A Fracture Mechanics Evaluation of BWR Shroud Mid-Core Horizontal Weld to Justify Continued Operation

Abstract

The Mid-core shroud weld (H4 weld) at a BWR plant was inspected during refueling outages in 1998 (RF06) and 2000 (RF07). A structural margin evaluation considering 2000 inspection results for this weld provided technical justification for continued operation to at least the fuel cycle ending in 2002. IGSCC mitigation measures were implemented during operation through 2002. Therefore, a factor of two improvement in the assumed crack growth rate in the depth direction has been applied for operation between 2002 and 2004. The objectives of this paper are to describe the structural evaluation methodology used and describe the results of the evaluation in support of continued operation of H4 weld to 2004. Structural margins for continued operation of H4 weld to 2004 were evaluated two ways. First, the limit load calculations were conducted for a configuration in which through-wall flaws were assumed in regions where the ID surface fluence in 2004 exceeded 3×1020 n/cm2. Since all of the areas taken credit for had a fluence less than the threshold value of 3×1020 n/cm2, a limit load evaluation constituted a complete structural margin evaluation and no linear elastic fracture mechanics (LEFM) or elastic plastic fracture mechanics (EPFM) evaluations were necessary. Secondly, LEFM and EPFM calculations were conducted for the assumed configuration in which through-wall cracking was assumed where the ID surface fluence exceeded 5×1020 n/cm2. The calculated safety factor for the nominal case was determined to be 4.98, which exceeds the required value of 2.77. To demonstrate structural margin in the LEFM regime, a configuration similar to the limit load nominal case was used except that through-wall cracking was assumed in regions where ID fluence exceeded 5×1020 n/cm2 instead of 3×1020 n/cm2. The calculated value of the highest stress intensity factor was 49.2 ksi√in that is less than the allowable value of 54.2 ksi√in. Additional evaluation with EPFM was also conducted to demonstrate higher available structural margins. The EPFM evaluation was conducted by first determining an equivalent single through-wall flaw to conservatively model the LEFM configuration. The applied J-integral values were calculated using the EPRI ductile fracture handbook. A conservative material J-T curve corresponding to a fluence level of 5×1020 n/cm2 was used in the evaluation. The EPFM evaluation showed the structural margin for this case to be 4.0, which exceeds the required value of 2.77. Based on the results of these limit load, LEFM and EPFM structural margin evaluations, it was concluded that the required structural margins will be maintained at the H4 weld for operation through year 2004.