Fast reactor focus: France
Learning from Phénix8 January 2010
After 35 years of operation, the French fast reactor Phénix shut down in October 2009. But it still has a role to play in shaping the design of future sodium fast reactors. EDF has outlined its requirements for a commercial SFR, and areas where further R&D is needed. By Caroline Peachey
With the shutdown of the Phénix reactor at the Marcoule nuclear site near Bagnols-sur-Cèze, EDF has moved beyond experimental fast reactors. “As a utility representative I would like to stress that the real decisions [on Generation IV reactors] will be made largely on economical grounds. The message is: don’t give top priority to sustainability or proliferation resistance and forget the economics,” said Jean François Sauvage of EDF, speaking at Paris’s Global 2009 conference in September. Sauvage is head of EDF’s future sodium reactor project, and a former director of Phénix.
Sauvage explained that Generation IV plants have to be economically competitive, not only with non-nuclear energy sources but also with Generation III light water reactors in terms of complete generation cost, including construction, operation & maintenance, fuel & fuel cycle, reprocessing, decontamination, decommissioning and repository costs.
“Beyond a prototype a utility cannot, and will not, take the risk of nuclear power plants that could suffer long shutdowns or premature closure due to technological choices and/or safety issues,” he said.
Under French legislation, a prototype sodium-cooled fast reactor, Astrid, is scheduled to come online in 2020. The final design features of the reactor are to be decided by 2012.
Sauvage’s message echoes one from a conference hosted by the British Nuclear Energy society in March 1974, the same year Phénix achieved full power. According to the May 1974 issue of NEI: “The London conference on fast reactor power stations concluded with lively discussion on just when – or if – the breeders would be sufficiently competitive with evolving designs of thermal reactor power stations to induce hard headed utility executives to embark on large breeder programmes.” We are still debating this same point today, and interested in the views of these ‘hard headed’ utility executives.
In a technical session at Global 2009 on 9 September over 100 people crammed into a small room to hear a paper on ‘Fast Breeder Reactor Development: EDF’s Point of View’, presented by Noel Camarcat and co-authored by Sauvage and three other members of EDF’s research and development department.
Camarcat begun by explaining how EDF plays the role of a utility, not a designer, nor a vendor, nor a government R&D centre, which means it cannot commit the same amount of human resources to R&D. Nevertheless the utility is spending around 5% of its R&D budget on the long-term SFR programme and has around 30 engineers working on the project.
EDF contributes about EUR4 million a year to the trilateral French SFR R&D effort with Areva and the French Atomic Energy Commission (CEA).
SFR research and development is currently carried out in five areas: core physics, safety, technology innovations, materials, deployment and fuel cycle scenarios. Camercat outlined EDF’s activities and the main results and aims of the SFR R&D programme.
• Core physics. Neutronics studies have been pursued with CEA and Areva to improve core designs. Camercat outlined the areas for improvement. These include:
-Reduction of the sodium void effect. The void coefficient in fast neutron reactors is positive, which means that reactivity increases if loss of sodium coolant occurs by boiling or gas intrusion.
-Improvement of the prompt temperature or Doppler effect. When the temperature rises, the neutron capture rate of uranium-238 increases, which proportionally decreases the core reactivity. This is an inherent core safety characteristic.
-Minimization of the loss of reactivity during cycle.
• Safety. EDF is working to improve the core designs for better behaviour during transients. It has carried out detailed safety methodology research, particularly for severe accidents, and has also worked on the preparation of methodology discussions with the regulator.
• Technological innovations. The Septen engineering division has proposed ways to mostly reduce costs and protect investment. One of them concerns an integrated heat exchanger/steam generator for loop-type reactors, which allows the conversion of energy from hot sodium heated in the core into steam using only one component, eliminating the need for secondary sodium circuits. (By comparison, previous reactors such as Phénix use two components with an intermediate sodium circuit for the same function.) This innovation is expected to result in lower capital cost with comparable heat exchange performances. However, since EDF does not have in-house technological construction or demonstration capabilities, development of this concept is being pursued by the other members of the trilateral SFR R&D programme.
• Materials. EDF has had modest involvement in the materials R&D programme and much of its work has formed part of collaborative European and French R&D projects. EDF’s research has focused on new materials for the core and the fuel assemblies, steel and alloy corrosion, the study of the chromium steels P91/P92, which could be put to use in the secondary circuits of a FBR.
• FBR deployment scenario studies in France. The deployment studies have been performed in the frame of the French energy policy. A two-stage deployment scenario for FBRs has been simulated with core physics and fuel cycle codes. In this scenario, the total electric power remains constant (60GWe) over the 21st century, and PWRs decommissioned between 2040 and 2080 are replaced with FBRs of equivalent capacity.
EDF is in the process of fully defining its requirements for a commercial sodium fast reactor and the level of verification that will be brought by the French prototype. Many of these requirements depend on the role of the prototype. Will it be a research reactor for concept validations and/or transmutation physics using innovative fuels? Or will it be a demonstration plant with industrial capability on a smaller scale, derived from utility requirements?
This is a decision for the French government, to be taken in 2012. However if the prototype is going to be an industrial demo, EDF will see it as the predecessor to a first of a kind (FOAK) plant. EDF will therefore wish to check whether it fulfils more detailed specifications in terms of safety, but more importantly in nuclear island layout, operations, in-service inspection and repair and maintenance.
Camercat outlined some of EDF’s main requirements for an industrial prototype.
According to EDF’s requirements the safety level of industrial FBRs should be equivalent to the level of Gen III LWRs.
The safety approach of the prototype should be as similar as possible to the safety approach of the commercial plant, Camercat said. What EDF does not want to see, for example, is a smaller core used in the prototype to increase the safety parameters. This would imply a major change in the safety analysis at the next phase of the programme.
Therefore the sodium void effect of core of the prototype must be of the same magnitude as the one of the commercial reactor, Camercat explained. For a core with height to diameter ratio of 0.25, a small core reactor (~2.6m3) will have a void coefficient of the order of 500 pcm (1 pcm is a thousandth of a percent) and that a large core reactor (15.5m3) will have a void coefficient of the order of 1300 pcm. In the case of an Unprotected Loss of Fluid Accident (ULOF), the small core can be stabilized by the prompt temperature coefficient due to the Doppler effect. But the larger core will not be, inducing early in the transient a power increase and a significant energy release. Therefore, to keep the same behaviour when going from prototype to commercial reactor, a larger core volume (at least 5.2m3) must be retained at the prototype scale to be able to extrapolate the safety analysis to the commercial scale.
In addition, the safety analysis for eventual intermediate sodium circuits will have to be redefined in agreement with the safety authorities for the practical elimination of large sodium fires and sodium-water reactions which could breach the containment of radioactive products.
Camercat said that the methodology for analysing the generalized water-sodium accident is still open during the initial R&D phase. It can either be included in the Design Basis Events (DBE) leading to a containment structure capable of withstanding a significant energy release or it can be included in the Beyond Design Basis Events (BDBE). If this were the case, prevention measures should be robust and mitigation measures should be implemented to limit the mechanical energy release and recriticality occurrence, in accordance with the defence in depth principle.
The prototype should be designed to demonstrate the capability to inspect the major supporting structure inside the reactor vessel.
EDF has carried out extensive technical reviews to collect, analyse and discuss operation feedback from Phénix. Although many of the recommendations resulting from this analysis are specific to pool-type reactors, in which the primary heat exchangers sit in the reactor, still EDF recommends:
• Complete core discharge should be possible for periodic or requested inspection of the bottom internal structures and in cases of an hypothetical sodium leak.
• The rotating plug of the reactor vessel should be designed to allow periodic inspection and replacement in case of fault or at end of life, should it be less than the facility lifetime.
• Decay heat removal should be made by simultaneous use of cooling circuits in the reactor pit and of circuits in contact with the hot pool in the primary sodium circuit. Natural and forced convection should be employed. Cooling fluids can be sodium, water, air, or others.
• A sodium intermediate storage should be built outside the reactor vessel for the discharged fuel assemblies.
Significant improvement will be needed in operational performances of future Gen IV FBRs compared with the first generation reactors that were built in the 1980s, Camarcat said. Availability must be “dramatically improved”, especially in comparison with the Creys-Malville, France Super Phénix, which was operated by EDF from 1986-1997. Availability factors of at least 80% will be needed for industrial plants. “Obviously the 80% availability factor is not expected at start up; there will be an initial learning curve,” Camarcat said. The 1,242MW Super Phénix reactor had a difficult life, being targeted by a fierce antinuclear campaign and closed down probably for political and economic reasons in 1997.
Energy conversion system
EDF has proposed an integrated heat exchanger/steam generator. Camercat explained although such an innovation cannot be brought at the industrial scale before 2030, R&D can be pursued on sodium and other fluid loops without requiring the nuclear heat source of the industrial prototype.
Important innovations can still be brought to previous generation secondary sodium circuits and steam generator designs, at a time compatible with the schedule of the prototype, he said. In conclusion Camercat noted that further work needs to be done to deal with the issues surrounding sodium technology, industrial implementation and economics. But there is still time: EDF is looking at main deployment criteria in the timeframe beyond 2050.
Based on presentations by EDF at Global 2009 held in Paris in September. The conference was organised by SFEN.Related ArticlesPhénix 1974-2009 France, Japan and the USA to cooperate on fast reactor demos Phenix rises Phenix rises for 250 million Euros