Rupture accident in a BWR offgas charcoal treatment system

Modern BWR offgas treatment systems often employ a charcoal adsorption technique to selectively delay release of the fission noble gases (xenon and krypton), thereby minimizing off-plant radiation exposures. However, thousands of curies can thus be stored in the charcoal beds and become a potential risk to the public in case of a charcoal-spill accident. This paper deals with the radiological consequences of such an accident by showing the ''weight'' of each parameter (plant or meteorological) as well as their inter-relationships. Main plant variables and layout factors influencing accident evolution are discussed, as well as meteorological conditions which also influence doses. Integrated doses at various times and various distances are presented in this paper. A discussion on possible ways of minimizing the effects of this type of accident is presented together with the implications for plant design (e.g. seismic analysis) and operation. The following main conclusions can be drawn: First, it seems worthwhile to spread the inlet offgas activity into several beds instead of one bed. Second, a ''tight'' layout around the charcoal beds would ensure a large protective adsorbent layer. (H.K.)