In the United States Pressurized Water Reactors, the reactor internals are subject to augmented inspections under the auspices of MRP-227 to ensure safe operation through periods of extended operation. Recent operating experience has revealed degradation of baffle-to-former bolts (BFBs) where emergent and lengthy shutdowns were required to replace non-functional BFBs. A pro-active approach to safely managing BFB degradation requires an integrated analysis approach that addresses uncertainties in non-destructive examination (NDE) techniques, and uncertainties in bolt degradation rate and bolt degradation location. These uncertainties present nearly unlimited permutations of potential degraded bolt conditions, requiring probabilistic treatment to demonstrate structural integrity of the reactor internals assembly over the next inspection interval.
In this paper, we present almost 10 years of experience with probabilistic structural analysis of bolted connections in nuclear reactor internals applications. Modeling techniques for bolt degradation, location of degraded bolts, and uncertainty in NDE techniques are discussed. Techniques for rapidly evaluating millions of bolt failure patterns against the structural design basis are discussed. These techniques permit definition of requirements for an initial examination, requirements for reexamination interval, and prescription replacement bolt patterns optimized to meet the operator’s financial and risk management objectives, while ensuring safety with rigorous demonstration of maintaining design basis.