Modelling BWR contamination buildup

As with PWRs, several attempts have been made to describe activity and contamination buildup in BWR plants by computerized models. These models are essentially based on materials balances for the iron transport and on materials and activity balances for the cobalt transport. The reaction rates and the reaction orders needed for the calculations in the partly very complex models are in most cases empirical values or even assumptions. The principles of some of the models have been outlined e. g. by Ishigure (1987) and by Alder et al. (1992). 

The General Electric model (Lin et al., 1981) is a comprehensive analytical description of the activity transport as well as of the activity buildup. Iron transport and cobalt transport are treated in separate sets of equations of balance. The block diagram of Co/ Co transport is shown in Fig. 4.51. In this model, several interactions are considered to exist between dissolved ions and corrosion product particles, including adsorption of ionic species onto the surfaces of the particles. Moreover, both particulate and dissolved species are assumed to be deposited onto the surfaces of the fuel rods, with corrosion product particles playing an important role in the deposition of the ionic species. The fuel rod deposits are assumed to consist of two layers, loosely-adherent and tenacious ones, and a certain amount 

In the GE model, differences in the characteristics of the corrosion product particulate material are emphasized in detail they are considered to arise from the different types of condensate cleanup systems (powdex or deep-bed) employed in the plants and from the option of forward-pumped heater drain systems. The model includes many empirical coefffcients obtained through laboratory experiments and, above all, by the aid of a large number of data from measurements at operating BWR plants. For this reason, the model is mainly applicable to plants of GE design (Alder et al., 1992). 

To summarize these short remarks on modelling of BWR contamination buildup, it can be stated that all the existing models suffer from the same problems as the corresponding PWR models (see Section 4.4.3.6.), i. e. primarily from the fact that some of the fundamental steps are still insufficiently known. To develop a causative model based on physico-chemical principles, further work is claimed to be necessary to elucidate the mechanisms and the kinetics of the steps involved in activity buildup (Alder et al., 1992). Finally, it is still an open question whether it is possible to reflect the very complex conditions in a real BWR plant satisfactorily by a mathematical model that can be applied in daily work. 

---