more than that in corrosion product (CP) in primary coolant and is accumulated inside the fuel pellets and fuel bars. In order to confine primarily the FPs, following 5 walls (multiple barriers) are provided.

1. Fuel pellets (1st wall)

Fuel pellets have good holding capability. All of the solid FPs and most of the gaseous FPs are held in the fuel pellets. Fuel temperature at normal operation is kept below melting temperature to preclude FP leakage out of fuel pellets by melting.

2. Fuel claddings (2nd wall)

Fuel claddings hold the gaseous FPs at normal operation. Integrity of fuel pellet are maintained by avoiding lack in cooling or fuel pellet and cladding interaction.

3. Reactor primary coolant boundary (3rd wall)

If FPs are released into the reactor coolant due to failures of fuel rods, the reactor primary coolant boundary consisting of the reactor pressure vessel, piping and isolation valves, etc. is maintained intact and FPs and corrosion products are confined.

4. Reactor containment vessel (4th wall)

If a LOCA occurs due to pipe breaks and fuels are damaged and FPs are released, the reactor containment vessel becomes an independent barrier. Containment vessel has a good leak-tight capability and can considerably reduce release of FPs to outside atmosphere.

5. 2nd containment vessel (Reactor building) (5th wall)

If FPs leak from the reactor containment vessel, they are confined inside the reactor building or the annulus structure; these are kept at a negative pressure, and filtered and release to environment is further reduced.

In addition, considerable distance is taken from NPP to surrounding environment area and radioactivity released is expected to be diluted. Including such an effect, radioactive substances are confined and sufficient measures are provided so that surrounding environment is not affected by radioactivity.

The basic objective of the Defense-In-Depth philosophy is to make the utmost efforts for reactor safety at each of the three levels. The objectives

and facilities are however not necessarily clearly identified from the beginning. By virtue of the research studies and experiences as described in Section 1.2, systems and equipment have been established nowadays in some aspects. The International Atomic Energy Agency (IAEA) has five levels of Defense-in-Depth, adding accident management and off-site emergency response to the three levels. These added levels may be regarded as included in level 3 in a broad sense.

2. Outline of Laws, Regulations and the Regulatory Framework

1. Domestic Framework

There are checking systems for NPPs at the stages of construction, operation and maintenance as described in the previous section. Nuclear power generating reactors are provided with unique safety in their design from the beginning and moreover they are equipped with safety devices in multiple layers. For the safety design to function effectively, electric companies and constructors carry out construction and operation ascertaining the material, structures and functions from each standpoint. Moreover strict checks are conducted by the government in accordance with the laws at each stage. The principal regulatory framework is explained in this section.

1. Permit to establish a power generating reactor

When an electric company constructs a NPP, it submits an application to the Ministry of Economy, Trade and Industry (METI) in accordance with the “Law for Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors” (in short, the Nuclear Reactor Regulation Law). »

When METI receives the application, it hears opinions of outside specialists and ascertains that the reactor is safe and has safety margins. After then METI consults with the Atomic Energy Commission (AEC) and the Nuclear Safety Commission (NSC).

The Atomic Energy Commission (AEC) examines and deliberates that there is no possibility that the reactor is used for other than peaceful use, there is no hindrance to

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NSRA, Japan