Starting up of New Steam Generator of N4 1450 MWe Plants

Abstract

The first N4 plant, CHOOZ B1, was commissioned in 1997. This plant of 1450 MWe capacity is equipped with four steam generators of a new design fitted with an axial economiser. The axial economiser principle essentially consists in directing all the feedwater to the cold leg of the tube bundle and about 90% of the recirculated water to the hot leg. A double wrapper along the downcomer allows cold water to enter tube bundle as in a boiler-type steam generator and prevents excessive vibration due to cross flow. The main result of this design is to enhance the heat exchange efficiency between the primary and the secondary sides and to increase the steam pressure as compared to a boiler steam generator having the same heat exchange area. As it is the common practice in France for the first plant unit of a new model, one of the four steam generators has been specifically instrumented in order to assess its actual operating characteristics and verify their consistency with the predicted values resulting from the design studies and from the qualification tests. The test programme and the dedicated instrumentation allowed measurement of essential S.G. thermal hydraulics parameters (saturation pressure, flow velocities, pressure drops, circulation ratios, temperature distribution,...) and assessment of vibratory behaviour of tube bundle. The results are in agreement with the tests carried out in the course of the steam generator development and qualification programme, notably a few years ago on the large 25 MWth MEGEVE test facility. They confirm adequacy of the axial economiser design. The paper describes successively : the specificities of the new steam generator design, the instrumentation and the test programme, the main results obtained from the numerous tests performed during plant start-up.