Stable salt reactor

The stable salt reactor (SSR) is a nuclear reactor design proposed by Moltex Energy Ltd[1] based in the United Kingdom and Canada.

A stable salt reactor core, with support structures in grey, heat exchangers and pumps in green, and fuel assemblies in red.

Moltex Energy proposes the SSR as a version of the molten salt reactor. This design aims to have improved safety characteristics and economics over traditional light water reactors. Stable salt reactors would not need expensive containment structures and components to mitigate radioactive releases in accident scenarios. The design of the SSR would preclude the type of widespread radiological contamination that occurred following the Chernobyl or Fukushima accident as hazardous airborne isotopes are chemically bound to the coolant. Additionally, the modular design would allow factory production of components and delivery to site by standard road transportation, reducing costs and construction timescales.

The fuel design is a compromise between light water reactor fuel assemblies and traditional molten salt reactor approaches in which the fuel is mixed with the coolant. In the SSR design, the liquid salt fuel mixture is contained within solid fuel assemblies that are very similar to current light water reactor technology. The fuel assemblies are then submerged in a pool of pure liquid salt coolant.

Technology
Single fuel tube showing 'diving bell' gas vent and an entire assembly.

The basic unit of the reactor core is the fuel assembly. Each assembly contains nearly 400 fuel tubes of 10 mm diameter with a 1 mm helical wire wrap filled to a height of 1.6 metres with fuel salt. The tubes have diving bell gas vents at the top to allow fission gasses to escape.

An unusual design feature of the reactor is that its core is rectangular in shape. This is neutronically inefficient compared to a cylindrical core but allows for simpler movement of fuel assemblies, and extension of the core as required simply by adding additional modules.

The assemblies move laterally through the core, with fresh assemblies entering at the sides in opposite directions, similar to the refuelling of CANDU reactors. They are raised only slightly to move them into an adjacent slot, remaining in the coolant at all times.

Modular construction
Reactor module, and 300 MWe reactor core with 2 modules.

The reactor core is composed of modules, each with a thermal output of 375 MW, containing 10 rows of 10 fuel assemblies, upper and lower support grids, heat exchangers, pumps, control assemblies and instrumentation. Two or more of these modules are assembled side by side in a rectangular reactor tank. A 1200 MWe reactor is possible in a tank that can fit on the back of a truck, making the technology significantly more compact than today’s reactors.

The modules (without fuel assemblies) are planned to be delivered to the construction site pre-assembled and pre-tested as single road-transportable components. They are installed into the stainless steel tank when the civil works phase is complete during commissioning.

The upper part of the reactor consists of an argon containment dome, incorporating two crane-type systems, a low-load device designed to move fuel assemblies within the reactor core and a high-load device designed to raise and lower fuel assemblies into the coolant, and to replace entire modules should that be necessary. All reactor maintenance is planned to be carried out remotely.

Fuel and materials

The fuel in the SSR is composed of two-thirds sodium chloride (table salt) and one-third plutonium and mixed lanthanide/actinide trichlorides. Fuel for the initial reactors is planned to come from converted conventional spent nuclear fuel from today’s fleet of reactors but in the case of the UK, could come from the stocks of civil plutonium dioxide from PUREX downblended and converted to chloride with added impurities to reduce any proliferation concerns.

Trichlorides are more thermodynamically stable than the corresponding fluoride salts and can therefore be maintained in a strongly reducing state by contact with sacrificial nuclear grade zirconium metal added as a coating on, or an insert within, the fuel tube. As a result, the fuel tube can be made from standard nuclear certified steel without risk of corrosion. Since the reactor operates in the fast spectrum, the tubes will be exposed to very high neutron flux and so will suffer high levels of radiation damage estimated at 100–200 dpa over the tube life. Highly neutron damage tolerant steels such as HT9 will therefore be used for the tubes. Other steels with fast-neutron tolerance could also be used depending on the local supply chain capabilities such as PE16, NF616 and 15-15Ti.

The average power density in the fuel salt is 150 kW/l which allows a large temperature margin below the boiling point of the salt. Power peaking to double this level for substantial periods would not exceed the safe operating conditions for the fuel tube.

Coolant

The coolant salt in the reactor tank is a sodium zirconium fluoride mixture. The zirconium is not nuclear-grade and still contains ~2% hafnium. This has minimal effect on core reactivity but makes the coolant salt low-cost and a highly effective neutron shield. One metre of coolant reduces neutron flux by four orders of magnitude. All components in the SSR are protected by this coolant shield.

The coolant also contains 1 mol% zirconium metal (which dissolves forming 2 mol% ZrF2). This reduces its redox potential to a level making it virtually non-corrosive to standard types of steel. The reactor tank, support structures and heat exchangers can therefore be constructed from standard 316L stainless steel.

The coolant salt is circulated through the reactor core by four pumps attached to the heat exchangers in each module. Flow rates are modest, approximately 1 m/s with resulting low requirement for pump power. There is redundancy to continue operation in the event of a pump failure.

Safety

The stable salt reactor was designed with intrinsic safety characteristics being the first line of defence. There is no operator or active system required to maintain the reactor in a safe and stable state. The following are primary intrinsic safety features behind the SSR:

Reactivity control

The SSR is self-controlling and no mechanical control is required. This is made possible by the combination of a high negative temperature coefficient of reactivity and the ability to continually extract heat from the fuel tubes. As heat is taken out of the system the temperature drops, causing the reactivity to go up. When the reactor heats up the reactivity goes down. Such large negative reactivity feedback allows the reactor to always be in a shutdown (subcritical) state when at temperatures exceeding 800°C. This provides security against all overpower scenarios, such as a reactivity insertion accident. The ability to shutdown is ensured through the removal of fuel assemblies along the core's edge to their storage inside the reactor tank. This renders the system subcritical. For the sake of having diverse and redundant safety systems, there also exist four fast-action boron control blades[2].

Non-volatile radioactive material

Use of molten salt fuel with the appropriate chemistry eliminates the hazardous volatile iodine and caesium, making multi-layered containment unnecessary in preventing airborne radioactive plumes in severe accident scenarios. The noble gasses xenon and krypton would leave the reactor core in normal operation, but be trapped until their radioactive isotopes decay, so there would be very little that could be released in an accident.[3]

No high pressures

High pressures within a reactor provide a driving force for dispersion of radioactive materials from a water-cooled reactor. Molten salt fuels and coolants have boiling points far above the SSR's operating temperature, so its core runs at atmospheric pressure. Physical separation of the steam generating system from the radioactive core by means of a secondary coolant loop eliminates that driving force from the reactor. High pressures within fuel tubes are avoided by venting off fission gases into the surrounding coolant salt.

Low chemical reactivity

Zirconium in pressurized water reactors (PWRs) and sodium in fast reactors both create the potential for severe explosion and fire risks. There are no chemically-reactive materials used in the SSR.

Decay heat removal

Immediately after a nuclear reactor shuts down, almost 7% of its previous operating power continues to be generated, from the decay of short-halflife fission products. In conventional reactors, removing this decay heat passively is challenging because of their low temperatures. The SSR operates at much higher temperatures so this heat can be rapidly transferred away from the core. In the event of a reactor shutdown and failure of all active heat-removal systems in the SSR, decay heat from the core dissipates into air cooling ducts around the perimeter of the tank that operate continually. The main heat transfer mechanism is radiative. Heat transfer goes up substantially with temperature so is negligible at operating conditions but is sufficient for decay heat removal at higher accident temperatures. The reactor components are not damaged during this process and the plant can be restarted afterwards.

A solution to the nuclear waste legacy

Most countries that use nuclear power choose to store spent nuclear fuel deep underground until its radioactivity has reduced to levels similar to natural uranium. Acting as a wasteburner, the SSR offers a different way to manage this waste.

Operating in the fast spectrum, the SSR is effective at transmuting long-lived actinides into more stable isotopes. Today’s reactors that are fuelled by reprocessed spent fuel need very-high-purity plutonium to form a stable pellet. The SSR can have any level of lanthanide and actinide contamination in its fuel as long as it can still go critical. This low level of purity greatly simplifies the reprocessing method for existing waste.

The method used is based on pyroprocessing and is well understood. A 2016 report by the Canadian National Laboratories on reprocessing of CANDU fuel estimates that pyroprocessing would be about half the cost of more conventional reprocessing. Pyroprocessing for the SSR uses only one third of the steps of conventional pyroprocessing, which will make it even cheaper. It is potentially competitive with the cost of manufacturing fresh fuel from mined uranium.

The waste stream from the SSR will be in the form of solid salt in tubes. This can be vitrified and stored underground for over 100,000 years as is planned today, or it can be reprocessed. In that case, fission products would be separated out and safely stored at ground level for the few hundred years needed for them to decay to levels similar to uranium ore. The troublesome long-lived actinides and the remaining fuel would go back into the reactor where they to be burnt and transmuted into more-stable isotopes.

Other stable salt reactor designs

Stable salt reactor technology is highly flexible and can be adapted to several different reactor designs. The use of molten salt fuel in standard fuel assemblies allows Stable Salt versions of many of the large variety of nuclear reactors considered for development worldwide. The focus today however is to allow rapid development and roll out of low-cost reactors.

Moltex Energy is focussed on deployment of the fast spectrum SSR-Wasteburner discussed above. This decision is primarily driven by the lower technical challenges and lower predicted cost of this reactor.

In the longer term the fundamental breakthrough of molten fuel salt in tubes opens up other options. These have been developed to a conceptual level to confirm their feasibility. They include:

  • Uranium burner (SSR-U) This is a thermal spectrum reactor burning low-enriched uranium which may be more suited to nations without an existing nuclear fleet and concerns about waste. It is moderated with graphite as part of the fuel assembly.
  • Thorium breeder (SSR-Th) This reactor contains thorium in the coolant salt which can breed new fuel. Thorium is an abundant fuel source that can provide energy security to nations without indigenous uranium reserves.

With this range of reactor options and the large global reserves of uranium and thorium available, the Stable Salt Reactor can fuel the planet for several thousands of years.

Economics

The overnight capital cost of the stable salt reactor was estimated at $1,950/kW by an independent UK nuclear engineering firm.[4] For comparison, the capital cost of a modern pulverised coal power station in the United States is $3,250/kW and the cost of large-scale nuclear is $5,500/kW.[5] Further reductions to this overnight cost are expected for modular factory-based construction.

This low capital cost results in a levelised cost of electricity (LCOE) of $44.64/MWh with substantial potential for further reductions, because of the greater simplicity and intrinsic safety of the SSR.

The International Energy Agency predicts that nuclear will maintain a constant small role in global energy supply with a market opportunity of 219 GWe up to 2040. With the improved economics of the SSR, Moltex Energy predicts that it has the potential to access a market of over 1,300 GWe by 2040.

Development

A patent was granted in 2014.[6] A design has been developed and a safety case is well under way, to begin formal discussions with nuclear regulatory authorities.

Moltex Energy will build a demonstration Stable Salt Reactor (Wasteburner) at the Point Lepreau nuclear power plant site in Canada under an agreement signed with the New Brunswick Energy Solutions Corporation and NB Power. [7]

References
  1. "Moltex Energy | Safer Cheaper Cleaner Nuclear | Stable Salt Reactors | SSR". Moltex Energy LLP. Retrieved 2017-01-25.
  2. "An Introduction to the Moltex Energy Technology Portfolio" (PDF). Moltex Energy. Retrieved 15 October 2019.
  3. "Gaseous Fission Products in the Stable Salt Reactor" (PDF). Moltex Energy Ltd. Archived from the original (PDF) on 2016-06-19. Retrieved 2017-01-25.
  4. Brooking, Jon (2015-01-01). "Design review and hazop studies for stable salt reactor". Cite journal requires |journal= (help)
  5. "Moltex Energy sees UK, Canada SMR licensing as springboard to Asia". Nuclear Energy Insider. June 28, 2016. Retrieved 2017-01-25.
  6. "Patent GB2508537A" (PDF).
  7. https://www.nextbigfuture.com/2018/07/moltex-molten-salt-reactor-being-built-in-new-brunswick-canada.html
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