Page 151 - Dynamics and Control of Nuclear Reactors

148 CHAPTER 12 Pressurized water reactors

Feedwater enters the steam generator through a nozzle above the tubes and flows into
the downcomer where it mixes with the recirculation flow. The saturated steam exit-
ing the steam separators and driers passes through piping and a nozzle chest to the
turbine.
A slightly different version of the U-tube steam generator also exists. This sys-
tem, called an integral economizer steam generator, features a chamber around the
tubes where they exit the steam generator. Part of the feedwater enters this chamber,
called the economizer, rather than flowing into the downcomer region. The water
entering the economizer is cooler than the recirculated water entering the downco-
mer from the steam separators and driers. The cooler water enhances heat transfer
from the primary water. Heated water flows from the economizer and mixes with
water from the downcomer. The improved heat transfer increases the efficiency
of the steam generator.
Section 10.8 addresses U-tube steam generator modeling.

12.5.2 Once-through steam generator (OTSG)
A once-through steam generator is basically a vertical shell and tube heat exchanger.
Also see Section 10.8. Fig. 12.8 shows a once-through steam generator.
A typical OTSG is about 73-ft tall (22.25m) and has a 13-ft (3.96m) shell diam-
eter. It has 15,000 to 16,000 vertical tubes with an OD of 0.625-in. (15.9mm). The
tube material is Inconel-600 with a tube thickness of 0.034-in. (0.864mm) [3]. More
details of OTSGs may be found in Ref. [3].
Hot leg water enters at the top, flows downward through the tubes then exits at the
bottom. Secondary water flows upward outside of the tubes, in the shell side of the
steam generator. Heat transfer in the lower part of the tube region brings the second-
ary water to the saturation condition and boiling ensues. As secondary steam con-
tinues upward, heat transfer raises the steam temperature above the saturation
temperature. The resulting superheated steam passes through piping to the turbine.
Section 10.8 addresses once through steam generator modeling.

12.5.3 Horizontal steam generator
There is also a third type of PWR steam generator. It is used in the Russian VVER
reactor. VVERs use horizontal shell-and-tube steam generators.

12.6 Reactivity feedbacks
Reactivity feedbacks occur in a PWR as a result of fuel temperature changes and
moderator/coolant temperature changes.
The fuel temperature coefficient, due to the Doppler effect, is always negative (see
Section 7.2). The fuel temperature reactivity dominates feedback in PWRs. Reactivity
change is due to the product of fuel temperature change and the fuel temperature