Progress in RI ISI Applications for WWER 440 and WWER 1000 Type Piping Systems

The technical publication contains a review of four risk informed inservice inspection pilot studies focused on WWER 440 and WWER 1000 type piping systems. All applications conducted in close co-operation with EPRI staff were based on modified EPRI RI ISI methodology for WWER type NPPs. Two of four pilot studies belonged to the EPRI methodology extension for Break Exclusion Region (high energy piping). All results and lessons learnt are shortly presented and discussed. The above RI ISI pilot studies were conducted for the following systems: 1. Primary piping including pressurizer piping surge lines at Dukovany NPP (WWER 440 type reactors, Czech Republic) 2. Low pressure emergency core cooling piping system (LP ECCS) at Temeli´n NPP (WWER 1000, Czech Republic) 3. No break zone (NBZ) application for steam line piping outside containment at Khmelnitskyi NPP (WWER 1000 type reactors, Ukraine). 4. No break zone application for steam line and feed water line piping outside containment at Temeli´n NPP (WWER 1000 type reactors, Czech Republic). Two studies are discussed in detail. The results for LP ECCS can be summarised as follows: from 115 welds in class 1 only 16 welds ranked in risk category 2 (RC2) with thermal fatigue potential, 42 welds in RC4 with no degradation mechanism identified (all welds with high consequence) and 57 welds in low RC6. From 671 welds in Class 2 only 19 welds ranked in RC5 with thermal fatigue or cavitation potential. The rest of 652 welds were ranked in low RC6 or RC7. The number of inspections applied were substantially changed from the point of inspections performed according to current ISI programme and proposed RI ISI examinations. The former 73 VT and PT examinations in the current ISI programme were proposed to be replaced by 12 UT inspections of which 5 are proposed as qualified mechanised UT examinations in case of identified potential to thermal fatigue damage mechanism. The pilot study at Khmelnitskyi NPP Unit 2 was ordered by IAEA. Based on the RI ISI pilot study results, nine of 67 Class 2 welds were selected for recommended qualified UT mechanized inspections. No Class 3 weld of total number 18 welds was included in the RI ISI programme. The risk impact acceptance criteria 1E−7 per system and 1E−8 per year for CDF and LERF were applied according to EPRI methodology in compliance with guidelines of the Czech Regulatory Body.