Page 144 - Dynamics and Control of Nuclear Reactors

12.4 The pressurizer 141

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The pressure in the primary system is maintained at about 2250lb./in ( 153 bars,
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1bar¼10 Pa) so that there is no boiling in the primary system. This is accomplished
by a pressurizer, which is connected to one of the hot legs. Sections 10.7 and 12.4 pro-
vide details of the pressurizer.
The combination of reactor core (thermal power) and the steam generator system
is called the Nuclear Steam Supply System (NSSS).

12.3 The reactor core

All three U.S. PWR manufacturers used similar reactor core designs. Differences
included fuel bundle designs and control rod designs.
A cylindrical vessel contains the fuel assemblies in PWRs. Fig. 12.2 depicts a
typical reactor vessel, showing reactor internals and control rod drive mechanisms.
A fuel assembly is shown in Fig. 12.3. The reactor vessel diameter is about 14ft., and
the height is about 44ft. The reactor vessel material is carbon steel (8-in. thick) with
an inside stainless-steel cladding (nominal thickness 7/32in.). The weight of the ura-
nium oxide fuel is approximately 82,000kg. A core support barrel is attached to the
reactor vessel flange, and it supports the core.
There are approximately 200 fuel assemblies in a typical 1150 MWe PWR. A fuel
assembly is a bundle of fuel pins (or fuel rods), typically in a 17 17 or 19 19 array.
The fuel pins form an open lattice structure. The fuel rod contains pellets of Uranium
oxide (UO 2 )( 0.4in. diameter x 0.6in. long) contained in Zircaloy cladding tubes
with a He-filled gap of 0.002in. The active core length is 12ft. and has a diameter
of about 11ft.
Water flows upwards through the core, to the vessel upper plenum, and to hot leg
piping that leads to the steam generator.
Mechanically driven control rods enter from above (see Fig. 12.4). There are
about 60 control rod assemblies (clusters) that occupy spaces in the fuel assemblies.

12.4 The pressurizer

A pressurizer is connected to the hot leg through a pipe (or a surge line). Also see
Section 10.6. Fig. 12.5 shows a typical pressurizer.
The purpose of the pressurizer is to control the pressure in the primary loop at a
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nominal coolant pressure of 2250lb./in ( 153 bars). The primary pressure is reg-
ulated by modulating heater power and spray flow from a cold leg. Fig. 12.6 shows a
pressure controller.
The overall height and diameter of the pressurizer in a four-loop plant are approx-
imately 52ft., 9in. (16.1m) and 7ft., 8 in. (2.3m), respectively and a total volume of
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1800ft (31m ). The pressurizer has electric immersion heaters with a total power of
1800kw. The maximum spray flow rate is 900gpm (57l/s) with a continuous flow