On Analysis Of Fault Mechanism & Reliability About Key Parts Of CRDM For QNPC-1

Due to the low-technology level and insufficient designing experience in that time, the Control Rod Driving Mechanism (CRDM) of QNPC-1, as a prototype of CRDM, designed and manufactured by Chinese themselves in 1970s. The experience of commissioning and operation indicates that there are many deficiencies on both itself and its control device and it needs a continue improvement later on. It is necessary to analyze the failure-mechanism of CRDM's key parts and its reliability, and it is also very meaningful to suggest useful and practical improvement before the replacement of CRDM in QNPC-1 recently.This article analyses the current condition of CRDM for QNPC-1 and its difference with the others firstly, and carries out the fault-analyzing of T4-3 rod-drop typical accident, the failure mechanism analysis about CRDM key parts, the calculation confirmation of gripper traveling. It concludes that the design of gripper traveling is reasonable, and there are two main factors resulted in movement failure of CRDM. One is power failure, mainly on AC phase-loss consisting of SCR failure, the failure of SCR's trigger circuit , there phases AC switches'failure and the failure of CRDM coil; the other is the dis-match of parts'movement due to the moveable parts'loosing resulted from the wore-out of grippers, link pins and their holes. There is a"AND"logic relation between these root-accident and the top-accident.This article also simulates the rod-drop accident and calculates its fault-probability by using of modern simulation technology and dynamical fault tree analyzing method. The conclusion is that the comprehensive fault-probability of CRDM is rather low, that is the CRDM's mechanical reliability is high and its movement reliability can be improved through part improvement. It also confirmed that power failure has high fault probability and pointed out the direction of improvement at the same time .Based on the above research outcome, the article suggests the detail improvements on CRDM control device, mechanical modification and management, and all these approaches have been realized in the last refueling outage of QNPC-1 at the end of 2007. The result is quite good.The research result of this article is also a good reference on the reliability analyzing and improvement about CRDM for the other nuclear power plants.