�

The Path to Zero Defects: EPRI Fuel Reliability Guidelines

Fuel reliability is critical to the safe and economic operation of nuclear power plants. High reliability depends on the structural integrity of the plant’s nuclear fuel rods, the metal-clad columns of uranium fuel pellets that power the nuclear reactor. Fuel failures, or a breach in the cladding, can lead to radioactive material leakage, lost plant generation, increased inspection and repair activities, premature removal of fuel assemblies, and increased radiation exposure. Recent U.S. fuel failures have cost individual utilities as much as $40 to $80 million. Moreover, fuel failures negatively influence public perception of nuclear power.

The nuclear industry has devoted considerable effort to understanding and eliminating the causes of fuel failures, and has adopted long-term strategies focused on operation with a defect-free core. Thanks to these efforts, the number of fuel failures per plant is significantly lower today than in past decades. However, while the industry has moved in the right direction, the number of fuel failures since 1990 has not markedly decreased.

Executive Summary

Developed in collaboration with utilities, industry organizations, and fuel vendors, a series of new EPRI reports capture state-of-the-art knowledge and describe best practices for eliminating fuel failures at nuclear power plants.

Fuel failures are trending down due to concerted industry efforts, but are not yet to zero.

�

EPRI Executive Summary: EPRI Fuel Reliability Guidelines

In 2006, the Institute of Nuclear Power Operations (INPO) set an ambitious goal to achieve zero fuel failures by 2010. In response, U.S. nuclear owners and operators backed a Fuel Integrity Initiative that emphasized the development of fuel reliability guidelines. Interestingly, while industry guidelines have been successfully used in a number of nuclear power areas, they have not been used substantially for fuel. The absence of guidelines can primarily be traced to the challenge of balancing industry best practices against the flexibility needed for effective fuel design and operation.

As a first step in moving toward zero fuel failures, INPO led the development of guidance documents summarizing current industry information to assist utilities in improving fuel integrity and performance.

Subsequently, EPRI’s Fuel Reliability Program led a coordinated effort to develop technical guidelines in five areas:

Fuel Surveillance and Inspection

Pressurized Water Reactor Fuel Cladding Corrosion and Crud

Boiling Water Reactor Fuel Cladding Corrosion and Crud

Pellet Cladding Interaction

Grid-to-Rod Fretting

The first three technical guidelines documents are now available; the last two will be finalized by August 2008. The documents capture industry state-of-the-art knowledge, providing specific guidance and good practices to help utilities avoid fuel failures associated with specific failure mechanisms. Although developed in response to a U.S. initiative, the guidelines are broadly applicable to multiple reactor designs around the world.

More than 70 utility experts and 26 vendor experts actively participated in developing the five guideline documents, along with EPRI, INPO, the Nuclear Energy Institute and industry consultants. All U.S. nuclear utilities and five international nuclear utilities have been involved in the review process via EPRI working groups and the Zero-by-2010 industry group. In all, about 200 people have had the opportunity to review these guidelines to ensure their accuracy and relevance to the fuel reliability issues at hand.

Tiered Recommendations

Each guideline document presents recommendations in three categories, consistent with industry practice formalized in the Nuclear Energy Institute’s Management of Materials Issues initiative (NEI 03-08):

Mandatory: Implemented at all plants where applicable.

Needed: Implemented wherever possible, but alternative approaches are acceptable

Good Practice: Expected to provide significant operational and reliability benefits, but implementation is left to the discretion of the utility.

The number of recommendations in the guidelines is substantive, but not onerous. Across the three completed guidelines documents, for example, EPRI has identified five mandatory recommendations, 15 needed recommendations, and numerous good-practice recommendations.

•

•

•

•

•

•

•

•

�

EPRI Executive Summary: EPRI Fuel Reliability Guidelines

Fuel Surveillance and Inspection – The ultimate goal of a successful fuel surveillance and inspection program is to ensure acceptable fuel performance to prevent fuel failures. These guidelines will help plant operators develop fuel surveillance and inspection programs that will identify margins in key fuel performance characteristics for currently operating fuel designs; assess margins in key fuel performance characteristics following changes in fuel design, manufacture and operation; and provide guidance on failed fuel action planning.

The Fuel Surveillance and Inspection guideline includes three mandatory recommendations:

Each utility shall establish a unit-specific surveillance and inspection program for non-failed fuel.

Each utility shall establish a program to prevent the reinsertion of failed fuel.

Each utility shall perform causal analysis to establish apparent cause of failure.

Needed recommendations include:

Perform baseline (“healthy fuel”) inspections, including visual and oxide measurements for both PWRs and BWRs, and grid-to-rod fretting wear measurements for PWRs.

Evaluate the need for inspections following significant changes or events such as changes in fuel design, water chemistry, core design and operational strategy, or other changes or anomalous events.

Enter baseline inspection scope into the EPRI Fuel Reliability Database.

PWR Fuel Cladding Corrosion and Crud – Fuel reliability recommendations for pressurized water reactors are derived from analysis of the four crud-induced corrosion failures in the United States since 1990. These guidelines capture state-of-the-art information relating to the impact that various changes in core design, assembly mechanical design and chemistry can have on corrosion product deposition on the fuel.

The PWR guidelines include one mandatory recommendation:

Utilities shall include a crud-induced corrosion risk assessment as part of the core design process for each cycle.

Five needed recommendations address core and fuel design changes, chemistry and plant operations, and startup and shutdown monitoring.

BWR Fuel Cladding Corrosion and Crud – Fuel reliability recommendations for boiling water reactors are based on BWR fuel operational experience over the last 30-plus years. The guidelines define approaches that utilities can take to ensure that cladding materials provided by the fuel suppliers meet quality requirements with respect to corrosion resistance; and provide recommendations on controlling water chemistry impurities and additives to minimize crud and cladding corrosion.

•

•

•

•

•

•

•

Heavy crud loading on fuel assemblies can lead to failures in pressurized water reactors.Heavy crud loading on fuel assemblies can lead to failures in pressurized water reactors.

“Fuel is the only component in our primary system that doesn’t come with an inspection and maintenance plan. ‘Run to failure’ is not an acceptable strategy.”

—EPRI Fuel Reliability Program Executive Committee

�

EPRI Executive Summary: EPRI Fuel Reliability Guidelines

The BWR guidelines include one mandatory recommendation, similar to that in the PWR guidelines:

Needed recommendations address cladding materials, chemistry parameters, and fuel duty, as well as fuel fabrication quality assurance and fuel handling.

Two More Guidelines – The two guidelines documents to be released this summer will take a more detailed look at two of the leading causes of fuel failures. The pellet cladding interaction (PCI) guidelines will help utilities assess their margins to PCI relative to current fuel vendor recommendations and plant-specific operating conditions. The recommendations are supported by operational

experience and EPRI and vendor fuel performance codes to ensure that stresses don’t exceed a threshold that could lead to fuel failures. The grid-to-rod fretting guidelines address the failure mechanism responsible for more than 70% of all fuel failures. The recommendations cover aspects related to fuel design, core design, and plant-specific flow conditions.

In addition to the five guideline documents described above, EPRI has developed a DVD on foreign material exclusion. The video presents examples and consequences of inadequate foreign material exclusion practices, as well as techniques and good practices for preventing foreign material from entering plant systems. Utilities can incorporate the 20-minute video into plant training materials useful to all plant workers, including management, plant staff, contractors and supplemental workers.

The Looming Deadline: Guideline Implementation

Achieving zero fuel defects by 2010 demands a concerted effort by utilities, fuel suppliers, and other industry organizations. U.S. nuclear power plants will have about six months to incorporate the EPRI guidance into their fuel reliability programs after guideline release. However, actual implementation of the mandatory, needed and good practices, particularly those associated with fuel design changes, will take longer.

EPRI stands ready to work with member utilities and industry partners to implement these guidelines and achieve the challenging goal of zero fuel failures by 2010.

©2008 Electric Power Research Institute (EPRI), Inc. All rights reserved. Electric Power Research Institute, EPRI, and TOGETHER…SHAPING THE FUTURE OF ELECTRICITY

are registered service marks of the Electric Power Research Institute.

Electric Power Research Institute3420 Hillview Avenue, Palo Alto, CA 94304-1338 • PO Box 10412, Palo Alto, CA 94303-0813 • USA

800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com

A corrosion failure in a boiling water reactor.

Utilities shall include a crud and cladding corrosion risk assessment for each cycle.

•