Analysis of Reasons for Non-Conformance of the Movable Latch Lock Plunger Releasing Current of Control Rod Drive Mechanism and Improvement Measures

As one of the most important equipment for reactivity control, Control Rod Drive Mechanism (CRDM), which is widely used in pressurized water reactor (PWR) nuclear power plant, has a series of important security functions. As an important component of the claw part in CRDM, the nonmagnetic shim materials have big influence on the movable latch lock plunger releasing current of CRDM. When cutting off the coil power, the nonmagnetic shim materials can block the magnetic circuit between the pole and latch lock plunger effectively and reduce the remanence suction force between magnetic pole and latch lock plunger, which can promote a quick latch lock plunger’s action and finish the step-jump and rod-release. In this paper, we introduce the background of non-conformance of the movable latch lock plunger releasing current of CRDM of PWR plant simply, and then we analyze the reasons of the non-conformance in detail, including a comprehensive analysis of various factors and a series of retest conclusions. Through specific analysis, the important role of nonmagnetic shims in CRDM and its big influence on movable latch lock plunger releasing current are proved. Based on research and test results, we show our optimized measures on the movable latch lock plunger releasing current in detail, from the perspective of technical specification for raw materials and improved processing technology during production. At last, one latch unit which occurred the movable latch lock plunger releasing current non-conformance and experienced a 1.7 million steps performance test is used in the following performance verification test. Being installed in this latch mechanism, our nonmagnetic shims finished an integral series of cold test, hot test and cold test after hot test. All of the test results meet our design requirements and all of the releasing current under cold condition is better than the results of 1.7 million steps performance test. Especially, the independently developed nonmagnetic shims improved the movable latch lock plunger releasing current significantly. In addition, other current results of hot test are equal to that of 1.7 million steps performance test. In conclusion, these optimized measures may not only provide data for solving the problem completely, but also provide reference for the manufacture of nonmagnetic shim materials in the future nuclear power plant.