ORNL Evaluation of Electrabel Safety Cases for Doel 3 / Tihange 2: Final Report
Oak Ridge National Laboratory (ORNL) performed a detailed technical review of the 2015 Electrabel (EBL) Safety Cases prepared for the Belgium reactor pressure vessels (RPVs) at Doel 3 and Tihange 2 (D3/T2). The Federal Agency for Nuclear Control (FANC) in Belgium commissioned ORNL to provide a thorough assessment of the existing safety margins against cracking of the RPVs due to the presence of almost laminar flaws found in each RPV. Initial efforts focused on surveying relevant literature that provided necessary background knowledge on the issues related to the quasilaminar flaws observed in D3/T2 reactors. Next, ORNL proceeded to develop an independent quantitative assessment of the entire flaw population in the two Belgian reactors according to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section XI, Appendix G, Fracture Toughness Criteria for Protection Against Failure, New York (1992 and 2004). That screening assessment of all EBL-characterized flaws in D3/T2 used ORNL tools, methodologies, and the ASME Code Case N-848, Alternative Characterization Rules for QuasiLaminar Flaws . Results and conclusions from the ORNL flaw acceptance assessments of D3/T2 were compared with those from the 2015 EBL Safety Cases. Specific findings of the ORNL evaluation of that part of the EBL structural integrity assessment focusing on stability of the flaw population subjected to primary design transients include the following: ORNL s analysis results were similar to those of EBL in that very few characterized flaws were found not compliant with the ASME (1992) acceptance criterion. ORNL s application of the more recent ASME Section XI (2004) produced only four noncompliant flaws, all due to LOCAs. The finding of a greater number of non-compliant flaws in the EBL screening assessment is due principally to a significantly more restrictive (conservative) criterion for flaw size acceptance used by EBL. ORNL s screening assessment results (obtained using an analysis methodology different from that of EBL) are interpreted herein as confirming the EBL screening results for D3/T2. ORNL s independent refined analysis demonstrated the EBL-characterized flaw 1660, which is non-compliant in the ORNL and EBL screening assessment, is rendered compliant when modeled as a more realistic individual quasi-laminar flaw using a 3-D XFEM analysis approach. ORNL s and EBL s refined analyses are in good agreement for the flaw 1660 close to the clad/base metal interface; ORNL is not persuaded that repeating this exercise for more than one non-compliant flaw is necessary to accept the EBL conclusions derived from the aggregate of EBL refined analysis results. ORNL General Conclusions Regarding the Structural Integrity Assessment (SIA) Conducted by EBL for D3/T2 Based on comparative evaluations of ORNL and EBL SIA analyses and on consideration of other results, ORNL is in agreement with the general conclusions reported by Electrabel in their RPV D3/T2 Technical Summary Note of April 14, 2015: More than 99 percent of flaws in D3/T2 meet the defined screening criterion, rendering them benign with respect to initiation in the event of a design transient. Refined analyses of non-compliant flaws from the screening assessment indicate that only 11 of the 16196 detected flaws have a critical reference-temperature material index (designated RTNDT) that implies the possibility of the initiation of cleavage fracture at some future time. For those 11 2 flaws, the calculated margin in RTNDT (a measure of acceptable embrittlement relative to end-ofservice-life conditions) is significant, being greater than 80 C. Fatigue crack growth is not a concern in the flaw-acceptability analyses. Primary stress re-evaluation confirms that the collapse pressure is more than 1.5 times the design pressure in the presence of defects detected in D3/T2. Sufficient conservatisms are built into the input data and into the different steps of the SIA; in some cases, those conservatisms are quantified and imply that additional margins exist in the SIA. Taken as a whole, the foregoing results and conclusions confirm the structural integrity of Doel 3 and Tihange 2 under all design transients with ample margin in the presence of the 16196 detected flaws.