Why To Use Sodium Fast Reactors?
The first nuclear reactor that produced electricity, in 1951, was a liquid-metal-cooled fast reactor. Since then, gradually, for industrial and economic reasons, pressurized water reactors (PWRs) have become the dominant reactors worldwide. Yet fast reactors have huge potential interests in providing carbon-free and virtually inexhaustible energy for the needs of humanity in the future:
Like current water reactors, they provide energy without producing CO2 or other potentially harmful pollutants. Besides, their chemical or gaseous releases are almost zero and their dosimetry is even lower than those of water reactors. Occupational exposures and environmental discharges during their operation are significantly less compared to any other reactor types.
Fast reactors operate with a fuel made of a mixture of depleted uranium, a material recovered from uranium enrichment process and plutonium extracted from the PWR spent fuels.
In 2020, more than 300,000 tons of depleted uranium were stored in France. These reactors, therefore, do not require uranium-enrichment facilities or uranium-mining activities.
Together with the closed fuel cycle, fast reactors allow the extraction of the energy from natural uranium to the greatest extent possible. With the quantities of depleted uranium and plutonium currently available, France would have thousands of years of carbon-free electricity production ahead of it.
In terms of nuclear waste, fast reactors are potentially capable of transmuting minor actinides and, consequently, allowing the production of final wastes with less radiotoxicity and decay heat in it.
In conclusion, we potentially have an almost inexhaustible low-carbon energy source, without the need for uranium mines, or uranium enrichment as the reactor can operate on already available and stored nuclear produces.
Sodium Fast Reactors in Europe
Based on the aforementioned enticing promises, France, Germany, and the United Kingdom embarked on the construction of SFRs in the 1970s: the “Phénix” SFR in France, the prototype fast reactor (PFR) in the United Kingdom, and the “KNK” SFR in Germany.
Jointly with Italy, these countries committed in the construction of the Superphénix reactor (1200 MWel). This was the European reactor with strong participation from Italy (33%) and a Germany/Netherlands consortium (around 16%). As it was subject to strong opposition from environmentalist groups and a lack of political support, the reactor was shut down in 1998 for purely political reasons, after a year of successful steady operation. Furthermore, Superphénix spent more than a third of its ten years of operation, able to operate, but shutdown only waiting for administrative authorizations (Fig. 1).
The gradual withdrawal of nuclear power in Germany and to a lesser extend in England led to the termination of fast-reactor development programs in these countries in the late 90's. Only France, backed by the success of its reprocessing activities, continued along this path with the Phénix reactor (250 MWel) kept in operation until 2009 and continuous research and development programs.
European Prospects of SFRs
Given the potential advantages of these reactors, the question naturally arises, why we do not see this type of reactor emerging in Europe today, with all the experience accumulated during the past years.
There are several reasons for this:
There are some technical difficulties specific to the SFR. Several experimental reactors (such as prototype fast reactor) had such technical difficulties, which led to their premature shutdown.
There are additional costs. By its design, an SFR is more expensive than a PWR, which is more compact and without secondary circuits. The additional costs are estimated to be in the range +30-50%. On top of this, additional costs have to be added, inherent in the prototype reactors with which the knowledge is accumulated for mass production.
In the USA, the non-proliferation issue led to the abolition of fuel reprocessing.
Only countries with reprocessing plants remained interested in this technology. In 2020, outside France, we find, mainly, Russia and Japan with reprocessing plants in operation or close to commissioning, which is crucial to close the fuel cycle of the reactor.
Uranium prospecting, from the 1960s onwards, led to significant discoveries of many high-rate mines. This drove the price of uranium to a historical low point. Further prospecting has almost stopped and some mines were even closed. Correspondingly, this motivation for fast reactors has temporarily disappeared.
It exists, at least in France, a strong opposition of environmentalists. This type of reactor, claiming to be able to operate without uranium mines and to produce our energy for thousands of years, based on the available nuclear wastes, has been a real red flag for environmentalists seeking to exit from nuclear power.
Lack of political support: under these conditions, political support collapses and, in particular, Superphénix was shut down after elections, as a pledge of a coalition of power with environmentalists.
The difficulty of defending long-term investments in a competitive economy, which rather demands short-term returns on investment.
In conclusion, the fast-reactor technology has strategic and ecological interests, but its development and deployement is delayed since it requires significant developments and long-term investments, hardly compatible with economic models in Western Europe. Under these conditions, Europe needs to maintain its skills, with projects like European SFR-safety measures assessment and research tools (ESFR-SMART), and overview on what could be an SFR in the future. This view is supported by the fact that the pressure to solve climate change fosters the reintroduction of efforts to implement new nuclear technology into the energy generation mix, thus the political interest can potentially change in the future.
Status of SFR in the World
During this time, several countries as Russia and China continue to build and operate this type of reactor. Russia has two reactors in operation BN-600 (600 MWel) (commercial operation from 1981) and BN-800 (800 MWel) started recently in 2016. China is building two reactors of 600 MWel and announced a 1000-MWel project. India is currently filling in sodium in the prototype fast breeder reactor (PFBR) (600 MWel) (Fig. 2).
All these countries will take some advantage if Europe doesn't maintain their significant and valuable knowledge in this technology.
The ESFR-SMART Project
In 1988, while the European Superphénix reactor was in operation, a new European (sodium) Fast Reactor (EFR) project, with a slightly higher power of 1500 MWel, was launched in collaboration between France, Germany, and the UK and was subsequently stopped by the shutdown of Superphénix reactor. However, a collaborative project on SFR (CP ESFR) was initiated a few years later to “groom” EFR options and integrate new technical developments. It is on this new basis that a project called ESFR-SMART started at the end of 2017 mainly to integrate the new safety rules resulting primarily from the Fukushima accident.
The ESFR-SMART project is what in the Anglo-Saxon world is called a “working horse.” Its role is to introduce, outside any constructive planning, new ideas for the future, which can be valuable guides for R&D. Unlike in a project as ASTRID, which initially had a construction schedule, one can introduce innovative ideas, even if their lower technological-readiness level would require development and time. For these new ideas, research is performed to check their general feasibility and the absence of major impossibilities. The project is not designed to necessarily create solutions, which can be readily used by a committed industrialist, requiring validation after numerous additional files submitted to the Nuclear Safety Authority (ASN), but as was earlier mentioned, rather narrow down further R&D directions to the most feasible and promising concepts.
In this sense, the five main goals of the project are as the following:
Produce new experimental data to support calibration and validation of the computational tools.
Test and qualify new instrumentations to support the reactor-protection system.
Perform further calibration and validation of the computational tools to support the safety assessment of generation-IV SFRs.
Select, implement, and assess new safety measures for a commercial-size SFR in Europe.
Strengthen and link together new networks, in particular, the network of European sodium facilities and the network of European students working on SFRs.
Increased Safety of the Reactor Design.
Since the previous CP ESFR project, the safety groups of the Generation-IV International Forum (GIF) have published new documents. In particular, a “task force” dedicated to SFRs proposed a set of rules to be applied for these reactors. More importantly, the Fukushima accident in 2011 led to the issuance of new rules for all reactors. These rules, not applied in the CP ESFR project, have been applied to the ESFR-SMART project.
All the analyzes and modifications proposed in ESFR-SMART are based on simplifications of the systems rather than adding new systems to the design, which is an important guarantee of safety. In general, safety authorities around the world tend to favor intrinsic and passive safety. Many passive arrangements have, therefore, been introduced to exploit the remarkable potentials of SFRs: low pressure, good natural convection, etc. In addition, the so-called practical elimination method was used to make by design, impossible to happen, the unacceptable incidents, identified today for SFRs.
In particular, the main simplifications compared to the outcome of the CP ESFR project, improving the safety of the reactor and reducing the costs, are the following and are described in this NERS journal:
Improved core design with low void sodium and the usage of corium-discharge tubes.
Passive-control rods allowing shutdown without any order from the control room.
Reactor pit as a sodium-retention barrier instead of safety vessel.
Elimination of reactor dome by several dispositions as welding the components to the slab.
Introduction of three separate residual-heat-removal systems of which two works in a completely passive manner.
Suppression of residual heat-removal systems inside the primary vessel.
Simplified secondary-circuit design with straight pipes between components.
Introduction of thermoelectric-electromagnetic pumps for increased passive safety and natural convection.
With all these dispositions the reactor has great thermal inertia and passive-safety features. Its shutdown can be engaged without intervention by the passively activated control rods. After shutdown, the only thing that is required is to open the hatches of the steam-generator casings or the sodium-air-exchanger flaps to cool it by natural air convection in a passive manner, even in a “Fukushima”-type situation. This means, that no cooling water or on-site electricity is needed for the safe shutdown of the reactor. These interventions are very simple and can be carried out with significant delays.
With measures like an 80-centimeter thick metal slab, a reactor pit that retains sodium leaks, a design preventing primary sodium leaks, residual-power evacuation systems outside the vessel operating passively, and a corium collector (called core catcher) able to cool the corium by natural convection through the surrounding sodium, the mitigation of severe accidents is reinforced and the design meets the new mitigation criteria for generation-IV reactors ensuring no short or long-term radioactive external releases (Fig. 3).
Thanks to the ESFR-SMART project, the preliminary feasibility of new safety measures in the SFR design has been assessed, meeting the new reinforced post-Fukushima safety criteria. This reactor brings about significant simplifications, incorporating feedback from previous European reactors and projects. These simplifications could bring safety improvements, cost savings, and ease of operation.
If one day Europe wanted to build a reactor on these bases, the points still to be developed and requiring R&D are the following:
Fuel technologies and innovative core performances
Components for prevention and mitigation of severe accidents
Confirmation of the proposed organization for the reactor pit.
Qualification of low-expansion materials and large-diameter bellows for the secondary circuit.
Industrial validation of the manufacturing method of the EFR-type thick slab.
This project makes it possible to have additional knowledge available for the development of these reactors which could potentially solve the energy problems of humanity, but are in 2021, whithout any short-term project in Europe.