The BWR RPV Internals Management Program of the German NPP Gundremmingen, Units B and C: Results and Conclusions

Due to the core shroud cracks reported from numerous BWRs including the German KWU type BWR Wuergassen, a RPV internals management program for the Gundremmingen NPP (KRB-II) has been initiated in 1994. Major steps and the main results of this program are presented. As an interim result, surface condition of the weld regions and controlled post weld heat treatment (PWHT) in order to reduce the residual stresses seem to play an important role for resistance to crack initiation and growth. To support these assumptions, results of related R&D projects of the German utilities (VGB) are presented.

A Logic Model Approach to Conceptual Design of a Scientific/Industrial Complex

We demonstrate how the use of logic models and approximate reasoning can lead to more effective conceptual design of a large-scale scientific/industrial complex. A deductive model is used to describe the relationships between the mission of a complex, the tasks that are the concrete expressions of the mission, and the capabilities and resources that allow the mission to be carried out. This deductive logic model provides the formal basis for integrating these relationships into a conceptual design and provides a framework to express design options that are an essential part of the description of the complex. A decision model for choosing among the design options is constructed using approximate reasoning. This AR model uses forward-chaining inference models to emulate the types of reasoning used by experts. This approach allows the analyst to use both numerical data when available and the qualitative knowledge that forms much of the information base used in conceptual designs of new and innovative complexes.

Statistical Considerations for Determining Extent of Piping Inspections for RBI or API-570 Driven Inspections

This paper outlines a method for calculating the number of inspection locations for process piping inspections. The method determines the number of piping inspection locations required for an inspection to detect a particular damage state within the confidence limits of the premised inspection’s reliability. It is intended to be used for piping inspections per API-570, “Piping Inspection Code” and in the application of risk-based inspection concepts presented in AP1-581, “Risk Based Inspection, Base Resource Document”. This method combines recognized inspection and piping engineering practices and random-field statistical tools to calculate the number of inspection locations in piping systems with probabilistic confidence level. This method has provisions for future applications when inspection data is known, or there is greater uncertainty in the distribution of the degradation or the reliability of the inspection data is different than those premised in this paper.

Life Time Management: A Practical Approach of Nuclear Power Plants

A comprehensive life time management has to take care of all safety and availability relevant components in nuclear power plants, with different intensity, of course. For instance, mechanical systems and components can be basically classified/ranked into three different groups: (1): The quality status of the components in this group has to be guaranteed on a pre-defined (high) level. (2): The quality status of the components in this group has to be maintained on its actual level. (3): Other components with no specific quality demands. Regarding the first group, integrity has to be guaranteed. Therefore it is necessary to monitor the possible root causes of degradation mechanisms during plant operation; thus the degradation effects can be assessed and — more important — controlled to maintain the safety standard on the demanded high level without any compromise. The monitoring of consequences of degradation mechanisms is being performed as an additional redundant measure. The requirements to maintain the quality status of the second group of components can be fulfilled by monitoring of the consequences of operational degradation mechanisms to be performed by preventive maintenance activities, in terms of tests, inspections and repairs, using either time dependant procedures or component condition orientated methods. For the third group of components, no preventive action is necessary. However, failures and malfunctions have to be assessed statistically to avoid a reduction of the required basic component quality. In the first two groups all safety relevant components and systems are included. Generally, aging management programs cover these two groups of components; life time management covers all of above groups. This paper concentrates on mechanical systems and components; it summarizes the practical approach to life time management as it is realized in German nuclear power plants. The application is discussed using dedicated examples.

Optimal Control of Performance Risk for Large Potentially Dangerous Systems (LPDS)

The paper considers the safety problem for large potentially dangerous systems (LPDS). Disruption of their normal operations may lead to casualties, ecological and property damage. Solution to the above problem is sought in the framework of risk control of LPDS during their normal operation, based on the principle of preventive actions. Risk is described as the product of conditional probability of failure and the overall consequences of such failure. Methods of brining down risk analysis problems to reliability problems are presented. They are based on the following: assessments of “cost of life” (as economic equivalent of casualty); simultaneous optimization of the LPDS and its safety subsystem (expansion of the object of optimization). Such an approach allows unification and merging of structural reliability theory and probabilistic risk analysis. A quantitative method of damage size (the first component of risk) assessment is described, based on computer modeling of a full group of scenarios of a structural failure developing into a full blown LPDS catastrophe. As a result of modeling, the destruction zones and the character, size and probabilities of all kinds of damage (casualties, ecological damage, loss of property) are assessed. It is proposed, as the main method of securing LPDS integrity and safety, to equip each LPDS with suitable monitoring/inspection/maintenance systems, designed as an instrument for controlling the second component of risk (conditional probability of failure), on the basis of a three-level (warning-alarm-failure) control policy. In the outlined format maintenance/repair is considered as optimal control of random degradation and renewal functions, interaction of which forms a certain regeneration process. Analysis of this process allows defining the optimal triggering levels of deterioration parameters or risk that minimize total expenditures of LPDS performance while ensuring its safety. The problem formulated above naturally embodies all existing maintenance methods (based on admissible performance time, rate of failure and on actual and prognosed system condition). Further, the problem of optimal cessation of performance is solved. It allows convoluting a multi-parameter problem into a one-parameter problem and defining the ultimate permissible level of conditional probability of failure. The described methods of risk analysis and control were used in residual lifetime monitoring systems for oil pumping aggregates and for main oil pipe line segments repair prioritization.

Using QRA to Make Maintenance Trade-Off Decisions

This paper discusses the concept of using quantitative risk assessment (QRA) techniques to make specific decisions regarding competing risks associated with specific maintenance policies. This paper is based on a project performed for a major international airline to assess the risk trade-off between a potential catastrophic event occurring during a maintenance procedure versus the possibility of inadvertently leaving a safety device in place following plane takeoff. The paper demonstrates the use of simplified event trees, human reliability analysis, and risk-based decision making.

Age-Related Degradation of Steam Generator Internals Based on Industry Responses to Generic Letter 97-06

This paper presents the results of an aging assessment of the nuclear power industry’s responses to NRC Generic Letter 97-06 on the degradation of steam generator internals experienced at Electricite de France (EdF) plants in France and at a United States pressurized water reactor (PWR). Westinghouse (W), Combustion Engineering (CE), and Babcock & Wilcox (B & W) steam generator models, currently in service at U.S. nuclear power plants, potentially could experience degradation similar to that found at EdF plants and the U.S. plant. The steam generators in many of the U.S. PWRs have been replaced with steam generators with improved designs and materials. These replacement steam generators have been manufactured in the U.S. and abroad. During this assessment, each of the three owners groups (W, CE, and B&W) identified for its steam generator models all the potential internal components that are vulnerable to degradation while in service. Each owners group developed inspection and monitoring guidance and recommendations for its particular steam generator models. The Nuclear Energy Institute incorporated in NEI 97-06, “Steam Generator Program Guidelines,” a requirement to monitor secondary side steam generator components if their failure could prevent the steam generator from fulfilling its intended safety-related function. Licensees indicated that they implemented or planned to implement, as appropriate for their steam generators, their owners group recommendations to address the long-term effects of the potential degradation mechanisms associated with the steam generator internals.

Ten Years Experience on Inconel Alloys Since the Bugey Leak in 1991

Base nickel alloys like Inconel 600 or 182 are particularly sensitive to stress corrosion cracking. This fact is well known since Corriou’works at the beginning of the sixties and its applications to the steam generator tubes in the seventies. For the RP vessel heads, the major fact of the nineties was the leak that occurred on one penetration in 1991 in the French NPP unit of Bugey. Several important decisions were taken after discover of this leak. First of them was to understand why it appeared so quickly, then test repairs for the Bugey case, then decide to replace all vessel heads considering that the repair solutions was to high cost. In parallel many developments were launched to establish laws for PWSCC and develop non-destructive methods to inspect the head penetrations. The conclusions obtained show the decision was good and no new leak happened on the VH penetrations.

Integrated Risk Evaluation for Probabilistic Safety Assessment Levels 1, 2, and 3 for Nuclear Power Plant Applications

This paper introduces a newly developed paradigm for performing an integrated risk evaluation using Probabilistic Safety Assessment (PSA) Levels 1, 2 and 3 results for nuclear power plant applications. This paradigm focuses on the issues of radionuclide release transport phenomena and source term determination from a severe nuclear reactor accident.

Industry Survey of Risk-Based Life Management Practices

The application of risk-based life management practices has generated considerable interest in industry. Plant safety and availability can be demonstrably improved through the application of risk-based methods alongside good plant management practices. The needs for and the experience of risk-based plant life management however, vary greatly across industry sectors. The principles of risk-based methods are documented and many risk-based techniques are widely available, but the practical interpretation of the principles and the use of the most appropriate techniques are subjects of great debate. This paper will identify technical and organisational requirements to implement risk-based methods. As part of this paper, a questionnaire survey has been carried out among companies to gain better understanding of the reality of plant life management and the needs of plant operators. This survey indicates that the benefits of risk-based methods for inspection (RBI) and maintenance (RBM) optimisation are recognised by different industrial sectors. There appears to be a lack of established and documented uniform RBI/RBM policy or guidance for application throughout the industry sectors. There is also an indication of insufficient resources and training to implement risk-based methods. Development of risk-based techniques by a competent team and an integrated user-friendly software based on a sound methodology remain as key issues. A case study is described of the application of RBI to an oil refinery process unit.