Prediction of the Time-to-nip-up for the Point Lepreau Fuel Channels

Abstract

Following the 1999 fuel channel inspection, concerns arose about the possibility of spacer nip-up in Point Lepreau Generating Station (PLGS). Nip-up is an off-design condition in which the spacer is forced into full circumferential contact with the inside of the Calandria Tube (CT) by Pressure Tube (PT) diametral expansion. Nip-up is not a structural integrity issue but needs to be assessed as it will stress the CT beyond the values in the CT stress analysis. Nip-up can become a maintenance issue since it will impair SLAR late in the life of the fuel channels. To deal with potential nip-up, NB Power Nuclear chose to demonstrate that it would not occur prior to refurbishment at185 kEFPH. To evaluate the potential for nip-up prior to 185 kEFPH, NB Power Nuclear undertook an updated analysis of the onset time for spacer nip-up in PLGS, which featured the following modelling innovations: 1. PT diametral expansion was predicted with an multivariable regression model; 2. The local deformation of the CT cross-section at the spacer due to spacer load was predicted accounting for CT elastic and creep deformations; 3. As-installed CT and PT dimensions were used to calculate the initial clearance of the spacer inside the CT. This paper presents a summary of the updated nip-up analysis which concluded that nip- up would not occur by 185 kEFPH in PLGS. On the basis of this analysis, no further work related to nip-up was undertaken by NB Power Nuclear.