Problèmes diphasiques rencontrés dans les générateurs de vapeur des chaudières nucléaires PWR Comportement transitoire aux faibles charges

Two-phase problems in the steam generators of PWR boilers Transient low-pressure effects

¹ EDF - Direction des Études et Recherches, Chatou
² EDF-SEPTEN, Direction de l'Équipement

Abstract

At rated operating conditions, emulsion passes through two-thirds of the secondary volume of the natural-circulation PWR steam-generators Framatome has produced for Electricité de France (EDF). The numerical data in Figure 1 are from an EDF-designed single-path numerical model. I - Tube corrosion Tube corrosion is accelerated in high heat-transfer flux areas in the vicinity of the tube plate by drying and re-wetting of the wall and sludge accumulation. Determined efforts have been made to obtain closer data on flow conditions at the foot of the tube cluster, by such means as eddy-current measurement, two-dimensional numerical two-phase models, comprehensive instrumentation of steam generators in operation, mock-ups, etc. The first three of these have successfully contributed to improvement of flow conditions at the foot of new French-designed steam generators. Tube contraction and subsequent crack formation can also be caused by tube plate corrosion due to unsatisfactory 'sweeping' of the tubes at the plate joints. 'Quadrifoliate' design of tube passages through the plates is expected to improve steam generator reliability. II - Possible hydroelastic instability of the tube cluster Fluid/tube cluster interaction is significant at arch supports. Two unknowns for evaluating stability margins are 1) normal velocity distributions at the largest radii and 2) margins resulting from vapour/liquid slip. III - Performance The usefulness of a 'cold-leg' économizer mainly depends on the effect it will have on heat-transfer and corrosion conditions at the foot of both branches of the tube cluster. Closer data on the subject may be obtainable from operating experience with generators provided with an economizer and from tests on water-steam mock-ups. Operating experience with steam generators and model tests have revealed the vital importance of the intermediate cyclone/drier area. By suitable layout design of this area it should be possible to ensure satisfactory steam quality from the generators with which EDF's 900 MWe boilers are provided. IV - Hydrodynamic stability of the circulation loop Whereas stability of the 900 MWe EDF steam generators has turned out to be adequate, the stability limits for the 1300 MWe generators are a little off rated conditions. Closer knowledge of the head losses in the circulation loop is required for calculation of these limits. Tests on a model with freon should enable detailed investigation of the problem. v - Level measurement Steam generator level measurement by recording differential static pressure across two tappings on the same vertical (Fig. 1) fulfils both regulating and safety functions. Development work is in progress as regards the representativeness of this system in the case of accidental depressurization. VI - Level regulation at low load The level-regulating system provided on Westinghouse steam generators prior to the end of 1975 - manually-operated at below 15 % load - was one of the main causes of unavailability during start-ups. This problem has been studied on a numerical axial two-phase steam generator simulation model designed by EDF. Level response to a steam or water discharge increment (Figs. 4 and 5) takes place in two stages, 1) rapid redistribution of the mass of secondary water in the generator and 2) evidence of global mass balance variation showing up in the measured level. The greater 'settling' and 'swelling' amplitudes at low load are due to the non-linear relationship between depression and steam content. The following improvements are based on operating experience, the numerical model and an analog model of two coupled steam generators (4) : a) A separate control system for the low-flow control valve (water and steam flow monitors are useless at below 15 % load). b) Dynamic adaptation of tendency action. c) Dependence of proportional regulator gain on feed temperature. d) Automatic switchover from 'low flow' to 'high flow' regulation. This regulation system is a vast improvement over the manual system. Owing to the limited gain of the level regulator, however, not all pre-positioning deficiencies of the 'low flow' valve can be recovered. Conclusions Schematic single-path representation of natural-circulation steam generators has enabled development of a satisfactory level-regulating system. Many different investigation methods have been applied in anti-corrosion research, as a result of which circulation rates have been increased and further problems have been solved or are expected to be so in the near future. Further means of investigation and additional tests are being devised for operating security and cost-optimization research.