Risk-Based Inspection
SIB-96-176

Over the last few years, there have been several industry initiatives to develop risk-based inspection programs as alternatives to currently required ASME Code, Section XI Inservice Inspection (ISI) programs. These efforts have been initiated by owners groups, the Electric Power Research Institute and ASME Boiler and Pressure Vessel Code Subcommittee XI. A working group has been established under Subcommittee XI to study the application of risk-based technologies to ISI and to develop the necessary Code changes to permit their use in lieu of current requirements. There are also a number of ongoing industry risk-based pilot studies to demonstrate the feasibility and effectiveness of the proposed methods.

The overall goal of these efforts is ultimately to allow the use of risk assessment, plus an understanding of component-specific degradation mechanisms, to establish an effective plant integrity management program which maintains plant safety, while at the same time reducing the industry and regulatory burden associated with current ISI requirements. Preliminary results from the pilot studies have shown that the application of risk-based techniques will allow nuclear plants to reduce the examination scope of current ISI programs by as much as 60% to 80%, without sacrificing safety. A brief discussion of the ongoing programs will follow.

Risk-Based Inspection of BWR Vessel Welds
This study was conducted under the auspices of the BWR Vessel and Internals Project (BWRVIP) to evaluate the current inspection requirements for boiling water reactor (BWR) pressure vessel shell welds and to provide an alternate, technically-justifiable inspection program. As shown below, several plates are used to fabricate both the cylindrical and hemispherical portions of BWR pressure vessels, resulting in several circumferential and longitudinal seam welds.

The study considered factors such as BWR vessel construction practices, prior inservice inspection results, BWR design and operational issues as related to vessel integrity, potential degradation mechanisms, and state-of-the-art in nondestructive examination (NDE) capabilities. It also included a detailed probabilistic fracture mechanics evaluation.

Evaluation of vessel fabrication practices, construction examinations, and preservice inspections established that BWR reactor pressure vessels were constructed to very high standards which maximized their initial quality. The results of in-service inspections performed to date support the conclusion that vessel seam welds are free from unacceptable fabrication defects, and that no significant flaws have developed during operation. A detailed review of current NDE criteria and technology was also performed, which concluded that the industry's ability to ultrasonically locate and size flaws in vessels has improved considerably, and is highly reliable at this time.

A review of design and operational issues, including vessel loadings and irradiation embrittlement effects, demonstrated the large inherent safety margins of BWR reactor pressure vessels. Differences exist which render the reactor vessels in BWRs less risk-significant than those in pressurized water reactors (PWRs), especially as related to the circumferential welds in the vessels. Specifically, BWR vessels are about twice the diameter of PWR vessels, with much larger annular spaces between the vessel and the reactor core. Irradiation embrittlement is therefore much smaller in BWRs. Also, loading conditions such as the pressurized thermal shock transient are not possible in BWRs because of the large steam region in the upper portion of the vessel. Thus, the most limiting condition with respect to low temperature pressurization of a BWR vessel is the vessel pressure test performed each outage. This is advantageous from a risk standpoint, since the reactor is already in cold shutdown during this test.

Finally, a probabilistic fracture mechanics analysis was performed to quantify these observations. This analysis concluded that the probability of BWR vessel failure was extremely low, and well within NRC safety goals, even with a significant reduction in the level of in-service inspections. In addition, this probabilistic analysis determined that the risk of either vessel leakage or failure from circumferential shell welds is orders of magnitude less than that associated with longitudinal shell welds. Summary results of this study are presented, and include the vessel failure probability and inspection costs of three ISI alternatives:

1. Current ISI requirements per ASME Code, Section XI and 10CFR50.55a (essentially 100% of all longitudinal and circumferential welds).
2. Eliminate circumferential welds from the ISI program.
3. Eliminate circumferential welds from the ISI program, and reduce longitudinal weld inspections to 50%.

Comparative Analysis of ISI Alternatives

On the basis of these results, the BWRVIP has submitted a Petition for Rulemaking to the NRC requesting that the requirements of 10CFR50.55a be modified to eliminate inspection of circumferential welds in BWR vessels (Program B). This program results in substantial cost savings, with virtually zero increase in vessel failure probability. Actions are also underway to implement similar changes in ASME Section XI.

EPRI Risk-Informed Inspection Evaluation Procedure for Piping Welds
Inspection of piping welds in nuclear power plants by current ASME Code requirements is very costly and time-consuming because there are literally thousands of welds that must be inspected periodically. The task is made even more difficult by augmented inspection requirements put into place by the NRC to address specific degradation mechanisms which have occurred inservice (e.g. erosion-corrosion and intergranular stress corrosion cracking (IGSCC)). Several efforts are underway by various owner's groups and EPRI to develop a risk-based inspection program for piping welds which would integrate and potentially reduce piping ISI requirements using risk-based techniques, while still maintaining an acceptable level of safety.

The EPRI risk-informed ISI (RISI) approach is a blended approach which utilizes both Probabilistic Safety Assessment (PSA) and deterministic insights. The PSA insights are founded on the logic structure of the plant's PSA, including fault tree and event tree models of the failure combinations which could cause undesired events. The evaluation process includes system boundary identification, failure modes and effects analysis (FMEA), risk evaluation and selection of inspection locations and examination methods.

The FMEA consists of a consequence evaluation and a degradation mechanism evaluation. The consequence evaluation focuses on the impact of a pipe segment failure on plant operation. Both direct and indirect effects are considered in the consequence evaluation including spatial effects and isolability of a break. Consequences are analyzed assuming a worst-case break (usually a large break). The degradation mechanisms considered include thermal fatigue, stress corrosion cracking, local corrosion, erosion cavitation and flow accelerated corrosion. The results of the consequence and degradation mechanism evaluations are used to divide the system into piping segments which are determined to have common degradation mechanisms and failure consequences.

The risk-informed evaluation then categorizes the risk of each segment as high, medium, or low. The integrity of the piping segments in the low-risk categories is monitored by periodic pressure/leak testing, operator walk-downs, and in-place leakage monitoring. For high and medium-risk piping segments, an appropriate scope of inspections is defined and inspection locations selected.

For each inspection location, an "inspection-for-cause" program is implemented to ensure that appropriate examination methods, procedures, acceptance criteria, and evaluation standards are applied to address the degradation mechanisms of concern for that location.

Two ongoing pilot programs to apply the RISI procedure to BWRs and PWRs have, so far, shown that this program can be used to establish an effective piping integrity management program, which reduces the number of inspections yet continues to maintain plant safety.

ASME Code Case N-560
Code Case N-560 has recently been approved by the ASME Boiler and Pressure Vessel Code Committee, which applies the above EPRI RISI approach to Category B-J welds in Class 1 piping. This Code Case permits reduction in inspections of Class 1 piping from 25% to 10% of piping welds using specific risk-based procedures, prescribed in the Case. The procedure requires an evaluation of consequence-of-failure and degradation mechanisms as defined in the EPRI RISI process. Application of this Code Case is estimated to save a typical plant approximately 300 man-REM and $1 million per inspection interval; relative to current ASME Code, Section XI requirements for Class 1 piping; with a net improvement in plant risk.

If you would like to obtain more information on risk-based inspection or SI's capabilities in this area, please contact SI.