1.1. OBJECTIVE
This publication provides information regarding good practices for the assessment and management of ageing related to buried and underground piping and tanks within a nuclear power plant. Specifically, the objectives of this report are to provide:
— State of the art information regarding ageing management of underground piping in nuclear power plants throughout their entire service life, including the after service period;
— Background material indicating the importance of ageing management programmes (AMPs);
— Practices and techniques for assessing fitness for service and for inspection, monitoring and mitigation of ageing related degradation of underground piping important for the safe, reliable and environmentally acceptable operation of nuclear power plants;
— A technical basis for developing and implementing a systematic ageing management programme;
— Guidelines that can be used to ensure that ageing management is taken into account during different phases of a nuclear power plant’s life cycle (i.e. design, fabrication and construction, commissioning, operation (including long term operation and extended shutdown) and decommissioning);
— Research materials related to ageing and lessons learned.
This report is intended for plant owners, operators, designers, engineers and specialists to:
— Establish, implement and improve AMPs for nuclear power plants;
— Facilitate dialogue between owners/operators and regulators when dealing with age related licensing issues;
— Consider ageing in the design of new plants and modifications and in approaches to mitigating ageing effects.
1.2. BACKGROUND
1.2.1. Safety aspects of buried and underground piping systems and tanks
Buried and underground piping systems associated with nuclear power plants have a number of safety aspects.
Their integrity is maintained at plants to ensure nuclear safety, equipment reliability and environmental protection.
Numerous nuclear power plants have experienced leakages of radioactive materials from underground structures (piping, tanks, etc.) that caused contamination of groundwater. For example, some 45 of 65 US plants have had leaks or spills involving tritium in excess of 740 Bq/L at some time during their operating history [1].
Although there were no identified risks to public health and safety, the findings raised questions regarding plant vulnerability to underground piping corrosion and its potential impact on the public and environment. As a result, several initiatives (e.g. the 2006 Groundwater Protection Initiative in the United States of America) were put in place, a number of task groups (e.g. the National Association of Corrosion Engineers (NACE) Task Group (TG 404) and the Electrical Power Research Institute’s (EPRI) Buried Pipe Integrity Group (BPIG)) were formed and several guidelines (e.g. the Nuclear Energy Institute’s NEI 09-14 [2]) were produced. To restore public confidence, utilities proactively started adopting these guidelines and associated milestones.
Codes and standards within IAEA Member States are not consistent in their treatment of buried piping.
Some jurisdictions have little or no specific guidance, while others have guidance in the form of regulations on the environment, the handling of chemicals or petrochemicals, or pressure boundaries.
1.2.2. Need for ageing management of buried and underground piping and tanks
The design life of most nuclear power plants was typically chosen to be 30–40 years. There are, however, economic benefits for utilities to extend plant service life (with 60 years or more being a quoted target), delayed construction schedules and/or decommissioning strategies that involve use of some plant structures as a ‘safe store’
for periods of up to 100 years. This can mean that buried and underground piping systems at plants often have to perform functions for a time period significantly greater than their initial design life.
A number of existing nuclear power plant programmes, such as periodic inspection and testing, surveillance, and preventive and corrective maintenance, contribute to proper ageing management of underground piping.
The effectiveness and efficiency of ageing management can be improved by integrating and modifying these programmes, as appropriate, within a systematic AMP.
The IAEA Safety Standards Series No. NS-G-2.12 [3] provides a systematic and integrated approach to managing ageing (see Fig. 2 of this publication). Development of a systematic AMP and the interaction of key elements of such a programme are discussed later in this report.
Ageing management of buried and underground piping and tanks is more difficult than that for above ground systems. Access for routine surveillance and maintenance is more difficult and expensive, resulting in fewer opportunities to detect, mitigate and address degradation. A well planned AMP will optimize inspections to minimize costs while maintaining appropriate oversight of the systems in question.
Environmental cleanup, transportation and disposal of contaminated soil and other costs connected to a reportable leak can result in a substantial expenditure in terms of time and money. Effective AMPs can help reduce or eliminate premature plant failures that cause spills or radioactive leaks and thus reduce potential damage liabilities. In addition, some structures may be required to be maintained per government regulations. Cathodic protection of related metallic structures is essential to maintain any metallic structure in a corrosive environment at the lowest life cycle cost.
1.2.3. IAEA programme on safety aspects of nuclear power plant ageing
To help Member States understand the ageing of systems, structures and components (SSCs) important to safety and the effective ageing management of these SSCs, the IAEA in 1989 initiated a project on the safety aspects of nuclear power plant ageing. This project integrated information on the evaluation and management of safety aspects of plant ageing that had been generated by Member States into a common knowledge base. The IAEA issued numerous publications that assisted Member States with their ageing management programmes. The programme has continued to be developed over the years. A summary of activities undertaken is included in Appendix I.
1.3. SCOPE AND STRUCTURE OF THIS REPORT
This report deals with underground piping systems that are part of a nuclear power plant. It addresses potential ageing mechanisms, age-related degradation and ageing management (i.e. inspection, monitoring, assessment and remedial measures) as well as condition assessments for the materials and components of underground piping systems, such as:
This report follows the structure of the generic AMPs defined in Ref. [3].
1 An electrical conduit is not typically formally part of industry developed buried piping programmes; however, it is similar in construction to, subject to many of the same degradation mechanisms (on the outside diameter), and can be inspected and repaired with some of the same methods as buried piping, and so is included in this publication.
That is, Section 2 introduces the generic AMP as it relates to underground piping and tanks, Section 3 describes underground piping and tank material and Sections 4 through 8 discuss each of the process steps of an effective AMP:
— Understanding ageing;
— Developing and optimizing the AMP;
— Plant operation;
— Inspection, monitoring and assessment;
— Maintenance and repair.
Within each section are both process steps and information that is specific to underground piping and tanks and that supplements the generic information provided in Ref. [3].
Finally, Section 9 summarizes the conclusions of this publication and provides recommendations for plant personnel.
Appendices cover IAEA ageing management activities, some specific material properties, ageing management practices within selected Member States and non-nuclear industries, and some sample programme health reports dealing with buried and underground piping.
1.4. TERMINOLOGY
Common terminology for ageing management in a nuclear power plant context is typically derived from industry sources (see, e.g., Ref. [4]). In an IAEA context ageing management is defined in the IAEA Safety Glossary [5] and annex I of Ref. [3] points to other typical publications. Figure 1 shows the relationships among terms describing actual life events on an event timeline (based on figure 4 of Ref. [4]).
FIG. 1. Lifetime periods of structures, systems and components.
Reference [5] defines ageing management as:
— Engineering, operations and maintenance actions to control within acceptable limits the ageing degradation of structures, systems and components.
Examples of engineering actions include design, qualification and failure analysis. Examples of operations actions include surveillance, carrying out operating procedures within specified limits and performing environmental measurements.
Life management (or lifetime management) is the integration of ageing management with economic planning: (1) to optimize the operation, maintenance and service life of SSCs; (2) to maintain an acceptable level of performance and safety; and (3) to maximize the return on investment over the service life of the facility.
This publication focuses on the service life period of SSCs.