Heat transfer from the primary to the secondary coolant circuit of a PWR nuclear power plant by means of a natural circulation U-tube steam generator (non-linear transient model and digital code UTSG)

There exists a lack of exact physical models which describe the transient situation of steam generators in a digital way. The most known models are realized on hybrid computers. Many of them are restricted to the description of the steam production within the generator. They do not include the simulation of the whole steam removal system, so that possible feedbacks coming from the secondary side of the plant can not be taken into account in a proper way. The demand for an overall digital plant model was the reason for the development of a new nonlinear transient model for vertical natural-circulation U-tube steam generators together with its special steam removal system. This type of steam generator is widely used within the primary coolant circuit of PWR nuclear power plants. Starting from well-known fundamental equations (balance eqs. for mass, energy, momentum and volume, Fourier eq. for heat transfer) given in form of partial differential eqs. and taking into account additional state eqs. the corresponding theoretical nonlinear transient model could be derived. Based on this model the digital code UTSG (the name being an abbreviation for 'U-Tube Steam Generator') has been established. In this paper only a short overview of the main aspects of the theoretical model, the digital code UTSG and the verification of the code can be given. A very detailed description of the model is given in a separate publication. (author)