Seeking the next-generation fuel

Competitiveness has become the major challenge to the nuclear industry at the end of the century. To respond to this challenge Framatome/Fragema and Framatome Cogema Fuels (FCF) have developed a new nuclear fuel assembly, the Alliance fuel assembly, to improve the utilisation of existing PWRs and to be used in the future European Pressurised Reactor (EPR).

The Alliance fuel assembly objectives are based on an in-depth analysis of the market needs of customers of Framatome and Fragema (particularly in Europe, China, South Africa), and of FCF (in North America). The analysis was the first stage in serving utilities’ future needs in terms of improvement of performance, economy, safety and reliability.

Although not expressed in the same order of priority at any given time, or with the same timing, it appears that utilities’ requirements for the coming years will be similar, and one fuel assembly design can be developed as an envelope for all cases. This trend towards standardisation is not only beneficial to the utility in terms of fuel assembly cost. It also allows experience feedback from a given product to be extended, and the knowledge of the product performance in the widest possible variety of operating conditions to be assessed.

Alliance is designed to reach fuel assembly burn-ups of at least 70 GWd/t and to provide high thermal-hydraulic and mechanical performance up to this level of irradiation. In developing the Alliance design, it was decided from an early stage to ensure there was no fuel assembly distortion up to high burn-up levels. In fact, high levels of performance allow the operator to implement more economical fuel management strategies and to operate with higher power peaking factors. Alternatively, the operator can accommodate a plant power increase, while keeping substantial operating margins.

Among the other aims of the Alliance design are to enhance reliability and geometrical stability under irradiation, make handling easy, and take into account environmental issues. It is a “clean” fuel assembly, decreasing in-reactor radiological doses and responding to a “no loose parts during fuel assembly repair” concept.

Finally, among the Alliance design targets is improved safety. This means anticipating the evolution of safety requirements regarding irradiated component mechanical properties. It also means meeting and exceeding the expectations of utilities participating in the Robust Fuel Programme managed by the US Electric Power Research Institute.

DEVELOPMENT

To develop the fuel, a joint team was set up with engineers from Framatome and FCF. The team members were already experienced in fuel assembly development projects, as well as in working in multidisciplinary teams. Their complementary cultures and the merging of their various experience, contributed greatly to the success of the project. The team was based in Lyon and it started work in May 1996.

Framatome has had a long term commitment to R&D, and its programme has in the past provided significant developments such as new alloys, new components and new fuel assembly concepts. The R&D programme is supported by the expertise of the Commisariat à l’Energie Atomique and its facilities, such as test loops, hot cells and research reactors.

Development has been roughly divided into two parallel programmes:

• Development of a zirconium-based alloy that replaces Zircaloy 4, known as the M5 advanced alloy.

• In-depth review and functional analysis of the whole fuel assembly design and of each of the fuel assembly components, in order to redesign and optimise nozzles, grids, connections, guide tubes, fuel rods, and to improve the fuel assembly concept.

As for all new product development, Framatome goes through very extensive testing, both out-of-pile and in research reactors. As soon as it is practical, the new products or concepts are irradiated in commercial reactors. The irradiation conditions are carefully chosen so they encompass the whole spectrum of existing operating conditions for PWRs.

Alliance also has the advantage of the extended operational experience feedback available through the 57 000 irradiated fuel assemblies that the partners have supplied to 97 reactors worldwide, which cover a large range of operational conditions – some very demanding. Framatome’s and Fragema’s major customers have been directly involved in developing this assembly, mainly through their contribution to the irradiation campaigns. As a result, the partners claim that Alliance is a fuel assembly with totally demonstrated performance.

DESIGN FEATURES

To meet high burn-up requirements and long term customer performance needs, the Alliance design is based on a new structure concept with a new structural grid design, and is a full-M5 fuel assembly. Some features already implemented in the AFA 3G design have been maintained so the Alliance design can gain the benefit of their performance improvements.

To validate the design and quantify its enhanced performance, extensive out-of-pile tests have been performed, specifically:

• Critical heat flux tests, to validate the thermohydraulic performance gain.

• Hydraulic tests on full scale mockups to confirm the component and assembly pressure drop and the fuel assembly behaviour under flow conditions, and especially to confirm the assembly’s good response to flow induced vibrations.

• Mechanical tests to verify the mechanical resistance of the components and assembly under various loadings.

The results of all these tests have confirmed the assembly’s anticipated performance and the behaviour of the new concepts that have been incorporated.

The cladding and assembly structure are both made of M5 alloy. This advanced zirconium–niobium–oxygen alloy, developed by Framatome and its industrial partners in French industry, shows significant improvements, when compared to the optimised low-tin Zircaloy 4. Specifically, oxidation is improved by a factor of 3-4, hydriding by a factor of 5-6, and creep and free growth by a factor of 2-3.

These benefits have been proven in a very large number of reactors with a wide range of operating conditions (reactor temperatures, linear power, heat flux, burn-up, fuel management schemes and fuel assembly design). The post irradiation data presently reach 62 GWd/t and more data will be available soon at higher burn-up.

The M5 advanced alloy gives extra margin on every design criterion: fuel rods and guide tubes, as well as grids, take advantage of its properties. In particular, the excellent creep and growth properties make it possible to optimise the fuel rod plenum length to accommodate the fission gas releases attendant on very high burn up.

FUEL ASSEMBLY STRUCTURE

Designing a fuel assembly for very high burn-ups is not just a matter of adapting an existing product by changing a few of its features. It is also more than putting together components that were individually optimised. It also requires an in-depth review of overall assembly behaviour, and many analyses of how those components will function together.

The totally new structure of the Alliance is designed to counter fuel assembly growth under irradiation and to allow perfect behaviour of the fuel assembly towards control rod insertion, even at burn-ups up to at least 70 GWd/t.

This has been obtained through the following design solutions:

• In the assembly M5 is used for all structural components, to take advantage of the excellent hydriding, creep and growth behaviour under irradiation.

• The fuel rods are seated on the bottom nozzle to enhance the lateral stiffness in the lower part of the assembly.

• The assembly has taller monometallic structural grids, which enhance thermal-hydraulic performance and contribute to the assembly’s lateral stiffness. Mid span mixing grids (MSMGs) are also proposed as an option, when they are needed for specific reactor operating conditions.

• Fuel rod growth and assembly growth are decoupled, minimising global fuel assembly growth, thanks to easier fuel rod slippage through the grid cells.

• The new guide-thimble to nozzle connection has a “quick disconnect” feature.

The Alliance structure also incorporates features that had already been implemented in the AFA 3G design. It has enlarged and thickened Monobloc guide-thimbles, with reinforced dashpots, and it has a Trapper bottom nozzle, which is effective in retaining loose parts and debris.

The Alliance structural grid, made of M5 alloy, is one of the main new features of the fuel assembly. It claims significant improvements, when compared to current designs:

• It has increased thermal-hydraulic performance, thanks to the use of large vanes. As the gain in DNB (departure from nucleate boiling) is more than 10%, an Alliance fuel assembly without mid span mixing grids has a level of performance equivalent to that of current designs with MSMGs.

• The assembly stiffness has been enhanced.

• The pressure drop has been optimised. Thanks to a specific spring design the drop is identical to that on current grids, even though the Alliance grid is taller.

Other features of the structural grid are:

• It has improved behaviour during handling operations. Welds connect the outside guide vanes with the inner straps, and this reinforces grid integrity during grid slippage along other grids.

• Dose reduction: because the springs are directly stamped into the M5 straps, there is no longer any Inconel in the structural grid.

The Alliance assembly can be equipped with MSMGs, located at the three highest heated spans. These grids make it possible to improve the thermal-hydraulic performance – although this is not very important in many cases, given the superior performance of the Alliance structural grid. This option can also be implemented in some reactors for compatibility with resident third party fuel assemblies that are already fitted with MSMGs.

The top nozzle is provided with a new Y4 turnquick disconnect connection. This type of connection halves the time required to assemble and disassemble the top nozzle, during both manufacturing and on-site repair work. In the design of this new connection, the connecting part remains captive inside the top nozzle. This has two other important advantages during on-site fuel assembly repair: on the one hand, there are no screws to be replaced and there are no irradiated screws to be disposed of as waste; and on the other hand, the risk of dropping the connecting screws is completely eliminated.

Alliance is fitted with Monobloc guide thimbles, made of M5 alloy, and developed by Framatome and Zircotube. These guide thimbles are larger and thicker than current thimbles, and the outside diameter is constant all along the tube. This is the best way to eliminate incomplete RCCA insertion events, as it stiffens the dashpot region of the fuel assembly. Contrary to other solutions in the industry, the Monobloc guide thimble is made from a single piece, and the dashpot itself has no welds nor insert sleeves.

The good performance of M5 in terms of corrosion, hydriding and irradiation-induced growth, associated with the Monobloc geometry – especially in the lower part – provides excellent resistance against deformation at high burn-up.

The Alliance is equipped with the Trapper bottom nozzle. This component consists of a ribbed supporting structure made of AISI 304, surmounted by a perforated plate made of AISI

660. It is designed to eradicate all fuel rod failures induced by debris, and its debris trapping effectiveness has been tested in flow loops. These tests have confirmed the excellent trapping effectiveness of this bottom nozzle, and also demonstrated that this design stops debris from migrating to the periphery of the nozzle, where it might escape and pass between fuel assemblies.

The Trapper also exhibits a reduced height and a lower pressure drop coefficient, when compared to previous designs, thus upgrading its thermal-hydraulic performance as well as its in-operation reliability.

In the same way, the top nozzle is optimised in height and pressure drop.

FUTURE PERSPECTIVES

The first Alliance fuel assemblies were loaded in an EDF reactor in mid 1999. There they have a target burn-up beyond 65 GWd/tU. They will reach this point in four 18-month cycles. In 2000, Alliance assemblies will be loaded in a US reactor and full batches of Alliance fuel assemblies will be available from 2003.

The partners believe that performance of the Alliance fuel assembly allows utilities to consider completely new fuel management strategies, which were not technically accessible until now. It provides new options when considering the best fuel management for a given reactor, as the operator can evaluate longer or very short cycles, use very small core fractions (so that he can employ fewer fuel assemblies per reload and a better distribution of the burn-ups among the population of fuel assemblies), or reduce reactor vessel fluence, especially for older reactors. All these elements contribute to the competitiveness of each single plant and, ultimately, of the global nuclear industry.