Chapter 5 Operation and Maintenance of pwr Plants

Stable operation and regular maintenance of nuclear power plants (NPPs) are essential to improve their performance and reliability. NPPs are operated and maintained under the Law for the Regulations of Nuclear Source Material, Nuclear Fuel Material and Reactors and the Electricity Utilities Industry Law. In accordance with the former, those who construct and operate nuclear reactors are required to prepare a document titled “Nuclear reactor facility safety preservation rules” and submit it to the national government for approval. The document covers key elements concerning the maintenance of nuclear reactor facilities, such as quality assurance, security control organization, managing control of operation, maintenance, keeping records and reporting, as well as radiation management and fuel control. In carrying out the measures in the document, necessary rules are detailed in particular areas, including operation and maintenance, and actual work is performed in accordance with them.

NPP operators are trained at the Nuclear Power Training Center to acquire technical knowledge and skills to ensure the safe and steady operation of the plants. Personnel directly responsible for day to day operation of the facilities (shift chiefs) are selected from those having high levels of specialized knowledge and technical skills and are required to pass official tests based on a national government system for the qualification of personnel in charge of the operation of NPPs.

Maintenance and repair personnel of NPPs are required to get on-the-job training on technical knowledge and skills needed for their work.

1. Plant Operation

There are three regular phases of operation for NPPs: shutdown for annual inspections, startup after inspections and normal operation. Normal operation, of the most concern among the three,

is mainly discussed here, together with functions of reactor control systems tightly connected to the plant normal operation. In the following sections, the reactor control systems, plant startup operation, plant normal operation and duties of operators, and plant shutdown operation are discussed in that order.

1. Reactor Control Systems

A PWR plant consists of a nuclear steam supply system (NSSS), which corresponds to the boiler system in a fossil fuel plant, and a turbine generator system, and their control systems. These control systems are basically independent of each other. In this chapter, the control system of the NSSS is designated as a reactor control system. The reactor control system is provided for control of the reactor plant during its normal operation and under transient conditions. The PWR plant control system is designed to adjust the turbine-generator output to the demands of the grid and to make the reactor follow the turbine-generator, taking advantage of self-regulating characteristics of the PWR plant Those features of the control system are expressed as “turbine master, reactor slave” or ’’turbine generator prioritized”.

Here, the self-regulating characteristics mean such characteristics of PWR plants as follows: When the turbine-generator load decreases, the position of the main steam control valves (governor valves) is controlled by the governor mechanism, and the steam flow rate to the turbine is reduced. The reactor control system is designed to accommodate plant transients without any reactor trip, in combination with the self-regulating characteristics of the reactor, and restore and maintain plant equilibrium conditions during design load changes. The design load changes are defined as the following, when the plant is operating with a power in the range of 15% to 100%:

Chapter 5

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NSRA, Japan