Chapter 2 Systems of bwr Nuclear Power Plants

Chapter 2

1. General Design Philosophy

Light water nuclear power plants in Japan are designed pursuant to the policy specified in the Nuclear Safety Commission Regulatory Guide*¹ “Reviewing Safety Design of Light Water Nuclear Power Reactor Facilities (NSCRG L-DS-I.O, last update on August 30,1990)".

Chapter 2 outlines how the actual plant design is conducted to meet this Regulatory Guide. Unique design features of the Advanced Boiling Water Reactors (ABWR) are provided in individual sections and also collected in Section 2.12 for the readers’ convenience.

1. Fuel Rods

Fuel rods of a light water reactor (LWR) consist of sintered pellets of slightly enriched uranium oxide housed in a zircalloy cladding. Most solid fission products produced from fission reactions remain in the ceramic fuel pellets, although part of the rare gases such as gaseous xenon and krypton diffuse in the sintered pellets and are released from them.

These released gaseous fission products are retained inside the fuel cladding, namely, the fuel pellets work as the first barrier against the radioactive fission products release to the environment. Sufficient care must be taken in its design and manufacturing to ensure its integrity.

The aforementioned Regulatory Guide (NSCRG LDS-I.O) requires fuel rods to be designed so that the acceptable fuel design limit is never exceeded during the normal operation and anticipated operational occurrences. The acceptable fuel design limit is defined as the limit which corresponds to the safety tolerance for reactor fuel damage, as set

^l) [Translator’s note] The key regulatory guides authorized by the Nuclear Safety Commission of Japan are downloadable from its homepage < http://www.nsc.go.jp/NSCenglish/>.

forth in association with the plant design, and allows the reactor facility to continue its operation.

The plant is designed so that the safety protection system will automatically actuate the reactor shutdown system for terminating the reactor operation in case an incident occurs during the reactor operation which may cause the fuel to reach the above-mentioned design limit.

2. Reactor Core

The reactor core which generates heat consists of fuel assemblies arranged in the reactor pressure vessel (RPV). The overall configuration of the core is cylindrical. Each fuel assembly consists of fuel rods in a square array.

During the scheduled outage of a nuclear power plant, some of fuel assemblies are replaced with fresh ones and the fuel integrities are examined as needed.

The thermal output of a reactor core is determined by the average thermal power value of all the fuel rods. Hie integrity of fuel rods is bound by the local maximum thermal power of a fuel rod. The ratio of these two values (peak to average) is defined as the power peaking factor and used as an index for integrity evaluation. Small power peaking factors mean flat power distributions.

During the normal operation, the reactor core is kept critical; fission reactions occur at a rate corresponding to the intended thermal power and the neutron production and consumption are balanced. If this balance shifts, the thermal power of the core also changes accordingly. The degree of such a change in power or neutron populations is indicated by a parameter called “reactivity.” The reactor core is designed in a manner such that the reactivity can be controlled by inserting the control rods into, or withdrawing them from, the core. The core reactivity also changes with the changes in core parameters such as fuel temperatures, coolant

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NSRA, Japan