HKNIC - Inside a Nuclear Island - The Steam Generator

Inside a Nuclear Island - The Steam Generator

Water flows from the reactor pressure vessel in three separate branches to three steam generators.

In a steam generator, the water flows first into the bottom of the generator at its "primary circuit" side, where it is driven into one end of a bundle of over 4,400 U tubes made of a nickel-chromium alloy. It then passes along the typically 10 metres of piping in each tube, and emerges on the other side before returning to the reactor at some 35º C cooler. The water in the primary circuit inside the U-tube bundle heats up the water on the outside, which is the side of the "secondary circuit".

At a lower pressure, the water in the secondary circuit is boiled to become steam. The wet steam rises through mechanical separators to remove moisture from the steam, so that the steam emerging from the top of the generator contains only 0.25% humidity, at 284º C and a pressure of 69 bar, to drive the turbine.

Each steam generator weighs about 330 tonnes when empty and is about 21 metres tall. It produces about half a tonne of steam per second.

Image Credit : EDF