Zircaloy severe accidents

Vapor—aerosol interactions which are assumed to take place in the primary system were studied in detail in the British Falcon test facility. This facility, which is schematically shown in Fig. 7.19. (according to Beard et al., 1991), has been specifically designed to investigate the transport and deposition behavior of fission products under severe accident conditions. To produce representative aerosols, fuel pellets containing simulant fission products as well as trace-irradiated fuel pellets, both cladded in Zircaloy, were heated up to 2000 K in a steam—helium atmosphere in the presence of bulk-core materials. Fission product transport could be studied along a pathway which was designed to represent the upper plenum, hot-leg structures and containment. Considerable efforts were made to ensure that, as far as possible, the thermal-hydraulic conditions represented those of a selected accident. 

In addition to destroying the UO2 matrix, fuel liquefaction accelerates the release of fission products from the fuel. However, minor alloying components or impurities can have large effects on such releases. For instance, tin, which is a 1% component of zircaloy, may act as a getter for tellurium resulting in significant holdup or retention of this fission product. Both fuel liquefaction and retention of tellurium in the presence of tin illustrate that chemical reactions are crucial to the understanding of severe accidents. 

R. J. Wilson, J. Araj, A. O. Allen, P. Auer, D. Boulware, F. Finlayson, S. Goren, and C. Ice, Report to the American Physical Society by the Study Group on Radionuclide Release from Severe Accidents at Nuclear Power Plants, Reviews of Modern Physics, 57, No.3, Part II (1988). 15. V. F. Urbanic and T. R. Heidrick, High Temperature Oxidation of Zircal-loy-2 and Zircaloy-4 in Steam, Journal of Nuclear Materials, 75, pp 251-261 (1978). 

Tellurium exhibits a chemical behavior which is nearly as complex as that of iodine, but only little is known about its reactions under the conditions prevailing in the primary system during the course of a severe reactor accident. As was discussed in Section 7.3.1.1., the formation of temperature-stable Zr-Te compounds retains tellurium for a certain period of time and results in a significant release from the reactor core only after extensive oxidation of the Zircaloy cladding. According to... 

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