Fuel & fuel cycle
Fuel failure phaseout1 September 2008
EPRI has developed a series of guidelines to help eliminate fuel failures at nuclear power plants, with the aim of achieving INPO’s goal of zero fuel failures by 2010.
Fuel failures have been traced to several different causes (see Figure 1). The most common are corrosion and crud, mechanical fretting wear (foreign material such as a piece of wire vibrating against the fuel rod surface), and pellet cladding interaction (PCI – stress buildup on the cladding due to contact with the fuel pellets and interaction with the aggressive radioactive environment on the inside of the fuel rod).
The total number of fuel failures, for both BWR and PWR plants combined, 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 (see Figure 2).
In 2006, the Institute of Nuclear Power Operations (INPO) set an ambitious goal to achieve zero fuel failures by 2010. In response, US nuclear owners and operators backed a fuel integrity initiative that emphasised the development of fuel reliability guidelines. In the first instance, INPO led the development of guidance documents summarising current industry information to assist utilities in improving fuel integrity and performance.
The Electric Power Research Institute’s (EPRI’s) fuel reliability programme, headed by programme manager Kurt Edsinger, has since led a coordinated effort to develop technical guidelines in the following five areas:
• Fuel surveillance and inspection.
• PWR fuel cladding corrosion and crud.
• BWR fuel cladding corrosion and crud.
• Pellet cladding interaction (PCI).
• Grid-to-rod fretting (GTRF).
The guidelines capture state-of-the-art industry knowledge, providing specific guidance and good practices to help utilities avoid fuel failures associated with specific failure mechanisms.
More than 70 utility experts and 26 vendor experts have actively participated in developing the guideline documents, along with EPRI, INPO, the Nuclear Energy Institute and industry consultants. In addition, all the US nuclear utilities and five international utilities have been involved in a review process facilitated by EPRI working groups and the Zero by 2010 industry group. In total, about 200 people have reviewed the guidelines to ensure their accuracy and relevance to fuel reliability issues.
David Schrire, nuclear fuel design and materials manager for Vattenfall Nuclear, who was involved in the working group for the PCI guidelines, said: “We have obtained some ideas for good practices that can be incorporated into our own procedures; ideas we would have otherwise been unaware of.”
Guido Ledergerber, head of nuclear operation for Kernkraftwerk Leibstadt, explained that although the guidelines have been developed for US utilities they could also benefit European utilities. “The very structured way those guidelines have been approached has convinced me that this is also great value to us,” he said.
Each guideline document presents recommendations in three categories, consistent with industry practice. These are:
• 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.
Fuel surveillance and inspection
The EPRI guidelines should help plant operators: develop fuel surveillance and inspection programmes 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 document includes three mandatory recommendations to: establish a unit-specific surveillance and inspection programme for non-failed fuel; establish a programme to prevent the reinsertion of failed fuel; and perform causal analysis to establish apparent cause of failure. Needed recommendations are to: perform baseline ‘healthy fuel’ inspections (for PWRs visual, oxide, and grid-to-rod fretting measurements, and for BWRs visual and oxide measurements); evaluate the need for inspections following significant changes or events (eg changes in fuel design, water chemistry, core design and operational strategy); and enter inspection scope into the EPRI Fuel Reliability Database (FRED).
PWR cladding corrosion and crud
Fuel reliability recommendations for PWRs are derived from analysis of the four crud-induced corrosion failures in the USA since 1990. These guidelines contain 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 to include a crud-induced corrosion risk assessment as part of the core design process for each cycle. Five needed recommendations are:
• Assess effect of core and fuel design changes on critical factors controlling crud deposition, and take action to reduce crudding risk.
• Minimise locally high steaming rates on small fuel rod surface areas.
• Maintain reactor coolant pH=7.0 while at full power xenon-equilibrium conditions. Beginning-of-cycle pH should be as high as achievable within industry experience and vendor specified lithium restrictions.
• Analyse reactor coolant during shutdown and startup at a frequency allowing reasonable estimates of nickel, iron and cobalt-58 releases and removal.
• Optimise plant operating parameters that can affect sub-cooled nucleate boiling at all times during operating cycle.
BWR cladding corrosion and crud
The BWR guidelines, based on BWR fuel operational experience over the last 30 years, 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 minimise crud and cladding corrosion.
The BWR guidelines include one mandatory recommendation to include a crud and cladding corrosion risk assessment for each cycle. Needed recommendations address cladding materials, chemistry parameters, and fuel duty, as well as fuel fabrication quality assurance and fuel handling. These are to:
• Provide fuel vendor with anticipated fuel operating and environmental conditions for the reload.
• Review vendor’s fuel fabrication quality assurance programme and planned quality control checks.
• Implement fuel handling procedures that provide for protection from mechanical damage and surface contamination until stored under water.
• Review vendor-proposed changes in cladding alloy chemistry or material processing specifications.
• Ensure that new zirconium alloys will meet the corrosion, hydriding and mechanical property requirements of fuel designed for high exposure applications.
• Maintain feedwater oxygen within BWR chemistry guideline limits to minimise flow assisted corrosion of carbon and low alloy steels.
• Assess risk of adverse fuel impacts before increasing quarterly average feedwater zinc concentration >0.5ppb or the cycle average feedwater zinc concentration >0.4ppb.
The grid-to-rod fretting guidelines, released in late July, address the failure mechanism responsible for more than 70% of all fuel failures. The mandatory recommendations are: perform an initial assessment of the fuel in-core margin to GTRF failure; during each cycle, evaluate the impact of changes in fuel design and operating conditions on GTRF resistance; and utilities that have experienced a GTRF failure with their current fuel design shall develop a GTRF action plan to determine activities necessary to eliminate GTRF failures.
There is one necessary recommendation that utilities with unknown margin to GTRF failure shall perform poolside examinations to quantify the available margin.
The PCI guidelines, due to be published later this year, 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 do not exceed a threshold that could lead to fuel failures.
Achieving zero fuel defects by 2010 demands a concerted effort by utilities, fuel suppliers, and other industry organisations. US nuclear power plants will have about six months to incorporate the EPRI guidance into their fuel reliability programmes after guideline release. However, actual implementation of the mandatory, needed and good practices, particularly those associated with fuel design changes, will take longer.