Figure 2.3.10 Jet pumps

LPFL nozzle

LPFL sparger

HPCF nozzle

fuel assembly

react or inte mal pomp

control rod drive housing

steam outlet nozzle (with flow limitter)

top head spray­vent nozzle

flange

steam dryer

steam separator

feed water inlet nozzte

feed water sparger

core shroud

control rod

core plate

Incore monitor housing

reactor pressure vessel support skirl

Hpcf sparger top Fuel guide—

₍

HPCF: High pressure core flooder LPFL : low pressure flooder

Figure 2.3,11 Cut-away view of an ABWR reactor pressure vessel

8. In-core flux monitors guide tube

The in-core flux monitor guide tubes extend from the in-core flux monitor housings mounted on the RPV bottom head, provide partial support for the core plate, and orient the in-core neutron flux monitors.

9. Channel box

The channel boxes that encase each fuel assembly provide a channel for the coolant flow for the fuel assembly. The boxes separate the cooling channel for fuel rods from those for the control rods and in-core monitor guide tubes. Four adjacent channel boxes form a guide channel for control rods. In addition, the channel boxes provide rigidity of fuel assembly and protect the fuel rods during their handling.

2. ABWR

In an ABWR, the conventional external recirculation system (including jet pumps and recirculation pumps) is eliminated and replaced by internal recirculation pumps installed inside the RPV. Figure 2.3.11 shows the internal structure of the ABWR RPV. Key differences between an ABWR and a conventional BWR RPV

are as follows:

1. The conventional top grid made of stainless steel beams is replaced by a machined grid made from a solid plate and it is attached to the conventional upper shroud.

2. The moisture separator for the ABWR features a smaller pressure drop than the conventional BWR, and also has a smaller outside diameter. The height of the steam separator standpipe is also shorter in an ABWR.

3. Because there is no outside recirculation loop, the potential for core uncovering has been eliminated analytically and the necessity of the high and low pressure core injection sparger system has been eliminated also.

2. Reactivity control system

The reactivity control system of a BWR consists of control rods, control rod drive system, and the standby liquid control (SLC) system. During normal operation, reactivity is controlled by controlling the neutron flux in the core by moving the control rods that contain neutron absorbing materials into and out of the core. Hie control rod drive system

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consists of the control rod drive mechanisms and control rod drive hydraulic system which insert or withdraw the control rods at a speed required for normal operation. In an emergency, the control rods are rapidly inserted into the core for a scram (rapid shutdown).

The SLC consists of a borated water tank, pumps, test tank, piping, valves, etc. The SLC is provided to insert negative reactivity to the core for shut down by injecting borated water in case the control rods cannot be inserted.

Brief descriptions of other systems are given below.

1. Control rod and control rod drive system

The control rods and the control rod drive system are designed to the following policies:

1. The ejection speed of the control rods must be limited in order to avoid rapid reactivity increase, even when the control rods drop out of the core for any reason.

2. The control rods can be inserted into the core under the design basis earthquake.

3. The control rod drive system must be capable of rapidly inserting the control rods into the core (scram) to prevent fuel failure during abnormal transients and accidents. This functional capability must be secured even if the power to the control rod drive system is lost.

4. All the control rods must be operable independently of each other and a failure of one control rod or one control rod drive mechanism must not affect the availability and function of the other control rods.

5. Each control rod is supported by its own control rod drive mechanism and the rod position is held by a positioning system.

6. The maximum withdrawal speed of the control rods is set at a value allowing the operators to appropriately control the reactor power in combination with their reactivity worth control.

7. In order to prevent a control rod ejection accident caused by a failure of the control rod drive flange or housing, due considerations must be given to the welding between the RPV bottom head and the control rod drive housing. There should be a sufficient margin against the maximum anticipated stresses and a housing support structure should be used below the

control rod drives to prevent ejection of control rods from the core, even when the control rod drive or the housing suddenly and completely is breached.

8. Each of the control rods and control rod drive systems are independently installed in the reactor vessel and hence, each of them can be separately removed and repaired when necessary.

9. The control rods and the control rod drive system must be testable periodically for their function to rapidly shut down the reactor.

The control rod and control rod drive system that are designed with the above policies have the following specific structures.

1. Control rod (CR)

Control rods consist of neutron absorbing material for reactivity control and structural materials. There are two types of control rods, namely, boron carbide control rods and hafnium control rods. A boron carbide control rod, as shown in Figure 2.3.12, consists of numerous stainless steel pipes filled with boron carbide powder (B₄C, a strong neutron absorber) that are housed in a cruciform sheath made of thin stainless steel plates. Each control rod is positioned in the space between four adjacent fuel assemblies, and a substantial number of control rods are distributed uniformly in the core at approximately 30 cm pitch.

A velocity limiter, a hydraulic structure with no moving parts, is attached to the bottom of each control rod blade so that, even when the control rod blade detaches from the coupling for any reason and falls out of core due to its own weight from its stacked position in the core, the falling speed of the control rod is limited to an acceptable value. Figure 2.3.13 (1) shows the velocity limiter; it is a parasol-shaped piston, able to move up and down with an appropriate gap from the control rod guide tube. The parasol-shaped velocity limiter has a small hydraulic resistance for rapid insertion of the control rod (scram) and a large hydraulic resistance against control rod drop out.

There are several types of hafnium control rods. An example is shown in Figure 2.3.13 (2). Hafnium plates are housed in a cruciform stainless steel case. Figure 2.3.14 shows an example of

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(neutron absorber)

Figure 2.3.12 Cross section of a control rod

Figure 2.3.13 (2) Hafnium control rod

Ball check valve

Figure 2.3.13 (1) Boron carbide control rod

Core pressure

Figure 2.3.14 Control rod for an ABWR

Coupling spud

RPV

Filter

CRD housing -

Thermal sleeve

Buffer

Index tube

Driving piston

Seal ring

Flange

Insertion

and scram

Uncoupling rod

Guide cap

Collet finger

Return spring

Withdraw

Piston tube

Seal ring Collet piston

Figure 2.3.15 Control rod drive mechanism

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reactor feed water system reactor water clean up system

from oudd cf tDOfasge <temiwrali?g or

condensate storage tank

(HCU)

scram discharge volume

Figure 2.3.16 Control rod drive hydraulic system schematic drawing

a control rod for an ABWR, The ABWR control rod has no velocity limiter. A control rod and its drive assembly are not separable unless they are rotated by 45 degrees around the axis. In case a control rod falls out of the core due to its own weight from its stacked position in the core, it falls coupled with the hollow piston of the control rod drive. The ABWR control rod drive limits the control rod free drop velocity by a special mechanism to provide large resistance against the control rod drop out

2. Control rod drive mechanism (CRD)

The CRD is a single hydraulic piston-type mechanism housed in the control rod drive housing extended from the RPV bottom head, and is attached to the bottom flange of the housing by bolting as shown in Figure 2.3.15.

3. Control rod drive hydraulic control system

Figure 2.3.16 shows the control rod drive hydraulic control system that operates the control rod drives. Key components of the system include the control rod drive water pumps, scram discharge volume, and hydraulic control units

(HCUs). The HCUs have various valves and an accumulator, and drive the control rods for insertion/withdrawal or scram.

For a reactor scram, both the scram inlet and outlet valves of the HCUs are opened, the accumulator pressure is transferred to the bottom of the main drive piston and the bulk of coolant above the piston is discharged into the scram discharge volume; these actions give the control rods a strong acceleration for insertion into the core for a reactor scram in less than a few seconds. If the accumulator pressure drops below the reactor pressure for any reason, the ball position of the ball check valve will automatically change and the reactor pressure will be applied to the bottom of the main drive piston to complete the reactor scram.

2. Standby liquid Control (SLC) System

The following design policies are applied for the SLC system.

1. The SLC system is completely independent from the control rod and control rod drive system with sufficient redundancy for its

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