ESFR-SMART project

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Figure 1. ESFR-SMART reactor concept with main components

The European Sodium Fast Reactor (ESFR) is a pool-type sodium fast reactor (SFR) with a proposed thermal power output of 3600 MWth and electrical output of 1500 MWe. The research was initiated on the reactor in 2009 as part of the Collaborative Project for a European Sodium Fast Reactor (CP-ESFR), which was based on the European Fast Reactor (EFR) research outcomes. This 4-year-long research project introduced the basic concepts of the reactor systems design and studied the reactor core behavior at nominal operation and accidental conditions. As a continuation of the research study, a new research project named European Sodium Fast Reactor Safety Measures Assessment and Research Tools (ESFR-SMART) was launched in September 2017.

Sodium-cooled Fast Reactors

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 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 type..
• Fast reactors allow extraction of the energy potential in natural uranium to the fullest possible extent. They can operate with fuel made of spent fuel of other reactor types: a mixture of depleted uranium from the spent fuels of the pressurized water reactors and plutonium from the reprocessing of the fuel, e.g., in France.
• These reactors do not require uranium enrichment facilities and greatly reduce the necessary uranium mining activities. For example, with the quantities of depleted uranium and plutonium currently available, France would have thousands of years of completely carbon-free electricity production ahead of it, assuming continuous reprocessing of the fuel.
• In terms of nuclear waste (i.e., accumulated minor actinides), the fact of operating the reactor by using waste produced by other reactors already reduces the overall quantity. These reactors are capable of transmuting actinides and therefore of producing final wastes with a much shorter lifespan than the wastes produced by water reactors, i.e., of the order of a few hundred years. This waste is, therefore, easier to store and manage or dispose of.

In conclusion, this technology provides an almost inexhaustible low-carbon energy source, without the need for uranium mines, as the reactor can operate on already available nuclear wastes, while those produced final wastes have a shorter lifespan and are therefore more controllable.

Sodium Fast Reactors in Europe

On the basis of the aforementioned enticing promises, France, Germany, and the United Kingdom embarked on the construction of sodium fast reactors in the 1970s: Phénix in France, PFR in the United Kingdom, and KNK in Germany. The gradual withdrawal of nuclear power in Germany and England eventually led to the termination of fast reactor development programs in these countries. Only France, backed by the success of its reprocessing activities, continued along this path with the construction of the Superphénix reactor. This is a European reactor with strong participation from Italy (30%) and a Germany/Netherlands consortium (around 5%). As it was subject to strong opposition from environmentalist groups and a lack of political support, the reactor was shut down in 1996 for purely political reasons., after a successful year-long steady operation. Furthermore, Superphénix spent more than a third of its ten years of operation, able to operate but shut down, waiting for administrative authorizations. A new project on a smaller reactor (600 MWe) called ASTRID was recently launched in France. The studies have enabled new advances in this field, in particular in the design of the reactor core. Sadly, the project was stopped in 2019, and currently, no large or medium-size commercial sodium fast reactor project is planned in France or in Europe before the end of this century.

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:

• Technical difficulties specific to the sector: Some experimental reactors (such as PFR) had technical difficulties, which led to their premature shutdown.
• Public opinion is generally negative in relation to plutonium, in particular in the USA, which, due to political reasons, led to the abolition of fuel reprocessing, where the plutonium is separated from the used fuel. Unavailable reprocessing leads to a lack of plutonium, which eventually ends the fast reactor development. Therefore, the fast reactor development has been stopped in the United States.
• 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, motivation for fast reactors has temporarily disappeared.
• The additional cost: by design, a sodium fast reactor is more expensive than a pressurized water reactor, which is more compact and without secondary circuits. The additional costs are estimated to be between 30–40%. On top of this, additional costs have to be added, inherent in the prototype reactors with which the knowledge is accumulated for mass production.
• Reprocessing and fuel fabrication are feasible but complex, coupled with the requirement of significant investments.
• The opposition of environmentalists: in France, this type of reactor, claiming to be able to operate without uranium mines and to produce energy for thousands of years, based on the available nuclear waste, has been a real red flag for environmentalists seeking to exit from nuclear power. Their opposition has been strong and constant.
• Lack of political support: under the above-mentioned conditions, political support collapses, and in particular, Superphénix was stopped after elections, as a pledge of a coalition of power with environmentalists.
• The difficulty of defending long-term investments in our world, which rather demands short-term returns on investment.

In conclusion, the fast reactor technology has ecological interests, but its development requires significant long-term investments, hardly compatible with models of rapid profitability. In addition, the environmentalists who could have been the defenders of the concept carried out, at least in France, a bitter fight against Superphénix. This opposition remains strong today and has led to a lack of support from recent French politicians. This explains why, in the short term, a solution that could potentially solve humanity's energy problems, in particular with regard to climate change, uranium supply, and waste management, is currently at a standstill. Under these conditions, Europe needs to maintain, with projects like ESFR-SMART, competence and an overview on what could be a fast sodium reactor 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 near future.

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 MWe, was launched in collaboration between Italy, Germany, and the UK and was subsequently stopped by the shutdown of the Superphénix reactor. However, a CP-ESFR project 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 with the aim of integrating 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 like 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 the following:

1. Select, implement, and assess new safety measures for the commercial-size ESFR
2. Produce new experimental data to support calibration and validation of the computational tools
3. Test and qualify new instrumentations to support the reactor protection system
4. Perform further calibration and validation of the computational tools to support the safety assessment of generation 4 SFRs
5. 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

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Figure 2. Main design differences between CP-ESFR project outcome (left) and ESFR-SMART design proposal (right)

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 sodium fast reactors 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, new concerning the CP-ESFR project, have been applied to the ESFR-SMART project. All the analyses 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 sodium fast reactors: low pressure, good natural convection, etc. In addition, the so-called practical elimination method was used to make it impossible, by design, for certain unacceptable incidents to happen, identified today in sodium-cooled fast reactors.. 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:

• Improved core design and the usage of corium discharge tubes
• Reactor pit as sodium retention barrier instead of safety vessel
• Elimination of reactor dome by strong confinement of primary sodium
• Introduction of 3 separate residual heat removal systems of which 2 work in a completely passive manner
• Simplified secondary circuit design with straight tubes
• Introduction of thermoelectric electromagnetic pumps for increased passive safety

Innovative safety measures of the reactor

ESFR core design

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Figure 3. ESFR core layout

The reactor core was largely inspired by the advances made by the ASTRID project.. For example, its design allows for a zero sodium void coefficient. Furthermore, at the level of the core reactivity control, in addition to the two sets of diversified control rods, a third set of rods were introduced to cause the reactor shutdown automatically should the coolant temperature at core outlet exceed a threshold of 600°C or the coolant flow rate through the core reach an inappropriately low level. Thus, the activation of the rods is completely passive and requires neither operator intervention nor action from the control room. Finally, hollow guides were introduced in the core to channel the course of the molten corium towards the core catcher in a hypothetical severe accident situation.

Design features retained from EFR project

The EFR project file is very valuable as it was mainly designed by people who had worked at Superphénix. Some of the options proposed in the project were new compared to Superphénix and were built on the basis of its construction and operation experience. Many of these ideas have been kept on ESFR-SMART, such as the thick slab (a metal slab 80 centimeters thick) vessel roof, and the modular design for the steam generators (since then taken over by Russian, Indian, and Chinese fast reactors).

Improvement of accidental sodium retention barrier for the primary circuit

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Figure 4. Proposed reactor pit design described in

All existing sodium fast reactors have a safety vessel around the main vessel to retain primary sodium in the event of a main vessel leak. If that were to be the case, the reactor would never restart, but it would still take more than a year to unload the core. Under these conditions and with the constant evolution of requests from the Safety Authorities, it would be necessary to answer the question of what would happen in the event of a leak from this safety vessel. It will then be necessary for the reactor pit to withstand a sodium leak. In this hypothesis, the project proposed an improved primary sodium retention barrier and organized the pit to take over the safety vessel’s functions.. This new organization has several additional advantages such as greater efficiency of the pit cooling circuit for the removal of residual heat..

Proposal to remove dome or polar table-type structures

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Figure 5. Sectional view of the ESFR-SMART primary circuit.

In the event of a serious accident, the projection of sodium towards the slab leads, by very conservative calculations, to the potential of sodium leaks through the reactor slab and to unacceptable overpressures, by sodium fire, inside the enclosure. This possibility led to the integration of a dome in the Superphénix reactor and a polar table in the ASTRID project, both intended to protect the enclosure. In ESFR-SMART, measures are taken from the design stage to avoid these leaks through the enclosure: thick slab, rotating plugs with frozen eutectic, components welded to the slab, integrated cold trap, etc.. Under these conditions, the elimination of these costly and cumbersome elements, for the operator, is justified. This elimination allows a great simplification of handling operations above the slab and therefore an improvement in operational safety.

New residual heat removal systems

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Figure 6. View of the three independent residual heat removal systems

To meet the safety criteria of the 4th generation sodium reactors, the residual heat evacuation is carried out by three independent systems (figure 6), each meeting the single failure criterion.. The first system, DHRS-1, is a circuit outside the primary system which takes hot sodium from the collector of the intermediate heat exchanger (IHX), cools it by natural convection in a sodium-air heat exchanger and returns this cold sodium to the central column of the IHX. This system works completely passively, being activated by simply opening the air inlet flaps and can provide 100% of the residual heat evacuation on its own. The system was specifically designed to be available even if the secondary circuits are damaged and the sodium is drained from them. The second system, DHRS-2, utilizes six secondary loops. If the feedwater water circuits are lost, the doors of the steam generator casings are opened. Natural air convection cools the sodium-containing heat exchanger modules. Calculations show that the power evacuated by the loop in natural convection requires some hours to remove all the required heat. The efficiency of the system can be increased by keeping on operating the secondary pumps at low speed and using forced air convection in the chimney. The third system, DHRS-3, is the cooling of the reactor pit with the use of an active system. Thanks to the elimination of the safety vessel, the achievable heat removal capacity is greatly improved and can, after three days, ensure the residual heat evacuation on its own.

With these three systems, ESFR-SMART meets the criteria to ensure the practical elimination of the "loss of residual power evacuation circuits" situation. It should be noted that there is no specific system in the primary vessel intended for the removal of residual heat. This has many advantages:

• Reduction of the primary vessel diameter
• Reduction of the number of slab penetrations
• Efficient cooling loop in natural convection of the reactor core
• Greater resistance against primary system-related accidents

Improved design of the secondary circuits

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Figure 7. Overview of the ESFR SMART secondary circuit with straight pipes.

All existing sodium fast reactors have secondary circuits with “flexible” pipes, or the expansion of the piping is compensated by expansion loops. This imposes a certain number of constraints: long lengths, small thickness, numerous welds. The experience feedback from the Phénix and Superphénix reactors shows difficulties for the operator: false alarms, potential sodium leak, difficulty in clearing these alarms, corrosion of the insulation by sodium, difficulties in supporting these pipes due to dilation, etc. In the ESFR-SMART project, the recommendation is to use straight pipes between fixed points of the secondary loops, with the implementation of certain elements, called bellows, compensating for thermal expansions. This is the option also chosen for the Russian BN-1200 project because the advantages are significant:

• Savings of around 65% on the pipes (their length, the number of welds, etc.)
• Gain of about 68% on the sodium volume and therefore on the corresponding circuits
• Anti-seismic fixing of the pipes facilitated
• Possibility of installing an offset thermal insulation, which leads to a gain in speed and safety of detecting a potential sodium leak, facilitating easier interventions
• Possibility to locate the components closer to each other leading to significant savings on secondary buildings

This option requires validation of large-diameter bellows of the same type as those already used on Superphénix exchangers or on modular steam generators.

General plant layout refinement

The newly introduced compact secondary circuits allow the possibility of circular disposition of secondary circuits around the primary vessel, visualized in figure 8.

This circular disposition facilitates the reduction of more than 50% on the secondary building's surface as shown in figure 9. Another benefit of the new circular layout is the fact that by allowing the steam generators to be positioned as close to the primary circuit as possible the piping length could be further reduced which potentially enhances the natural convection in the systems and brings further cost reductions for the power plant design.

The overall outlook of the reactor systems is illustrated in figure 10. The steam generator casings are situated towards the turbine building side of the reactor, giving enough room for the handling of fuel assemblies or other bigger equipment to the reactor hall from the handling building side. This arrangement is beneficial as it does not hinder the handling tasks in the primary building.

The final layout of the plant with its turbine and handling buildings is shown in figure 11. We can notice that a twin reactor could be added on the left to use the same handling building and minimize the final cost.

• Figure 8. View of a circular disposition of the 6 secondary circuits around the reactor pit
  Figure 8. View of a circular disposition of the 6 secondary circuits around the reactor pit
• Figure 9. Comparison of initial layout and new layout with circular disposition of compact secondary loops
  Figure 9. Comparison of initial layout and new layout with circular disposition of compact secondary loops
• Figure 10. General view showing the primary and secondary circuits
  Figure 10. General view showing the primary and secondary circuits
• Figure 11. View of the general layout of the plant with the turbine building on the right and the handling building on the left
  Figure 11. View of the general layout of the plant with the turbine building on the right and the handling building on the left

Reinforced severe accident mitigation

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Figure 12. View of the core catcher located under the core visible in figure 5

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) which cools 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 external releases.. Furthermore, a pre-sizing of a core catcher, made of molybdenum, has been proposed in , where it was shown that even around 25% of the melted core can be safely kept and melt coolability is ensured with the proposed device.

An intrinsically and passively safe reactor

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 without significant delays. Calculations related to the cooling by natural air convection around the steam generator modules have been carried out in .

Conclusion

Thanks to the ESFR-SMART project, pre-calculations and a set of new safety measures are already available for the design of a sodium fast reactor meeting the new reinforced post-Fukushima safety criteria. This reactor brings about significant simplifications, incorporating feedback from previous reactors and projects. These simplifications 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 few and would relate mainly to:

• Industrial confirmation of the proposed organization for the reactor pit and its oil cooling circuit.
• 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 a database available for the future for the development of these reactors, which could potentially solve the energy problems of humanity but are totally obstructed today and without a short-term future in Europe, due to mainly political reasons explained before.

See also

A video on the ESFR-SMART project featuring the reactor itself.

ESFR-SMART project

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