At the top of the shroud, a shroud head is flange-joined to the shroud, on which an array of standpipes are welded with a steam separator unit at their top. Hie shroud head forms the top core plenum, where mixtures of steam and water from individual fuel assemblies are well mixed and homogenized before being transferred to the steam water separators through the standpipes,

2. Top guide and core plate

Hie top guide is a grid-form structure, made of stainless steel plate beams assembled at right angles to each other. It is welded to the top of the core shroud and provides orientation and lateral support for fuel assemblies. It supports the top ends of in-core monitor guide tubes as well.

The core plate is a perforated circular plate with beam supports made of stainless steel, bolted to the bottom of the core shroud. Hie core plate provides lateral support for control rod guide tubes, fuel assemblies through fuel support pieces and in-core flux monitor guide tubes (Figure

2.3.6).

3. Control rod guide tube and fuel support piece

The control rod guide tubes are supported by the control rod drive housings mounted on the RPV bottom head; these tubes act as guide channels for the control rod vertical movement in the core. Their top ends are interlocked with the core plate. A fuel support piece at the top of each control rod guide tube bears the weight of four fuel assemblies as a group.

At the outermost periphery of the core where there are no control rod guide tubes, fuel assemblies are supported by the support pieces directly attached to the core plate.

Figure 2.3.6 shows the core and the core support structure, whereas Figure 2.3.7 illustrates a fuel support piece structure,

2. Other internal components a. Steam separator

The steam separator and steam dryer assemblies separate steam from steam-water

steam separator

shroud head

stand pipe

jet pump diffuser

from recirculation-

pump

core shroud

jet pump nozzle

suction

mixing room (throat)

upper plenum

top guide

core plale

lower plenum

to recirculation pump

— reactor pressure

Vessel (rpv)

Figure 2.3.5 Core shroud and coolant flow in RPV

top guide

Figure 2.3.6 Core and lower core structures

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3. Jet pump

While external recirculation lines are provided for forced coolant flow into the core (see Section 2.4.1) jet pumps are utilized to reduce the coolant flow from the vessel to the outside recirculation line. Twenty jet pumps are located in the annulus between the reactor pressure vessel and the core shroud as shown in Figure 2.3.10 and they are connected to the recirculation flow to drive the coolant into the core for forced convection. Saturated water from the steam separators and the steam dryers above the core flows downward to the recirculation annulus between the vessel and the core shroud, and is mixed with the feed water from the feed water sparger. Part of the mixed coolant flow is then taken from the vessel by the two completely independent external circulation lines, pumped to a higher pressure by the (two) recirculation pumps and discharged through the jet pumps nozzles at high speed. The rest of the mixture is suctioned by the low pressure zones at the jet pump nozzle outlets induced by the high speed discharge of the recirculation water (driving fluid). The driving flow and the suction flow are well mixed in the throat of jet pumps before the mixture passes through a diffuser (where the pressure is recovered), and then flows into the

central region fuel support piece

peripheral region fuel support piece

Figure 2.3.7 Fuel support piece structure

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core. The coolant discharged from the jet pumps enters into the lower plenum of the vessel (Figure 2.3.5), and flows into individual fuel assemblies through the orifice at each fuel support piece shown in Figure 2.3.7. The coolant is heated by the fuel elements while it flows through the fuel assembly, and becomes a two-phase steam- water mixture.

The core shroud, RPV lower section and the jet pumps form an integrated container surrounding the reactor core. This container is not directly connected to the external recirculation piping. Therefore, even if the external recirculation piping is breached, the core can be re-flooded by the emergency core cooling system. The jet pumps (the throats of the jet pumps) are installed at an elevation that allows for flooding the core up to two-thirds of its height

4. Feed water sparger

A feed water sparger is a circular header with elbow nozzles attached on the RPV inner surface of the reactor vessel. It mixes the feed water entering the vessel via feed water inlet nozzles and high temperature coolant returning from the steam separators and steam dryers.

5. Core spray sparger

Core spray spargers are provided to inject spray cooling water into the core inside the shroud from the core spray system in an accident They are installed near the top of the core shroud.

6. Reactor pressure vessel head spray nozzle

The head spray nozzle is located at the top of the RPV upper dome where cooling water is injected from the shutdown cooling system into the RPV upper dome in order to relief the residual pressure in the vessel during the reactor shutdown.

7. Core differential pressure monitor and standby liquid injection nozzle

The core differential pressure monitor and the standby liquid injection nozzle are a coaxial double pipe that penetrates the RPV into the reactor core. In the core, the internal and external pipes separate, reaching below and above the core plate to measure the pressure drop across the core plate. The pipe reaching below the core plate is also used for emergency injection of borated water into the core.

Figure 2.3.8 Moisture separator unit

Figure 2.3.9 Steam dryer

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