Publication Laka-library:
The zircaloy - uranium dioxide reaction (1984)
| Author | B.D.Johnston |
| Date | September 1984 |
| Classification | 6.01.3.10/73 (NUCLEAR SAFETY - REACTORS - GENERAL) |
| Front | 
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From the publication:
1. INTRODUCTION The probability of a severe accident which degrades the core in an LWR is extremely remote, however it is necessary to understand the behaviour of the interaction between fuel and core components in such an accident. This is the motivation for the study of such reactions as that between UO2 and Zircaloy. The chemical affinity of Zircaloy and UO2 is such that although they are 'compatible' at normal operating temperatures, they are thermodynamically unstable and chemical interaction will take place at higher temperatures, partially because a - Zr can dissolve a large amount (up to 30 at. %) of oxygen. Thus it is proposed in this report to discuss the Zry - UO2 reaction in the context of the high temperatures and phenomenological sequence of events encountered in severe fuel damage and core melt scenarios. Core melt studies have identified the oxidation of Zircaloy by steam as being the major chemical reaction in the melt, because of hydrogen generation and clad embrittlement. This reaction is not considered in detail here, although it will be referred to quite often in the context of its influence upon the Zry - UO2 interaction. This latter reaction is considered likely to be the second most important chemical reaction in the melt. In both these reactions the diffusion of oxygen into Zirconium is the rate determining step, and as shown by experiment, Zircaloy absorbs oxygen from UO2 practically as readily as it does from steam. Comparative diffusion data for these two reactions are shown in Figure 14. Despite the fact that UO2 (unlike steam) is not sufficiently oxidising to produce a scale of ZrO2 on the substrata metal, the practical consequence, in terms of the clad, of the UO2 - oxidation process is the same, namely embrittlement of the Zircaloy by the formation of the oxygen rich a - Zr phase and by the increase of the oxygen content of the ß - Zr phase. The main object is to review what is known about the complex Zry - UO2 ternary corrosion process in terms of its reaction conditions, kinetics, phase formation and associated melting temperatures.
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