Comparison of 'VERLIFE' and Russian methodologies for reactor pressure vessel integrity assessment

Currently used methodology for the assessment of the reactor pressure vessel (RPV) integrity in the Czech Republic is based on the 'Unified Procedure for Lifetime Assessment of Components and Piping in WWER NPPs, VERLIFE' developed within the Project of the 5th Framework Programme of the EU by a group of experts from WWER operating countries. This methodology was compared with the recent Russian methodology for the RPV integrity assessment 'Methodology of Determination of the Residual Lifetime of the Reactor Pressure Vessels of WWER Reactors During Operation, MRK-SChR-2000'. Both methodologies are based on the same principle-comparison of stress intensity factor KI with its allowable value. This is evaluated for postulated crack defined according to the methodology. The assessment is performed for a set of emergency events of the pressurised thermal shock (PTS) type. In both methodologies, critical temperature of brittleness Tk was taken as the index temperature on which the assessment is based (but also reference temperature T0 and Master Curve approach can be applied within the VERLIFE methodology). Series of sensitivity analyses was performed to assess the influence of individual parameters entering the RPV integrity assessment, which are prescribed differently within both methodologies. The parameters were as follows: crack shape, crack depth, residual stresses, curve of allowable values of stress intensity factor (together with crack front length and shallow crack adjustments) and integral vs. point-by-point assessment. The sensitivity studies were performed for large break LOCA transient. Finally, the assessments of three selected PTS events were performed according to both methodologies and the results were compared. The VERLIFE methodology was found to be more conservative in comparison with the Russian one. (authors)