leak detection systems, such as radiation monitors, sump water level gauges and condensed water level gauges, are installed to detect, at an early stage, any leakage of the primary coolant from the reactor coolant pressure boundary.

4. Engineered Safety Features

Pipe rupture accidents, typically a LOCA, may damage substantial numbers of fuel rods, resulting in the risk of external release of significant amount of radioactive materials. Engineered safety features (ESFs) are installed to mitigate and/or prevent such accidents. ESFs include the emergency core cooling system (ECCS), which injects emergency cooling water into the reactor core, and the reactor containment system, which confines the radioactive materials released from the reactor coolant system.

The ESFs must have higher degree of reliabilities than other reactor ordinary systems, to meet the requirement of the single failure criterion. To check the on-demand operability of these safety systems, design precautions are made to enable periodical surveillance tests during the normal operation and the reactor shutdown periods. The ECCS is designed with independency and redundancy, ensuring its satisfactory functions in a LOCA even if a single failure of an active component is assumed for a short term before injection starts following an accident, or if either a single failure of an active component or a single failure of a passive component is assumed for a long term cooling after an accident. The ESF components installed inside the primary containment vessel (PCV) are designed to withstand the adverse environmental conditions in an accident.

In order to secure electric power supplies to the ESFs and safety-related systems, two physically and electrically independent on-site emergency.power supply systcms are provided in the BWR power plants, namely, emergency diesel generators and battery banks. The on-site emergency power supply systems are designed assuming the single failure criterion in the event of loss-of-off-site power. The emergency power supply systems are periodically inspectable and testable.

The reactor containment facility is the final barrier for preventing the fission product releases

to the environment and ensuring the safety of the public in the event of a failure of the primary coolant system.

The PCV is designed to ensure that the design limit for the leak rate of radioactive materials is not exceeded. The leak rate tests on some pre­designated penetrations, such as for piping, electric cables and air-locks, can be conducted individually or in a small group of them. Hie PCV penetrations for pipes, etc. are equipped with isolation valves, which protect the containment boundary and should also be tested regularly for operability.

To prevent rapid and unstable brittle ruptures of ferritic structures of the PCV boundary, under any duty conditions, the toughness of the materials is closely examined at the time of their selection.

5. Instrumentation and Control System

A BWR plant is controlled by the reactor control system and the safety protection system to ensure reactor safety in all its operation modes: normal operation and under any abnormal situations, including reactor shutdowns upon malfunctions and/or miss-operations or mitigation measures of an accident such as a pipe rupture. The safety protection system is comprised of the normal and the emergency reactor shutdown systems and the ESFs. The reactor instrumentation and process instrumentation systems provide necessary parameters for reactor control and protection, such as neutron flux distributions, temperatures, pressures and radiation levels of the reactor system. An automatic alarming system is installed for early detection of, and countermeasures for, an abnormal event. Several interlock systems are designed to block an abnormal event due to malfunctions and/ or miss-operations, and to prevent development of equipment failures that could possibly lead to an accident. Further, computer-aided systems assist operators by providing them the plant operation conditions straightforwardly.

The reactor control system maintains the operating parameters at normal values during normal reactor operations and controls them automatically upon the designed load changes on demand and the anticipated external disturbances. The system is not required to have reactor protection functions, but to reduce the duty

2--;

NSRA, Japan