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The Mitsubishi Nuclear Instrumentation System (NIS) is an advanced design on increased reliability and maintainability. The Mitsubishi NIS allows for easier calibrations and error detection by reducing number of components and manual adjustments. The Mitsubishi NIS is implemented as part of the MELTAC digital nuclear Instrumentation and Control (I&C) platform, and includes neutron detectors that continuously measure reactor neutron flux.

System Configuration​

The NIS is ​composed of neutron detectors, source range preamplifiers, wide range preamplifiers, NIS cabinets and an audible alarm system.

  • Neutron detector: Located in reactor containment, and sends neutron flux signals which are proportional to reactor output, to the NIS cabinets directly or via Source Range (SR)/Wide Range (WR) preamplifier.
  • SR/WR preamplifier: Amplifies pulse signals from SR and WR detectors and send them to the NIS cabinets
  • NIS Cabinets: Contain modules that receive signals from the SR, Intermediate Range (IR), Power Range (PR), and WR detectors and convert pulse or current signals to appropriate signals to send to the Reactor Protection System (RPS), and sends the power range average neutron flux signals to the Reactor Control System (RCS).
  • Audible Count Rate System: Receives pulse signals from the SR detector and converts them to the audible signals to issues audible alarms at volume proportional to the neutron flux in the containment vessel.

System Features

  • Major Circuits are housed in cabinets and accessible form the front allowing for easier maintenance and inspection​.
  • Module-type instruments are contained in the NIS cabinets and are easy to maintain through test points on the front side of module.
  • Indication from the detectors (SR, IR and PR) and alarm displayed on the cabinet.

Detector Range

Since neutron flux varies over a wide range between reactor shutdown and normal operation, the Mitsubishi NIS has capabilities for measuring in the following range.
  • ​Source Range: 1 to 106 cps (count per second) 
  • Intermediate Range: 1 x 10 -11 to 5 x 10 -3 A
  • Power Range: 1 to 120 % RTP (reactor thermal power)

These ranges overlap each other to ensure full-coverage monitoring. In addition, a Wide Range (1 to 109 cps) is provided to measure all ranges during accidents.