HEAT-TRANSFER CHARACTERISTICS OF HORIZONTAL STEAM-GENERATORS UNDER NATURAL CIRCULATION CONDITIONS

This paper deals with the heat transfer characteristics of horizontal steam generators, particularly under natural circulation (decay heat r emoval) conditions on the primary side. Special emphasis is on the inh erent features of horizontal steam generator behaviour. A mathematical model of the horizontal steam generator primary side is developed and qualitative results are obtained analytically. A computer code, calle d HSG, is developed to solve the model numerically, and its prediction s are compared with experimental data. The code is employed to obtain for VVER 440 steam generators quantitative results concerning the depe ndence of primary-to-secondary heat transfer efficiency on the primary side flow rate, temperature and secondary level. It turns out that th e depletion of the secondary inventory leads to an inherent limitation of the decay energy removal in VVER steam generators. The limitation arises as a consequence of the steam generator tube bundle geometry. A s an example, it is shown that the grace period associated with pressu rizer safety valve opening during a station black-out is 21/2-3 hours instead of the 5-6 hours reported in several earlier studies. (However , the change in core heat-up timing is much less - about 1 h at most.) The heat transfer limitation explains the fact that, in the Greifswal d VVER 440 station black-out accident in 1975, the steam generators ne ver boiled dry. In addition, the stability of single-phase natural cir culation is discussed and insights on the modelling of horizontal stea m generators with general-purpose thermal-hydraulic system codes are a lso presented.