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Prevent occurrence of abnormal conditions

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Occurrence of ’ abnormal event

Prevent abnormal event progression

Occurre- nce of

accident

Preventabnormal release of radioactive materials to the surroundings

Designed with safety margin (measure against earthquakes)

Fail safe (actuation to safety side)

Interlock (prevention of miss- operation)

(shutdown)

(cool) (confine)

[Source] Tokyo Electric Power Company, "Present Status of Nuclear Electricity Power Generation", 2006

Figure 1.4.1 Defense-in-Depth philosophy

  1. Design of reactor with unique safety

  2. Design based on ergonomics to prevent mal­operation and mal-function

•Adopt fail safe system

•Adopt fool proof (e.g. Interlock system)

  1. Check, Inspection of Equipment

• Installation of equipment at construction, elaborate tests and inspection of functions

•Daily check after start of operation, tests, inspection according to laws

  1. Prevention of Expansion of an Abnormal Event (Level 2)

If a failure or abnormality occurs in spite of exertion for 1st level, detect it early by various monitoring devices, decide, generate signals necessary to prevent expansion and progression to accidents (Safety and Protection System), inset rapidly all control rods upon signals received .and terminate surely nuclear fission (Reactor shutdown system), actuate devices to maintain integrity of pressure boundary as necessary, remove decay heat being generated after reactor shutdown and bring reactor to stable shutdown states.

Hie following are measures taken at this level.

  1. Detect the failure or abnormality quickly and surely (Safety and Protection System)

  2. Implement stable shutdown reactor (Reactor Shutdown System)

  1. Prevention of Abnormal Release of Radioactive Materials and Mitigation of Consequence (Level 3)

If an event is not ended by efforts at level 2 and there is a fear that radioactive materials are released outside systems, functions to “cool” reactor and “confine” radioactive materials should be provided to mitigate radioactivity release, and the consequence.

  1. Emergency core cooling system

If a pipe connected to the reactor vessel breaks and water level of reactor is reduced, following reactor shutdown, Emergency Core Cooling System (ECCS) should inject water into reactor to cool the reactor and maintain water level and remove decay heat from the reactor and prevent radioactive material release. The ECCS system is designed applying single failure criterion, redundancy or diversity and independency to assure the reliability of its function.

  1. Confinement of radioactive materials by five barriers

If an abnormal state is progressed, leak of primary coolant continues and fuel is damaged, radioactive materials should be confined and prevented to be released directly to the environment by various measures. Fission products (FP), which contain radioactivity much

NSRA, Japan

1~10