Nuclear Energy: Metal-cooled reactors

In this third installment of my series about reactor types, I will be looking at metal-cooled reactors. These are two of the six candidate designs of the Generation IV International Forum. It might seem counter-intuitive to cool something with molten metal, but if the liquid metal is cooler than what is supposed to be cooled with it, there is no reason why it shouldn’t work. And in fact, a lot of experience has been gained with these types of coolants.

Generally, these reactors are supposed to operate with low system pressures an no phase change of the coolant. There are designs that use sodium as coolant. These so called Sodium Fast Reactors (SFR) are a candidate design for the GenIV International Forum. The second GenIV metal-cooled candidate, the LFR uses lead or a lead-bismuth mixture of coolant. Liquid metal-cooled types of reactors are generally operating in the fast neutron spectrum and allow for rather high breeding ratios. Although sodium boils at 890°C and pure lead at around 1700°C, the outlet temperature of these reactors is limited to about 500-550°C. The reason for this are thermal safety margins to cope with transients and, especially in the case of lead, corrosion issues.

Most people are familiar with sodium from school experiments, where it is shown to react violently with water. But it is surprisingly non-corrosive to the structural materials and fuel elements. Fuel is often a metallic U/Pu alloy or U/Pu nitride, but they can also work with oxide fuels.

There are also two primary design types, that are under consideration. The pool-type and the loop-type. In the loop-type, the primary coolant is pumped through a heat exchange that is outside of the reactor vessel, while the pool-type features a heat exchanger inside the vessel, through which a secondary coolant is pumped.

Historical developments

There have been a number of metal-cooled reactors operating in the world. I think it is fair to say their performance was not always stellar.

There have been SFRs in France, the 233 MWe Phénix and and the huge 1242MWe Superphénix. For the latter, construction began in ’76, was completed in ’86. The reactor was decommissioned in ’97, after a lot of technical difficulties. The capacity factor was low. Like in the in less than 10%. France was later developing the ASTRID (Advanced Sodium Technological Reactor for Industrial Demonstration), a 600 MWe reactors as successor to the Superphénix, but canceled the project recently.

Germany built a 327 MWe fast breeder. Building started in ’72 and it was ready to be commissioned in ’85. However, it was never even fueled, let alone taken online. The plant was decommissioned and the remains were auctioned off. Today, it is an amusement park with hotels.

Japan was operating the Monju loop-type SFR fueled with MOX. It had a power output of 280MWe/714MWth. After reaching criticality in 1994, a sodium leak shut it down in late ’95. A cover-up scandal delayed restarting the reactor for 15 years. It started up again in May 2010, to be shut down again in August of the same year, after another accident. As of June 2011, it had only generated electricity for one hour since its first testing two decades prior. Decommissioning has started and is expected to be completed in 2047!

In the US, the 1.2 MWth EBR-I (’51-’64) and the 62.5MWth EBR-II (’64 -’94) were quite successful experiments with this technology. The experience of the EBR-II lead to the development of the Integrated Fast Reactor (’84-’94), for which funding was pulled by congress before its completion.
GEH picked up the remains and developed the commercial S-PRISM from this program.
More on this later. During the development, fuel processing technologies have also been developed. The so called pyroprocessing allows for recycling of 95% of the spent nuclear fuel. Recently, this has been increased to 97%, which reduces the needed time to store the processed fuel to a maximum of 1000 years.

The only country that seems to have had some success with SFRs is Russia. It is operating the BN-600 since ’80 and the BN-800 since 2014. The latter is scheduled to be fueled completely with MOX fuel during the next scheduled refueling.
It’s estimated that the BN-800 cost 2.1 billion dollars to build. Russia is developing an even larger reactor of the same principle design, the BN-1200. This design is supposed to deliver electricity at $US 0.022/kWh. An initially rather aggressive deployment schedule was postponed indefinitely.

China is building a 600 MWe SFR, the CFR-600, which seems to be based on the Russian BN-600.

Lead-cooled reactors are less commonly used. There is some experience with them on Russian submarines. Russia is also working on the BREST-300 a lead-cooled reactor.
The commercial companies envisioning a lead-coolant are hoping that the less reactive coolant decrease system costs, because no sodium-fire suppression systems are needed. But lead is surprisingly aggressive to steels. Today, there are two types of steel that could be used in a lead-cooled reactor. A Russian developed, silicon-alloyed steel and a Swedish aluminum-alloyed steel. Both form protective films on the surfaces, which reduces corrosion issues so significantly that a commercial use is anticipated.

It is fair to say that the economic advantages hoped to be realized by the low-pressure systems have failed to realize in each and every of the attempts.
But despite these difficulties, there are quite a few commercial companies trying to develop metal-cooled reactor designs ranging from 1.5MWe up to 1200MWe.
We will look at them here.

Oklo

Oklo is a company operating out of California, which is building a tiny 1.5 MWe fast-spectrum, heat-pipe reactor. Heat pipe reactors are using a metallic coolant in a low pressure environment. This makes the metal inside it, which is often sodium or potassium, boil off. In a quite literal sense, the heat pipe reactor is a boiling metal reactor. Oklo’s design is the first “advanced fission reactor design” to have been accepted for a combined license application review by the NRC. There is rather scant information about the reactor itself. From the NRC application, it is clear that the reactor will run on a metallic-HALEU-zirconium alloy fuel. Other information has been requested to be withheld.
The outer appearance of the reactor has been designed carefully in an effort to increase public acceptance of nuclear technology.
The reactor seems to be sized to supply remote locations with energy.

ARCNuclear

ARCNuclear is developing a 100 MWe pool-type SFR. The reactor will run for 20 years on each fueling. The hope is that this reactor can provide base load power as well as load following capacities. 100 MWe is small enough to be incrementally fit into developing grids. The use of this type of reactors is also envisioned to be used for water desalination, shale oil extraction and hydrogen production.
ARCNuclear considers the reactor to be highly proliferation-resistant and deems it suitable for deployment anywhere in the world. In a podcast about the reactor, ARC’s CEO Don Wolf explained that such a pool-type SFR was under consideration to be deployed in Libya. The long times between refueling allows for fuel handling equipment to be removed from the site. Additionally, the fuel is below opaque sodium, which makes extracting it harder, if no specialized equipment is used.
Some of the senior participants in the EBR-II experiment are on ARC’s technical team.
The ARC-100 is designed to be “walk away” passively safe against many types of accidents, like a complete loss of power to the station.
The EBR-II demonstrated these already in a real system.

GEH

GEH was involved in the development of the IFR in the US. After funding was pulled from the project, GEH started the development of the PRISM (Power Reactor Innovative Small Nuclear). There seem to be different version of the proposal, ranging from 311 MWe to 380MWe. The reactor is supposedly small enough physically to enable the factory production of a lot of the nuclear island components. The design features passive shutdown heat removal, passive post-accident containment cooling and passive accomodation of anticipated transients without scram (ATWS).

It seems that double reactors are the preferred deployment method. Seismic isolation is achieved by one platform for two nuclear steam supply systems. Given that the the design was completed somewhere between 1995-2002, it is unclear if it conforms to post-Fukushima requirements.

he S-PRISM was designed with the specifics of scaling of sodium-cooled systems in mind. The complexity of SFRs tends to scale with their size. A large, 3600 MWth reactor would need 6 heat exchanges due to the specific heat of sodium and limits on the piping diameter. The one-on-one arrangement of the S-PRISM simplifies operation and the size of the reactor building.
It’s estimated that 6 S-Prism reactors in 3 blocks would offer an 93% availability factor, which is 6% points higher than for a monolithic 6 loops plant.

TerraPower

Bill Gates is backing TerraPower, which is a nuclear development company. They are currently developing two distinct reactor types, the Molten Chloride Fast Reactor, at which we will look in a future post and the Traveling Wave Reactor (TWR).

he TWR is a pool-type SFR with an elaborate In-Vessel Fuel Handling Machine that swaps expired fuel rods from the center of the core for fresh fuel rods from the outer edge.

The TWR is a breeder capable of using depleted uranium, the tailing of the uranium enrichment process as fuel. Placed at the outer part of the reactor, it gradually absorbs neutrons during the operation to be converted to Pu-239, which is itself fissile and can sustain the fission reaction.
TerraPower was planning on deploying a 600MWe prototype in China. But this project got canceled due to rising political tensions between China and the US. The commercial size of this reactor is expected to be almost 1200 MWe.
It was announced that TerraPower and GEH are teaming up to develop a concept called “Natrium”. This is a 345 MWe SFR coupled to a molten salt heat storage which converts the this system into a 500MWe peaker plant.
This represents at least the second cooperation between these companies, which are also collaborating to pursue a Public-Private Partnership to design and construct the Versatile Test Reactor for the US Department of Energy.

LeadCold

The Swedish company LeadCold is developing a lead-cooled reactor. The enabling technology is the already mentioned aluminum-steel alloy, that forms a protective oxide layer to protect the structural material from the corrosive lead. This material enabled the development of the SEALER (Swedish Advanced Lead Reactor). There are currently two version under development, the “arctic” and “UK” versions. The former reactor is envisioned to be used in remote locations. It’s fueled with 2.4 t of 19.9% enriched uranium and produces between 3 and 10 MWe, which leads to refueling every 30 or 10 years respectively.

The UK Version is a larger, 55 MWe version, fueled with 19.8t of 11.8% enriched uranium.

A SEALER-UK united is estimated to cost on the order of £160M.
The spent nuclear fuel can be recycled or the whole unit can be brought to geological storage, with the fuel encased in solid lead.
The founder of the company, Janne Wallenius, has given an interesting interview about how safety margins for the structural material for transients limit the operating temperatures to about 550°C. If the coolant got too hot, it could damage the protective layer and corrode the reactor vessel rather quickly.

Westinghouse

Westinghouse is currently developing a lead-cooled fast reactor. The reactor is supposed to be modular to enable fast building schedules. The chemical inertia of lead relative to sodium eliminates the need for plant protection systems against coolant leaks. The usage of Westinghouse’s EnCore fuel is envisioned for this system.
The capability of storing thermal heat to supply peak demand is emphasized. The design has a thermal power rating of 950 MWth and features a “Brankine“-cycle, which is a condensing supercritical CO2 thermodynamic cycle. The plant is designed to operate at up to 650°C, which seems to be the highest temperature of the designs listed here.

There is a vague reference to a weld-overlay for the reactor vessel given as a reason for the higher temperature range. The references are about “advanced Al2O3” and “nanoceramic coatings”. It remains unclear, how practical the building materials really are.

Westinghouse is also developing a heat-pipe reactor, called eVinci, that’s supposed to be transportable. As in the case of Oklo, this reactor is cooled by boiling metal and therefore included here. This reactor is under consideration in the Department of Defense’s Pele program, as a candidate for a nuclear reactor that can be deployed at forward facing military bases. For military applications, the reactor will have to use TRISO fuel.
This reactor is operating in the thermal spectrum and is moderated by hydrogen, that is bound to a solid, which decomposes, if the operating temperature gets too high. This mechanism is actually included in the preliminary safety case of the reactor as a way of reducing reactivity.

Accelerator Driven System

Our last entry on the list, the Multi-purpose hYbrid Research Reactor for High-tech Applications (MYRRHA) does not really qualify as being a reactor, because it cannot sustain a chain reaction in the core. At least in one of the two possible configurations.

This system needs neutrons “from the outside” to get critical. These neutrons are generated by a process called spallation, when highly energetic protons hit a heavy nucleus. In the case of MYRRHA, a particle accelerator fires protons on a lead-bismuth target, which does double duty as coolant.

The neutrons generated by the spallation process are enough to make the reactor critical. When the particle beam is activated, MYRRHA is a lead-bismuth-cooled fast reactor, fueled with solid MOX elements. The system is supposed to demonstrate the feasibility of accelerator driven systems and some of their alleged benefits, like the effective burning of actinides. The major advantage of such systems is that turning off the particle beam immediately decreases the reactivity of the reactor. This is seen as a stronger guarantee against certain kinds of accidents, than any critical reactor can give.
The challenges for accelerator driven systems are largely the same as for “normal” fast systems, like operating in a really hard neutron environment.
In fact, the spallation neutrons make the neutron spectrum even harsher.

Additionally, these systems will have to maintain a complex particle accelerator with high enough availability factors to create a viable energy system. It remains to be seen, if these challenges can be overcome.

Summary

The development of LMFRs is still ongoing despite numerous financial setbacks. The range in power of developed reactors is quite impressive. It remains to be demonstrated that the low-pressure systems really offer a competitive advantage, if either fire suppression systems have to be installed for sodium-cooled power plants or special alloys have to be qualified for use in nuclear systems as is the case for lead-cooled reactors.
Their fast-spectrum enables a far better fuel utilization, but the manufacturing of solid fuel elements, specialized pumps and building materials still constitutes a significant cost. The recycling methods for nuclear waste from SFRs are well advanced and could change the conversation about nuclear waste.