Behavior during severe accidents

G. A. Berna, C. M. Allison, and L. J. Siefken, SCDAP/MODl/VO A Computer Code for the Analysis of LWR Vessel Behavior During Severe Accident Transients, IS-SAAM-83-002, Revision 1, July 1984. 

Beahm, E. C., Weber, C. F., Kress, T. S., Shockley, W. E., Daish, S. R. Chemistry and mass transport of iodine in containment. Proc. 2. CSNI Workshop on Iodine Chemistry in Reactor Safety, Toronto, Can., 1988 Report AECL-9923 (1989), p. 251—266 Beard, A. M., Bowsher, B. R., Nichols, A. L. Interaction of molecular iodine vapour with silver—indium—cadmium aerosol. Proc. International Symposium Severe Accidents in Nuclear Power Plants, Sorrento, Italy, 1988 IAEA-SM-298/108, Vol. 2, p. 201—213 Bell, J. T. Chemistry of iodine and cesium, in M. Silberberg (Report Coordinator) Technical Bases for Estimating Fission Product Behavior during LWR Accidents. Report NUREG-0772 (1981), Chapter 5... 

Such interactions can only occur, however, when the volatile fission products and the primary aerosols appear simultaneously in the primary system, in spite of the large differences in their volatilization behavior. As was discussed above, uniform thermal-hydraulic conditions do not prevail within the reactor core during a severe accident (for example, the peripheral fuel rods may fail relatively late in the accident sequence, at a point when a large part of the central rods may already be molten) and it can be assumed that the broad time-envelope of significant release of structural aerosols will encompass the release of the volatile fission products. However, as was mentioned in Section 7.3.1.2., the amount of primary aerosols formed and the timing of their formation depend highly on the specific accident sequence this is particularly true for the control rod materials. 

In general, it can be assumed that the reaction between silver and iodine species in the gas phase, as well as the reaction of iodine vapor with silver aerosol or with silver deposited on the primary circuit surfaces, is only of minor significance for iodine behavior in the course of a severe accident. The main reasons are the rather short residence time of the silver aerosols in the gas phase, the fact that iodine and silver volatilization from the reactor core may differ considerably over time and, finally, the small proportion of elemental I2 and of HI (compared with the Csl fraction) assumed to be present in the gas phase during transport through the primary circuit. In contrast, Agl formation is expected to proceed to a significant extent later on in the containment sump water (see Section 7.3.3.3.3.). 

Johnson, I., Farahat, M. K., Settle, J. L., Johnson, C. E., Ritzman, R. Downstream behavior of fission products. Proc. Symposium on Chemical Phenomena Associated with Radioactivity Releases During Severe Nuclear Plant Accidents, Anaheim, Calif 1986 Report NUREG/CP- 0078 (1987), p. 3/53-65... 

No nuclear accidents with severe off-site consequences have occurred in the United States. Other types of events have occurred that may indicate how people would respond to a nuclear accident. Objections to citing public behavior during nonnuclear emergencies for purposes of radiological emergency planning can and have been expressed. 

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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