Chapter 3 Systems of pwr Nuclear Power Plants

1. General Design Philosophy

In Japan, NPPs are designed according to the policy specified in the “Guide for Safety Design of Light Water Nuclear Power Reactor Facilities” (set by the Nuclear Safety Commission on August 30, 1990). The primary purpose of the guide is to establish protection for the general public. In other words, safety measures are provided in order to assure structural protection against hazardous fission products (FPs) and to ensure safe operation of the nuclear reactor. In this chapter, the applications of these safety guidelines to the actual plant design are outlined.

1. Fuel Rods

Fuel rods of a light water reactor consist of sintered pellets of slightly enriched uranium oxide(some containing gadolinium, etc.) housed in a zircalloy-4 (or zirconium-based alloy) cladding. Although most of the FP inventory remains in the ceramic fuel, a part of the gaseous FPs such as xenon and krypton, diffuse from the fuel pellets and are retained in the metal cladding. Therefore, the fuel cladding as a barrier, plays an important role in preventing FP release to the primary coolant system and sufficient care must be taken in its design and manufacturing to ensure its integrity.

The fuel rod design must be so comprehensive that the acceptable fuel design limit is never reached during the normal operation and abnormal transients. This limit is set at the level of fuel damage which is acceptable from a safety design standpoint while providing the conditions for continuation of the nuclear reactor system operation. Therefore, if any phenomenon appears during the reactor operation which causes the reactor conditions to reach the acceptable fuel design limit, the emergency shutdown system will act by dropping control rods to terminate the operation.

2. Reactor Core

The reactor core in which heat is generated consists of fuel assemblies cylindrically arranged in a reactor vessel (RV). Each fuel assembly consists of fuel rods in a square array. In reactor core design, a limit is set on the local maximum thermal power value of a fuel rod in order to maintain its integrity. The ratio of the maximum power value to the average power per unit length of a fuel rod is defined as power peaking factor and it is used as an assessment index. A smaller power peaking factor, implying a flat power distribution, is preferable from viewpoints of safe operation and efficient fuel utilization.

During normal operation, the reactor core is under the critical condition where, fission reaction occurs at a rate corresponding to the fixed power and, the production and the consumption of neutrons are in balance. If this condition changes, the power will also change. The degree of such a change in power or neutron population is called reactivity. Reactor core design provides reactivity changes either by moving control rods in and out of core or by adjusting the boron concentration in the coolant as chemical shim (chemical control). The core reactivity is also changed with any change in core parameters such as fuel temperature, moderator temperature (coolant) and void fraction. The reactivity change due to a unit change in each of these parameters is defined as a reactivity coefficient For example, the core reactivity change corresponding to a unit change of fuel temperature is called the Doppler coefficient. The others are moderator temperature and void coefficients. In discussion of the pressurized water reactor (PWR) core, these three coefficients are put together and expressed as a single coefficient, namely, the power coefficient Any increase in power leads to fuel temperature rise which in turn causes a minus reactivity change due to the Doppler effect

Chapter 3

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NSRA, Japan