Stratification and the operational aspects of nuclear-power plants

Operating experience indicates that thermal stratification is a significant event unaccounted for in original nuclear-power-plant designs. With the aging of nuclear plants, the long-term effects of thermal stratification on several piping systems have been exhibited in through-wall cracks, damaged supports, and thermal fatigue. This paper describes a comprehensive program developed and implemented at Virginia Power's North Anna and Surry pressurized water reactor power stations to address thermal stratification of the pressurizer surge line. Field inspections, temperature and displacement measurements synchronized with various plant events, and analytical evaluations indicate that stratification can be satisfactorily accounted for without undue restrictions on plant operations and with only minor hardware modifications to the pipe supports and pipe whip restraints. The fatigue evaluation rigorously considered measured stratification profiles, specified operating conditions, striping, and additive thermal stratification cycles