Advanced Nuclear Power Jan2002

Advanced Nuclear Power - The Magazine of Framatome ANP

Worldwide, nuclear plant operators and utilities expect three primary qualities from the fuel that they load into their reactor cores: excellent fuel utilization, good operational flexibility and a high degree of fuel reliability. The outstanding performance of Framatome ANP's fuel assemblies for BWRs is proof of the great progress made in recent years in meeting all three of these expectations. This applies especially to our ATRIUM™ 10 product line.

Broad-Based Operating Experience
By August 2001, Framatome ANP had supplied BWR fuel assemblies to a total of 51 plants located in Europe, the US and Asia: 10 BWRs built by Siemens and 41 reactors supplied by other vendors. This BWR fuel operating experience encompasses over 46,000 fuel assemblies with fuel rod arrays ranging from 6x6 to 10x10.

The frequent switch to more advanced fuel assembly designs with different rod arrays - which always has been a typical feature of BWR plants - has resulted in earlier fuel designs being successively replaced, with product cycles averaging around six years. This continuous development focused on enhancing fuel economics through increased burnup potentials, large operating margins, and a high degree of operational flexibility. Today's optimum was achieved in the spring of 2001, when four ATRIUM 10 lead fuel assemblies reached their target assembly burnups of 71 MWd/kgU after eight operating cycles at a European plant - the current world record in assembly average burnup for light water reactor fuel.

So far, ATRIUM 10 fuel assemblies have been supplied to a total of 19 plants in Europe, Asia, and the US giving Framatome ANP operating experience with nearly 4,000 fuel assemblies of this type and, thanks to ongoing supply contracts, the experience will continue to be enhanced. This applies particularly to European BWRs that have been supplied almost exclusively with ATRIUM 10 fuel assemblies since 1998. In the US, all BWR reload fuel currently being delivered is of the ATRIUM 10 design.

Continuous Increase in Discharge Burnups
To cut down on fuel cycle costs, nuclear fuel development work focused on finding ways to reduce the volume of spent fuel requiring disposal. By optimizing the key factors - fuel enrichment and material reliability - the target burnups for today's fuel assemblies could be pushed as high as 65 MWd/kgU. In the US, the Nuclear Regulatory Commission (NRC) to date has restricted burnups to 62 MWd/kgU peak rod. Design modifications that accompanied the transition from 8x8 rod arrays to the ATRIUM 10 with its 10x10 array served, on the one hand, to provide the design margins necessary for extended burnup and, on the other, to increase fuel utilization.

The progress made is especially reflected by the figures for batch average discharge burnup. In the last 20 years, average annual discharge burnups have risen from less than 30 MWd/kgU to 45 MWd/kgU.

Further increases in burnup planned for fuel assemblies now being loaded into reactors are supported by the wealth of experience that Framatome ANP has accumulated from fuel operation in the high-burnup range.

Greater Operational Flexibility
Plants must have the flexibility to respond to the needs of the power market for nuclear power to remain economical. As far as the reactor core is concerned, this means that operating modes such as load following, frequency control and stretchout operation must be possible. Flexibility also means that plants must be capable of altering their in-core fuel management strategies on short notice. Framatome ANP's ATRIUM 10 fuel assemblies, with their large margins to operating limits, are ideal for meeting these requirements.

Even Higher Reliability
Despite the increasing demands being placed on nuclear fuel today, fuel assembly reliability nevertheless has continued to increase in recent years. Average annual fuel rod failure rates have been under 2x10^-5 since 1991, with the average for the period from 1993 to 2000 even dropping as low as 0.7x10^-5. (Note that these failure rates include failures from all causes, including debris.) According to the data available so far, 2001 can be expected to be yet another year of excellent fuel reliability.

Two major contributing factors in this area were the introduction of new cladding materials and improvements in fuel pellet quality. Thanks to these advances as well as the excellent capabilities of today's core monitoring systems, the probability of fuel rod failures being caused by pellet-clad interaction (PCI) was further reduced.

To prevent debris-induced fretting, Framatome ANP developed various kinds of debris filters. All ATRIUM 10 fuel assemblies, for example, are now equipped with a lower tie plate with an integral debris filter - either SmallHole or FUELGUARD - to ensure maximum reliability.

Further Performance Improvement With ATRIUM 10XP The enhanced properties of the next generation, ATRIUM 10XP, will further improve economic performance. While the greatly improved thermal-hydraulic stability provides more operating flexibility, a higher fuel weight helps optimize fuel cycle costs. Lead assemblies will be loaded in a European plant in 2002.