Or pump

explosive valve

Figure 2.3.17 SLC system flow chart

moving components.

2. The SLC system is capable of keeping the reactor sub-critical at room temperature (20°C).

(0)1116 SLC system is can be tested periodically for its functional capability by using its test line and the test tank, even during the plant normal operation. Motor operated or explosive valves are utilized on the injection line and they are testable during plant outages.

4. The temperature in the SLC tank is maintained well above the precipitation limit of the neutron absorber. The SLC system brings the reactor to cold shutdown within about 30 minutes by injecting the neutron absorbing material into the core from the RPV bottom, when the cold shutdown of the reactor is not achievable due to the failure of control rods. Sodium pentaborate is used as the absorbing material. Borated water (sodium pentaborate solution) is stored in the SLC tank at temperatures above 15“C.

Hie SLC system is shown in Figure 2.3.17. Hie SLC system is remotely manually actuated from the main control room.

3. Advanced control rod drive mechanism (FMCRD)

In an ABWR, the conventional hydraulic control rod drives are replaced by the electro-hydraulic fine motion control rod drives (FMCRDs) as shown in Figure 2.3.18. Each control rod is

normally positioned by a step motor installed at the bottom of the spool piece. In this drive mechanism, the step motor rotation is converted to fine axial motion of the control rod either upward or downward by means of a ball nut screw and in a reactor scram, the control rod is rapidly inserted by hydraulic forces. Figure 2.3.19 illustrates the control rod drive hydraulic control unit (HCU) of an ABWR. Since the control rods are electrically driven in normal operation, the ABWR does not need the stabilizing valves and directional control valves used in the conventional HCU. In addition, the discharge volume has been eliminated for the scram water is returned to the core.

Since the function of the HCU in an ABWR is limited to providing a sufficient amount of water during a reactor scram, one HCU unit is capable of scramming two control rod drives, thus simplifying the reactor scram system.

Furthermore, the injection line of the SLC system of an ABWR is connected to the discharge line of the high pressure core injection system (HPCI), by which borated water can be directly delivered into the core.

3. Core Design

1. Thermal-hydraulic design

Core size, which is represented by the number

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2 — 30

Bayonet coupling

Internal support

Reactor pressure vessel

Hollow piston

Scram water inlet

Ball check valve

Separation probe switch

Spool piece

Step motor

Synchro

signal generator

Labyrinth seal

Guide pin roller

Ball screw

Ball nut

Figure 2.3.18 Advanced control rod drive mechanism

reactor pressure vessel

Figure 2.3.19 Control rod drive HCU for an ABWR

of fuel assemblies, is determined by the average thermal output of one fuel assembly, once the rated reactor thermal output for the specified electricity output is determined. The average thermal output of a fuel assembly is set at a value so that the maximum linear power density limit is met by all fuel rods in the core and those located at the maximum local output region of the core. Once the average thermal output of a fuel assembly is determined, the number of fuel assemblies required to obtain the rated thermal output can be calculated.

Next, the core flow rate is determined for cooling the fuel assemblies. Flow distribution to individual fuel assemblies is adjusted by the orifice in the fuel support piece at the inlet of the fuel assemblies (see Figure 2.3.7). Two orifice designs are used: one for the core peripheral region where the local thermal output is low, and the other for the central region where the output is relatively high. The minimum critical power ratios for the fuel assemblies in both regions are kept near the same level. Thus, the coolant flow in the core is effectively distributed while satisfying the criterion set for the minimum critical power ratio. Another condition for the flow distribution

pattern is that the pressure loss across each fuel assembly is kept equal for all fuel assemblies.

2. Nuclear Design

1. Fuel enrichment

The fuel enrichment (U-235 content) in a BWR core varies approximately between 1 and 5 wt% determined by the following factors: neutron leakage from the core to the surrounding reflector region, superheating and boiling of moderator, fuel temperature increase, production of zenon and samarium as neutron absorbers, and reactivity depletion due to fuel burn up.

2. Reactor shutdown margin

The excess reactivity of the core is controlled by the control rod position in the core and by burnable neutron absorbing material gadolinia (Gd₂O3) in the fuel. With burn up, the reactivity worth of the gadolinia poison decreases, compensating for the reactivity loss due to fuel depletion. Control rods together with the gadolinia poison are able to sufficiently control the maximum excess reactivity of the core.

The maximum excess reactivity stands when the reactor is at room temperatures with no xenon accumulation. The reactor must be sub-critical at the above condition, even when a control rod of

2-31

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