWR and the public safety in a LOCA can be maintained.

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