assumed for nuclear power plants.

1. The ECCS is installed to mitigate the situation that arises, assuming the core becomes uncovered due to an outflow of the reactor coolant. The ECCS is automatically actuated to keep core cooling for a long time period and thus failures of a large number of fuel rods are prevented.

2. Decay heat generated in the core for a long time period can be removed from the reactor by the residual heat removal (RHR) system.

3. An on-site emergency power supply system is installed to ensure necessary power supplies when the offsite power supply is lost

4. The PCV encompassing the reactor pressure boundary is provided to contain, over a long period, radioactive materials that would otherwise be released from the reactor and to let them decay.

5. Safety systems are installed, such as a containment spray system, which condenses steam in the PCV, to decrease the vessel pressure, and to remove proactively airborne radioactive iodine using the decontamination effect of the spray.

6. The flammable gas control systems (FCSs) are also installed and are actuated on demand, in case flammable gasses are accumulated in the PCV.

7. The PCV atmosphere, possibly containing radioactive gases (mainly radioactive iodine), released from the PCV are filtered for sure removal of radioactive gases before being discharged from the elevated main stack.

All these engineered safety feature (ESFs) are designed to maintain their high reliabilities and confirm them by periodical tests.

2. Emergency Core Cooling System (eccs)

1. Roles of the eccs

As mentioned in section 2.7.1, the occurrence frequency of a pipe break of the primary coolant system of the BWR is considered to be sufficiently low, since the possibility of an accident has been excluded by adopting appropriate standards and criteria at each stage of design, manufacturing, construction and operation.

However, in order to make doubly sure, the ECCS is installed to prevent large scale failures of fuel cladding and to remove the decay heat generated in the reactor core over a long time, assuming an occurrence of a LOCA due to a pipe break at the reactor pressure boundary.

2. Criteria for design and evaluation of eccs

Hie ECCS is designed pursuant to the Regulatory Guides ’’Reviewing Safety Design oiSLight Water Nuclear Power Reactor Facilities (NSCRG LrDS-I.O)" and "Evaluating Emergency Core Cooling System Performance of Light Water Power Reactors (NSCRG DSE-I.02)" authorized by the Nuclear Safety Commission of Japan (NSC), the technical standards specified by the then Ministry of International Trade and Industry (MITI), and other relevant rules.

1. Criteria for preventing large-scale failures of fuel claddings

Temperatures of fuel claddings and local metal­water reactions shall be limited to the following values for preventing the fuel cladding failure development and for keeping the core geometry Peak cladding temperature: below 1,20013 .

Local metal-water reactions: less than 15% of the fuel cladding thickness

2. Criteria for maintaining the integrity of the PCV Metal-water reactions shall be limited

to consuming below 1% of all the zircaloy contained in the core structures in order to avoid spontaneous combustion of hydrogen and oxygen, due to the hydrogen generated from the metal-water reactions in the core.

3. Criteria for long-term decay heat removal

The reactor core geometry shall be maintained and the decay heat from the long half-life isotopes shall be removed so that the core coolability is maintained for a long term.

3. Design policies for the eccs

The ECCS meets the following requirements of system functions, redundancy and independency in order to secure necessary safety levels in any pipe breaks composing the reactor coolant boundary, under the superposed conditions of: a single failure of an active component; and a loss-of-offsite-power.

1. ECCS functions

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difference between the PCV and the RPV decreases to about 2 Mpa [gauge].

In a large pipe break accident that promptly decreases the reactor pressure, the LPCS sprays water from above the core to cool it, together with the low pressure core injection (LPCI) system and the high pressure core spray (HPCS) system. In a small or medium piping break when the RPV pressure does not decrease by the break, the LPCI system is actuated for cooling the core, while the RPV pressure is quickly decreased by the ADS.

This system is automatically actuated by a signal “low reactor water level (level 1)” or “high drywell pressure”. The system sprays the suppression pool water atop the fuel assemblies from the nozzles of the above-core spray sparger header (Figure 2.7.3). Sprayed water covers the core to the height of two-thirds of the core in the static water head upon a recirculation line break. After reflooding, spilled water from the top of the jet pumps is discharged from the break portion to the drywell. This discharged water is stored in the bottom of the drywell before it returns to the suppression chamber when the water level in the drywell reaches the height of the vent line inlet Returned water is recirculated as spray water, ii) Low pressure core injection (LPCI) system

The LPCI system is composed of three motor driven pumps, piping, valves, and the I&C systems (Figure 2.7.4). This system injects coolant into the core when the pressure difference between the PCV and the RPV drops to about 1 Mpa [gauge]. In a large pipe break accident, the LPCI system cools the core, together with the LPCS system and the HPCS system. In a small or medium pipe break, the LPCI system cools the core, together with the ADS. The LPCI system works in one mode of the RHR system which removes the decay heat in a plant shutdown mode.

This system has three independent loops with one low-pressure injection pump for each (Figure 2.7.4) and it is also actuated automatically by the same signals “low reactor water level (level 1)” or "high drywell pressure” as the LPCS system

The LPCI system injects the suppression pool water directly to the reactor core shroud. The LPCI system cools the core by flooding the

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Figure 2.7.2 ECCS network (BWR-5)

Figure 2.7.3 LPCS system configuration

dry well

LPCI mode

Figure 2.7.4 RHR system configuration in LPCI mode

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