Fuel | Failures
Learning from failures6 October 2010
Over the last 30 years the number of fuel failures has fallen dramatically, and the common causes of these events have changed, according to a report from a European Union body set up to improve communication, regional collaboration and sharing of lessons learned on operational events. By Manuel Martín Ramos, M Nöel and Christiane Bruynooghe
The use of higher enrichment and higher burnup fuel, mixed cores with assemblies of different designs, more challenging core power management, and shorter outages has created the need to carefully consider the performance and safety of the fuel materials and designs under these conditions. A great effort has been put in to improve the design and manufacture of the fuel assemblies, which has generated better performance and higher operational margins, in spite of the more demanding operational conditions. The result of these counteracting trends is that fuel performance has improved significantly over the years, but there are still some concerns that need to be addressed, such as debris and grid-to-rod fretting, overpower, and corrosion.
The Institute for Energy has prepared a detailed report on fuel related events reported over the last 30 years to the Incident Reporting System (IRS) database which is jointly operated by the International Atomic Energy Agency and the Nuclear Energy Agency .
Roughly half of the events reported (mainly events related to changes in geometry, calculation errors, power instabilities, as well as some corrosion and overpower events) did not cause leakage or rupture of the fuel rods. The other half resulted in leaking or broken fuel rods. These were mainly events related to fretting, hydriding, and pellet-cladding interaction. About 3% of the reported events yielded fuel melting due to severe heating of the fuel rods.
The main root causes of most of these events were fuel assembly or component design deficiencies, or the inadequate design and performance of singular interventions like fuel cleaning or systems decontamination. Human errors are also of great importance as they are the predominant cause of overpower, oscillations and calculation errors, and a very important source of debris in the reactor coolant system that increases debris fretting-related fuel failures. (See Fig. 1 & Table 1).
Widespread operational strategies and operational experience feedback have been demonstrated to be very effective in reducing the frequency of in-core fuel related events. Some of the main phenomena important in the past are no longer a concern. Operational and surveillance procedures in foreign material exclusion, coolant chemistry control, and actions to enhance human performance need to be further enhanced. Systematic inspection of fuel assemblies and close monitoring of operation would allow better detection of potential failures in advance. Additionally, fuel design and manufacture shall continue improving to cope in particular with fretting and corrosion.
The project was carried out by the European Network on nuclear power plant operational experience feedback (EU Clearinghouse on OEF), which was established in 2008. One of the technical tasks of the European Clearinghouse is performing in-depth analysis of in-core events to learn from them [2-7].
Ten EU regulatory authorities are currently participating in the EU Clearinghouse on OEF for NPPs: Finland, Hungary, Lithuania, the Netherlands, Romania, Slovenia and Switzerland. (Six more countries are observers). Close cooperation has developed with IRSN of France and GRS of Germany.
First, the IRS database was queried to identify the fuel-related events. Results were screened to remove irrelevant entries. Second, the events were grouped by cause. During the process, a total of 169 reports were analysed, from which 85 event reports corresponded to about 103 in-core events. The other 64 were related to fuel handling or storage, and were not analysed here.
The events have been selected based on their importance from the point of view of operational experience exchange, rather than pretending completeness by reporting every single event. This limits the statistical significance of the sample used.
Despite that proviso, the exchange of operational experience has been shown to be a fundamental tool to enhance fuel safety.
Manuel Martín Ramos, M. Nöel and Christiane Bruynooghe, European Clearinghouse on OEF for NPPs, Institute for Energy - Joint Research Centre - European Commission, P.O. Box 2, 1755 ZG Petten, The Netherlands. References have been omitted for space but are available on www.neimagazine.com/fuelfailuresFilesTable 1