Introduction  The RGL system (Rod Position Indication and Rod Control System) is one of the important control systems in nuclear power unit, its performance is directly related to the availability of nuclear power unit when the double-hold fault happen in the RGL system, the entire control rod group will be locked, and the control rods cannot move in time during the transient test, load reducing operation or load raising operation of the unit, which may lead to the shut down of the reactor.

  Method  Based on the experience feedback of the double-hold fault of the RGL system in a CPR1000 unit, this paper analyzed and researched the cause of the double-hold fault from the perspective of design principles.

  Result  The analysis results indicate that the moment controller in the controller software misjudges the state of the MG rack, and thus outputs the wrong signal, which leads to the triggering of the double-hold fault.

  Conclusion  Software defect is an important factor causing double hold fault of RGL system.