CSNI Specialist Meeting on Nuclear Aerosols in Reactor Safety

This paper gives the perspective on fission product behavior and source term research of the Project Nuclear Safety (PNS) at the Karlsruhe Nuclear Research Center (KfK). It tries to demonstrate the conceptional background of the KfK core melting program, which has been started in 1973, and which is scheduled to be terminated by 1986. The paper also summarizes the main findings of the SASCHA program, with the aid of which the enveloping fission product release from the primary system into the containment during a PWR core melt accident has been investigated. Within more than 10 years of PWR core meltdown research in depth we developed an understanding of the most relevant phenomena which forces us at the time being to describing the complex physical relationships as easy and as plausible as possible, based on uncontested laws of nature, rather than to running the program more and more into refined specifications. Especially with respect to aerosol generation, transportation, removal, resuspension, and release into the environment, this attempt led to the following conclusions: According to the present state of knowledge the tools are not at hand for calculating core degradation in a detailed manner and, starting from that basis, for calculating the element specific activity release from the primary circuit as a function of the time in a manner which would be reliable from the scientific point of view. However, the calculation can be replaced by a plausible and, at the same time, physically justifiable estimate of the upper limit. The fractions of release from the fuel determined in the experiment are undoubtedly in the range of 70% to 100% for the radiologically most important elements I, Cs, Te. The reduction in release from the primary circuit due to deposition is 50% at the maximum. A considerable portion resuspended must be deducted from that value. The retention of iodine and aerosol particles in the safety containment amounts to several orders of magnitude (up to 5). Likewise, the decrease in the population dose by spread and dilution in the environment and due to other parameters attains several orders of magnitude (up to 7). Consequently, particle retention by a factor of 2 or 3 in the primary circuit is negligible. Our present knowledge is completely satisfactory for analyzing the so-called source term in core melt accidents. The wish to develop more detailed codes related to core degradation and to activity release from the primary circuit has many understandable causes. However, there is no single technical reason in favor of spending much money in order to materialize this wish.