The design criterion for the minimum critical power ratio (MCPR) is determined based on the following rationale. The largest value of the change in the minimum critical power ratio AMCPR, induced by various anticipated transients, is added to the transient criterion. For example, if the transient criterion on the minimum critical power ratio and the maximum AMCPR are 1.07 and 0.13, respectively, the design criterion on the minimum critical power ratio under the normal operation conditions will be set at 1.20.

The design criterion of the maximum linear heat generation rate (MLHGR)of the fuel is set at 44kW/m (for an 8x8 fuel assembly) under the normal operation. As long as the maximum linear heat generation rate is kept below 44kW/m, the plastic strain of the cladding will never reach its transient criterion of 1.0 % even if an increase in linear heat generation rate under any abnormal transients is considered. The maximum linear heat generation rate is defined as the maximum local thermal power output per unit length of a fuel rod in the core.

These fuel design criteria for the minimum critical power ratio and the maximum linear heat generation rate under normal operation are considered as the design criteria for the core reloading pattern (thermal-hydraulic design criteria). They are also the criteria when monitoring the core during plant operation (referred to as the operational criteria).

2. Reactor and Reactor Core

1. Structure of reactor and reactor core

i) Overview

A reactor consists of the reactor pressure vessel (RPV) with the core placed at its center, core support structures and other internal components. This section outlines the core, core support structures and other internal components (Refer to Section 2.4.2 for the RPV). Figure 2.3.4 shows the internal structures of a BWR and its core.

These structural elements are designed with the following design policies:

• Heat generated in the fuel must be appropriately removed;

■The strength and functions required are fulfilled

under various anticipated loads;

•The core coolant flow rate, neutron flux and other operational parameters can be measured and monitored;

■Main structural materials are selected by considering the core environment, including its corrosive nature; and

•Refueling is easy and smooth.

2. Core

The core accommodates vertical fuel assemblies arranged in a cylindrical configuration. The core generates heat at the high power-critical condition under sustained fission chain reactions by adjusting the control rod insertion ratio and coolant flow rate, transfers the heat to the coolant and generates steam to drive the steam turbine­generator.

2. Core support structures

1. Core shroud

The core is surrounded by a cylindrical, stainless steel core shroud, which separates the in-core up flow of the coolant from the coolant down flow through the annulus between the shroud and the RPV. Figure 2.3.5 illustrates the core shroud and the coolant flow in the RPV.

steam outlet nozzle

control rod drive housing

incore monitor housing

recirculation water inlet nozzle

vent nozzle

head spray nozzle

core spray inlet nozzle x

low pressure core injection (LPCI) inlet nozzle ~(

core spray sparger

lop fuel guide jet pump fuel assembly

flange

slearn dryer

steam separator

core shroud

control rod

feed waler inlet nozzle feed water sparger

reactor pressure vessel support skirt

Figure 2.3.4 Internal structure of a reactor pressure vessel (cut out view)

; spare nozzle

-— core plate

recirculating water outlet nozzle

NSRA, Japan

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