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4.3 STEAM GENERATOR

The Nuclear Steam Supply System (NSSS) of KNSP uses two steam generators for transfer

of heat from the RCS to the main steam system, One steam generator is located in each loop.

As shown in Fig. 4-9, each steam generator is a conventional, PWR, inverted U-tube steam

generator with an integral economizer which operates with the reactor coolant on the tube side

and secondary coolant on the shell side.

Reactor coolant, heated in the reactor core, flows through the 42-inch diameter hot legpiping to the inlet plenum of the steam generator. From the inlet plenum, reactor coolant flows

upward through the tubes and back down again to the outlet plenum. The reactor coolant

transfers some of its energy to boil the steam generator water through the walls of the steam

generator tubes. The reactor coolant exits the tubes, is collected in the outlet plenum, and

leaves the outlet plenum through two, 30-inch I.D. cold leg pipes to the suction of each of the

loop?reactor coolant pumps. The major components of steam generator are illustrated in Fig.4-10.

Fig. 4-9 Steam Generator Cutaway

Fig. 4-10 Steam Generator Components

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The steam generator provide steam for the turbine generator and supply temperature

control of the reactor coolant system during heat-up, at operating conditions and during cool-

down. Also, it provide for a heat sink in the event of a loss of all AC power.

For enhancing the steam generator thermal effectiveness, the concept of integral

economizer is adopted in the steam generator. Fig 4-11 shows a cutaway view of the economizer

region and flow path through it.

Feedwater is introduced through sparger ring to mix with the recirculating water flow in the

downcomer channel and is also introduced into a separate but integral section of the steamgenerator. A semi-cylindrical section of the tube bundle, at the cold the tube bundle by vertical

divider plates. Feedwater is introduced into this section and preheated before discharge into

the evaporator section.

Fig. 4-11 Flow Path of Economizer

With the fixed reactor coolant conditions, the use of an economizer enables the steam

generator to operate a higher steam pressure without an increase in heating surface. Also the

economizer increases the cold leg side temperature into contact with the primary outlet or cold

leg side of the tube bundle. Thus, the thermal effectiveness is increased (Fig. 4-12).

Fig. 4-12 Thermal Effectiveness of Economizer Concept