BWR Feedwater Nozzle Evaluation
SIB-96-141, Rev. 1

Introduction
The feedwater nozzles of boiling water reactors (BWRs) have been identified as the most limiting component in the reactor pressure vessel (RPV) from a stress and fatigue point of view as a result of the severe thermal duty experienced by these components. This severe cycling occurs as a result of returning the relatively cold condensed steam back to the hot reactor. The location at which these two different temperature extremes meet is the feedwater nozzle. Cyclic duty arises as a result of the temperature differential, and also as a result of fluctuations in the temperatures of both fluids caused by various modes of reactor operation.

Increased awareness of the severe cyclic duty experienced by BWR feedwater nozzles arose in the late 1970s, when cracking in the feedwater nozzles was identified as a generic industry issue. Due to leakage past the feedwater sparger inlet piping, high cycle fatigue cracking was initiated as a result of unsteady transient mixing and temperature cycling between the relatively colder feedwater and the relatively warmer reactor water in the region between the sparger inlet piping and the reactor vessel nozzle. In response to this issue, analyses and testing were conducted to develop hardware modifications and improved low flow feedwater controller characteristics to eliminate the problem. The NRC response to this generic issue (identified as Generic Technical Activity A-10) was provided in NUREG-0619. It included requirements for periodic inspection of the feedwater nozzle and spargers, usually every other refueling outage for UT inspection of all feedwater nozzle bore regions and inside blend radii, every fourth refueling outage for visual inspection of sparger flow holes and welds in sparger arms and tees, and every ninth refueling outage (or 135 startup/shutdowns) for internal PT examination of at least one nozzle with the sparger removed.

As a result of these regulatory requirements, significant effort has been put into the inspection, analysis, and repair of BWR feedwater nozzles. Structural Integrity (SI) has been involved with these efforts since the identification of the cracking problem, and offers several analytical solutions, as described below.

Leakage Monitoring System Review
As a result of the NRC staff position in NUREG-0619, leakage monitoring systems (LMS) were developed and installed at several plants. The systems consisted of thermocouples attached to the feedwater nozzle at a position downstream of the sparger inlet seals such that the effects of any leakage could be monitored as a reduction in measured temperature. The monitoring systems were based on the principle that any leakage would tend to reduce the feedwater nozzle temperature when any significant leakage was occurring. Many of these systems have identified potential leakage flows during the course of operation that have exceeded acceptable limits.

As a result of this, SI has performed several projects to obtain a better fundamental understanding of the effects of by-pass leakage on the safe-end outside surface temperatures where the LMS thermocouples are located. By using more rigorous heat transfer analysis to quantify the effects of both primary and secondary sparger inlet seal leakage, improved LMS calibration curves have been developed that provide better correlation between leakage and the measured temperatures. In most cases, these improved correlations have eliminated the identified leakage concerns. In some cases, the evaluation has identified dislocated or damaged temperature sensors which, when repaired, have returned the measured leakage to acceptable levels.

ASME Code Design Stress Analysis
The cracking experienced by many BWRs has led to nozzle machining and replacement of the fittings adjacent to the nozzles. SI has been contracted by several utilities to design, fabricate and/or install replacement hardware for these situations. SI has been particularly involved in preparing Design Reports that provide the design and analysis details for the replacement modifications, including a complete set of stress analysis calculations which demonstrate that the design complies with the requirements of Section III of the ASME Code.

Stress and Fatigue Reassessment
SI has performed several detailed assessments and re-evaluations of the fatigue analysis conducted for the feedwater nozzle. These reassessments have been performed to show that additional cycles may be accommodated beyond those considered in the fatigue analyses of record. This has been accomplished through evaluation for both system transients that affect the overall nozzle and safe-end region, as well as for rapid thermal cycling that may affect the fatigue usage in the nozzle bore and blend radius regions. Typically, a set of modified plant cycles are developed that bound the number of plant operating conditions for the remainder of the plant life.

Fracture Mechanics Evaluation
SI has routinely performed fracture mechanics evaluations in response to NRC Generic Letter (GL) 81-11. GL 81-11 was written to amend some of the hardware modification requirements specified in NUREG-0619, and allowed plant-specific analyses to be performed to justify not implementing the hardware modifications. Typically, these analyses consist of a fracture mechanics evaluation, where fatigue crack growth is calculated for postulated flaws under a specified set of loads using linear elastic fracture mechanics (LEFM) techniques. The intent of these analyses is to demonstrate that the growth of an assumed flaw is less than the allowable for the design life of the plant, thereby demonstrating acceptable structural margins.

Recent efforts by the BWR Owners' Group (BWROG) have been made to revise NUREG-0619 requirements. The basic premise of the BWROG work is that the cracking phenomena has been eliminated (through improved thermal sleeve/sparger designs), and ultrasonic inspection methods have significantly improved over those available in the 1980 time frame. Thus, when the improved ultrasonic inspection techniques are used, examination frequencies can be reduced for those plants demonstrating long-term structural acceptability. A key input for applying these new criteria are the results of plant-specific fracture mechanics analysis. The new BWROG requirements require plant-specific fracture mechanics analyses and long-term demonstration that actual plant duty is bounded by the duty used in the plant-specific evaluation.

FatiguePro Fatigue and Crack Growth Monitoring
On-line tracking of fatigue and fatigue crack growth has been routinely performed by SI using the EPRI FatiguePro fatigue monitoring software. FatiguePro is an EPRI-licensed software product developed by SI to address long term fatigue and cyclic duty tracking in nuclear power plants. This product was developed especially for components like the feedwater nozzle, where it is difficult to show long term structural acceptability for the design life of the plant (or for a license renewal period). This capability has been used to successfully track fatigue (both low cycle and high cycle) and fatigue crack growth of actual flaws in BWR components for more than ten years.

The crack growth analysis module available for FatiguePro performs on-line prediction of the fatigue growth of flaws (actual or postulated). Calculation of the fatigue crack growth is a major feature which has been added to the FatiguePro system's capabilities, since a basic input assumption for most ultrasonic examinations is that cracks of some defined size may have escaped detection. With this capability, the crack growth module of FatiguePro allows

in-situ evaluation of flaws (either actual or postulated) to ensure that they remain within allowable structural margins, thereby justifying longer inspection intervals or continued operation without repair. In particular, the fatigue crack growth module can be used to perform the fracture mechanics assessment recommended by the alternative rules to NUREG-0619 developed by the BWR Owners' Group.

If you would like more information on any aspect of BWR feedwater nozzle evaluation or SI's capabilities in this area, please contact SI.