AND SUSTAINABLE DEVELOPMENT
J. Bouchard
Commissariat à l’énergie atomique, Gif-sur-Yvette, France
The aim of this presentation is to provide an insight into the challenges that lie ahead for the development of fast reactors.
From the moment when the first fast reactor — EBR1 — lit up the city of Arco right up to Superphenix, by far the largest fast reactor ever built, there have been 40 years of fast reactor development, mainly centred on sodium cooled systems, leading to the successful operation of such plants. Therefore, the question could arise about the need for more R&D and the relevance of new prototype designs.
There have been two major development steps in the history of fast reactors. During the 1960s and 1970s, their development was undertaken following concerns related to the energy supply, resulting mainly from the oil crisis, as well as from the need to use uranium resources more efficiently. In the 1980s, however, demand for nuclear energy declined after the Three Mile Island and Chernobyl accidents, as well as from the belief that fossil energy was plentiful and would remain cheap. It took about 20 years to realize that nuclear energy would expand, owing to the energy and climate challenges the world was faced with, and with that, the need for fast reactors became obvious in order to account for the constraints of such expansion.
Currently, however, the context has changed since the 1970s, and the development of fast reactors needs to be made on a new basis, taking into account new criteria linked to economy, safety, reliability, resource saving, waste minimi-zation and physical protection against terrorism or proliferation. Such huge technological challenges also require that the new fast reactor designs be developed internationally, within multinational cooperation frameworks. Such is the goal of the Generation IV International Forum (GIF), which is a gathering of the major key actors in the field of R&D, cooperating for the sustainable development of nuclear energy.
A new way of thinking has emerged from this new context: the awareness that a global solution is required, accounting not only for fast reactors and their associated fuel recycling, but also for full burning of actinides created in both
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light water reactors and fast reactors. This will provide a solution for resource scarcity as well as for waste and proliferation concerns.
Another important point is the economy. The European Fast Reactor project elaborated at the end of the 1990s is still the most advanced design for a large breeder. It incorporates feedback experience from the construction and operation of Superphenix, as well as various improvements aimed at cost reduction.
Subsequent studies showed that though economic competitiveness with third generation advanced light water reactors for electricity production could be attained with the EFR, such a result would require larger investments, making capital cost a major obstacle to the development of reactors based on such a design. Hence, improvements on the economy of fast reactors are today mainly focused on reduction of capital cost.
It is one of the reasons why discussions on the technological choices have been reopened. The first topic is the coolant, which is the major technology driving force for reactors, in particular for fast reactors that cannot use the simplest one, namely water. Sodium was unanimously chosen by scientists and engineers in the 1960s due to its numerous advantages (high conductivity, low viscosity, compatibility with steels, low cost). It has, however, shown its limitations (reactivity with air, opacity). Consequently, the debate has been reopened, especially within the GIF, on other possible coolant options. Moreover, in the event of a large expansion in the use of fast reactors resulting from a wide increase in nuclear energy demand worldwide, it is not mandatory, and even may not be reasonable, that all these fast reactors rely only on sodium technology.
Lead as a coolant is another option being studied by numerous laboratories, mainly in Europe, because of its ‘quiet’ behaviour with air and water. Gas is also considered to be an alternative to sodium in France and a longer term challenge as it could allow for high temperatures and thus possible use for industrial applica-tions other than electricity production.
Of all the different coolant technologies, not one can be singled out as the best one, because each one of them has its own advantages and drawbacks.
Though there is common agreement that sodium technology is already mastered, one should always keep in mind that alternative or backup solutions are required.
Two basic designs have been developed for sodium cooled fast reactors: (i) the pool (or integrated) and (ii) the loop concepts. The former one has been adopted for Phenix and Superphenix: operation of these plants was successful and allowed for extensive feedback experience for the fuel as well as for the technology, especially in the case of Phenix. Demonstrations were made, in particular in the field of actinide burning. Most difficulties encountered during the Phenix operation were overcome over a relatively short period of time and without too much difficulty, such as small sodium fires and problems with steam generators and with intermediate heat exchangers. Most difficulties encountered
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In addition to the lessons learned from Phenix and Superphenix, findings were also obtained from the satisfactory operation of the Russian BN600, which has been running for 30 years now. Other reactors are being built around the world based on the same integrated concept, namely, the CEFR in China, PFBR in India and BN800 in the Russian Federation.
Nevertheless, besides these projects, which will allow us to gain greater experience on the integrated concept, we still have to find ways to reduce the capital cost. Innovative solutions are thus being investigated, which could also improve the features of such reactors, especially their safety. In particular, new options are being looked into for the energy conversion system (gas or super-critical CO2).
The loop concept is mainly studied in Japan, as the work performed in Germany was interrupted some 20 years ago. The Monju prototype should restart soon and its comeback is awaited by the international fast reactor community as an important tool for future development. In parallel, works are ongoing on the JSFR project to explore all the possibilities offered by the loop concept.
When comparing both concepts, loop and pool, the same conclusion can be drawn as when comparing different coolants. Neither one of them can be considered as the best concept, each one offering certain specific benefits but each also having some drawbacks. Selection should not be made prematurely and investigations of several concepts should be carried out, in order to offer alternative solutions.
The same logic has prevailed since the 1960s between the boiling and pressurized water reactors, thus providing the utilities in particular with freedom of choice. In the loop and pool concepts, investigations should be continued, with a common goal of reducing capital costs, enhancing safety, etc. Some of the developments performed today are common to these different design choices, in particular those related to the fuel.
Various possible fuels are being investigated, namely metal, carbide, nitride and oxide. Most of the past experience has been accumulated on oxide, which is still the reference choice for existing reactors or those that are under construction.
Metal fuel was developed in the United States of America (EBR2). Each type of fuel has intrinsic advantages. Carbide and nitride enable the combination of high breeding gain and large margins with respect to melting (gain in performances or in safety), but they are more risky to handle (pyrophosphoricity). Feedback experience may lead to a preferential choice, most probably oxide for near term development, but various options should remain open. Progress has been made for concepts using metallic cladding for the fuel (cladding with no swelling),
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which seem to point to oxide dispersion strengthened steel as the best solution for the future.
On the safety front, numerous studies are being carried out on the basis of the new requirements mentioned earlier. For example, a strategy has been set up to deal with severe accident management. Provisions are being made for mitigating the core melting risk and, in the event of a core meltdown, for preventing high energy density accident sequences.
Most of the works mentioned so far have concerned sodium cooled fast reactors. As regards concerns gas cooled fast reactors, seen as an alternative to sodium cooled fast reactors, one goal sought is to obtain both the advantages of such reactors for the fuel cycle as well as the advantages of high temperature reactors for applications other than electricity production. This is an important challenge and all the work performed so far has mainly been limited to paper studies; hence the need to build the first experimental gas cooled fast reactor of limited power, around 50 MW(th), to test the technology solutions. A project to build such an experimental gas fast reactor is currently being examined in Europe, and a decision should be made by 2012. The choice of fuel for the gas cooled fast reactor is the major difference with liquid metal coolant systems, mainly due to temperatures but also because of the volume occupied by the gas, which requires a compact fuel with high resistance. Also in the case of the gas cooled fast reactor, safety studies will be of utmost importance, and extensive work is already being carried out, such as, for example, the analysis of gas cooled fast reactor fast depressurization accidents.
Another fast reactor studied is the lead cooled fast reactor. More basic developments are still required, as no final choice has been made as to which coolant would be the best one between pure lead and lead–bismuth alloy. The goal is to obtain a coolant without the same chemical risk posed by sodium.
However, lead poses other problems (corrosion, need for operation at higher temperatures).
Various prototype construction projects exist round the world. Among them, let us mention the French sodium cooled fast reactor prototype project, ASTRID. It is a fourth generation prototype scheduled to be operational by 2020.
Technological studies are being carried out and a decision to pursue this project will be made in 2012.
In conclusion, it is important to recall the present context, which requires international cooperation for R&D as well as for prototype/experimental reactor construction. Though there exist several national prototype construction projects, harmonization is of great importance in order to avoid duplication and seek complementarities. Pooling of efforts and sharing of R&D tools and construction capabilities will allow for optimization of means.
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73 In the safety and security (non-proliferation and physical protection) areas, it is important to establish international standards, owing to the fact that these matters are largely congruent among the international community. This will allow for the establishment of reference regulatory practices and regulations, as well as international consensus on common (or compatible) high level safety and security objectives.
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