Predicting the Impact of CRDM Thermal Sleeve Flange Wear in Westinghouse Pressurized Water Reactors

Abstract

In Dec. 2017, a four-loop 1300 MW Electricite de France (EdF) plant in France, Belleville Unit 2, experienced a complete wear through and separation of one of their thermal sleeves at a rodded control rod drive mechanism (CRDM) location. During low power physics testing and rod drop testing, the plant had difficulty stepping the control rod into the core. The rod was freed by exercising the drive rod but was then stopped prior to full insertion during the rod drop test. The failure to insert the rod was caused by the worn thermal sleeve flange remnant. In response to this operational experience, Westinghouse notified the U.S. Nuclear Regulatory Commission (NRC) of this defect pursuant to the requirements of title 10 of the Code of Federal Regulations (10 CFR) Part 21 and published a nuclear safety advisory letter (NSAL) that provides details on the thermal sleeve flange issue and inspection recommendations. In their notification, Westinghouse determined that there was no immediate safety concern, but a substantial safety concern may be possible in the unlikely event that there is interference with the movement of more than one control rod. This paper summarizes the NRC staff's safety analysis of this issue. The staff conducted detailed probabilistic and risk analyses and followed up with a smart sample inspection of the industry's thermal sleeve inspection programs to verify the analysis assumptions. The staff found the risk of core damage to be low, their assumptions appropriate, and the industry was following the details of the NSAL appropriately.