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The 's Aircraft Reactor Experiment was primarily motivated by the compact size that the technique offers, while the 's Molten-Salt Reactor Experiment aimed to prove the concept of a nuclear power plant which implements a thorium fuel cycle in a breeder reactor. MSRs are considered safer than conventional reactors because they operate with fuel already in a molten state, and in event of an emergency, the fuel mixture is designed to drain from the core to a containment vessel where it will solidify in fuel drain tanks.
This prevents the uncontrolled nuclear meltdown and associated hydrogen explosions as in the Fukushima nuclear disaster that are at risk in conventional solid-fuel reactors. Another advantage of MSRs is that the gaseous fission products Xe and Kr do not have much solubility in the fuelsalt, [a] and can be safely captured as they bubble out of the molten fuel, [b] rather than increasing the pressure inside the fuel tubes over the life of the fuel, as happens in conventional solid-fuelled reactors.
Relevant design challenges include the corrosivity of hot salts and the changing chemical composition of the salt as it is transmuted by the neutron flux in the reactor core. MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons they have not been deployed. MSRs, especially those with the fuel dissolved in the salt, differ considerably from conventional reactors. Reactor core pressure can be low and the temperature much higher.
In this respect an MSR is more similar to a liquid metal cooled reactor than to a conventional light water cooled reactor. MSRs are often planned as breeding reactors with a closed fuel cycle—as opposed to the once-through fuel currently used in U. Safety concepts rely on a negative temperature coefficient of reactivity and a large possible temperature rise to limit reactivity excursions. As an additional method for shutdown, a separate, passively cooled container below the reactor can be included.
In case of problems, and for regular maintenance, the fuel is drained from the reactor. This stops the nuclear reaction and acts as a second cooling system.
Neutron-producing accelerators have been proposed for some super-safe subcritical experimental designs. The temperatures of some proposed designs are high enough to produce process heat for hydrogen production or other chemical reactions. MSRs offer many potential advantages over current light water reactors: [8]. FHRs cannot reprocess fuel easily and have fuel rods that need to be fabricated and validated, requiring up to twenty years [ citation needed ] from project inception.
FHR retains the safety and cost advantages of a low-pressure, high-temperature coolant, also shared by liquid metal cooled reactors. Notably, steam is not created in the core as is present in BWRs , and no large, expensive steel pressure vessel as required for PWRs.
Since it can operate at high temperatures, the conversion of the heat to electricity can use an efficient, lightweight Brayton cycle gas turbine. Much of the current research on FHRs is focused on small, compact heat exchangers that reduce molten salt volumes and associated costs. Molten salts can be highly corrosive and corrosivity increases with temperature.
For the primary cooling loop, a material is needed that can withstand corrosion at high temperatures and intense radiation. However, operating experience is limited. Materials for this temperature range have not been validated, though carbon composites, molybdenum alloys e. A workaround suggested by a private researcher is to use the new beta-titanium Au alloys as this would also allow extreme temperature operation as well as increasing the safety margin.
Fluorine has only one stable isotope 19 F , and does not easily become radioactive under neutron bombardment. Compared to chlorine and other halides, fluorine also absorbs fewer neutrons and slows " moderates " neutrons better. Low- valence fluorides boil at high temperatures, though many pentafluorides and hexafluorides boil at low temperatures. They must be very hot before they break down into their constituent elements.
Such molten salts are "chemically stable" when maintained well below their boiling points. Fluoride salts dissolve poorly in water, and do not form burnable hydrogen. Chlorine has two stable isotopes 35 Cl and 37 Cl , as well as a slow-decaying isotope between them which facilitates neutron absorption by 35 Cl. Chlorides permit fast breeder reactors to be constructed. Much less research has been done on reactor designs using chloride salts.
Chlorine, unlike fluorine, must be purified to isolate the heavier stable isotope, 37 Cl , thus reducing production of sulfur tetrachloride that occurs when 35 Cl absorbs a neutron to become 36 Cl , then degrades by beta decay to 36 S.
Lithium must be in the form of purified 7 Li , because 6 Li effectively captures neutrons and produces tritium. Even if pure 7 Li is used, salts containing lithium cause significant tritium production, comparable with heavy water reactors.
Reactor salts are usually close to eutectic mixtures to reduce their melting point. A low melting point simplifies melting the salt at startup and reduces the risk of the salt freezing as it is cooled in the heat exchanger. Due to the high " redox window" of fused fluoride salts, the redox potential of the fused salt system can be changed. Fluorine-lithium-beryllium " FLiBe " can be used with beryllium additions to lower the redox potential and nearly eliminate corrosion. However, since beryllium is extremely toxic, special precautions must be engineered into the design to prevent its release into the environment.
Many other salts can cause plumbing corrosion, especially if the reactor is hot enough to make highly reactive hydrogen. To date, most research has focused on FLiBe, because lithium and beryllium are reasonably effective moderators and form a eutectic salt mixture with a lower melting point than each of the constituent salts. Beryllium also performs neutron doubling, improving the neutron economy.
This process occurs when the beryllium nucleus emits two neutrons after absorbing a single neutron. Thorium and plutonium fluorides have also been used. Techniques for preparing and handling molten salt were first developed at ORNL. Oxides could result in the deposition of solid particles in reactor operation. Sulfur must be removed because of its corrosive attack on nickel-based alloys at operational temperature. Structural metal such as chromium, nickel, and iron must be removed for corrosion control.
The possibility of online processing can be an MSR advantage. Continuous processing would reduce the inventory of fission products, control corrosion and improve neutron economy by removing fission products with high neutron absorption cross-section, especially xenon.
This makes the MSR particularly suited to the neutron-poor thorium fuel cycle. In some thorium breeding scenarios, the intermediate product protactinium Pa would be removed from the reactor and allowed to decay into highly pure U , an attractive bomb-making material.
More modern designs propose to use a lower specific power or a separate thorium breeding blanket. This dilutes the protactinium to such an extent that few protactinium atoms absorb a second neutron or, via a n, 2n reaction in which an incident neutron is not absorbed but instead knocks a neutron out of the nucleus , generate U. Because U has a short half-life and its decay chain contains hard gamma emitters, it makes the isotopic mix of uranium less attractive for bomb-making.
This benefit would come with the added expense of a larger fissile inventory or a 2-fluid design with a large quantity of blanket salt. The necessary fuel salt reprocessing technology has been demonstrated, but only at laboratory scale. Reprocessing refers to the chemical separation of fissionable uranium and plutonium from spent fuel. In the United States the regulatory regime has varied dramatically across administrations.
A systematic literature review from concludes that there is very limited information on economics and finance of MSRs, with low quality of the information and that cost estimations are uncertain. In the specific case of the stable salt reactor SSR where the radioactive fuel is contained as a molten salt within fuel pins and the primary circuit is not radioactive, operating costs are likely to be lower.
While many design variants have been proposed, there are three main categories regarding the role of molten salt:. The use of molten salt as fuel and as coolant are independent design choices - the original circulating-fuel-salt MSRE and the more recent static-fuel-salt SSR use salt as fuel and salt as coolant; the DFR uses salt as fuel but metal as coolant; and the FHR has solid fuel but salt as coolant.
MSRs can be burners or breeders. They can be fast or thermal or epithermal. Thermal reactors typically employ a moderator usually graphite to slow the neutrons down and moderate temperature. They can accept a variety of fuels low-enriched uranium, thorium, depleted uranium , waste products [22] and coolants fluoride, chloride, lithium, beryllium, mixed.
Fuel cycle can be either closed or once-through. The reactor can adopt a loop, modular or integral configuration. Variations include:. The molten salt fast reactor MSFR is a proposed design with the fuel dissolved in a fluoride salt coolant.
They have been studied for almost a decade, mainly by calculations and determination of basic physical and chemical properties in the European Union and Russian Federation. When steady state is achieved in a MSFR, there is no longer a need for uranium enrichment facilities. MSFRs may be breeder reactors. They operate without a moderator in the core such as graphite, so graphite life-span is no longer a problem.
This results in a breeder reactor with a fast neutron spectrum that operates in the Thorium fuel cycle. MSFRs contain relatively small initial inventories of U.
MSFRs run on liquid fuel with no solid matter inside the core. This leads to the possibility of reaching specific power that is much higher than reactors using solid fuel. The heat produced goes directly into the heat transfer fluid. In the MSFR, a small amount of molten salt is set aside to be processed for fission product removal and then returned to the reactor. This gives MSFRs the capability of reprocessing the fuel without stopping the reactor.
This is very different compared to solid-fueled reactors because they have separate facilities to produce the solid fuel and process spent nuclear fuel. The MSFR can operate using a large variety of fuel compositions due to its on-line fuel control and flexible fuel processing. The core's shape is a compact cylinder with a height to diameter ratio of 1 where liquid fluoride fuel salt flows from the bottom to the top.
The return circulation of the salt, from top to bottom, is broken up into 16 groups of pumps and heat exchangers located around the core. The fuel salt takes approximately 3 to 4 seconds to complete a full cycle. At any given time during operation, half of the total fuel salt volume is in the core and the rest is in the external fuel circuit salt collectors, salt-bubble separators, fuel heat exchangers, pumps, salt injectors and pipes. During operation, the fuel salt circulation speed can be adjusted by controlling the power of the pumps in each sector.
The intermediate fluid circulation speed can be adjusted by controlling the power of the intermediate circuit pumps. The temperature of the intermediate fluid in the intermediate exchangers can be managed through the use of a double bypass.
This allows the temperature of the intermediate fluid at the conversion exchanger inlet to be held constant while its temperature is increased in a controlled way at the inlet of the intermediate exchangers. The temperature of the core can be adjusted by varying the proportion of bubbles injected in the core since it reduces the salt density. As a result, it reduces the mean temperature of the fuel salt.
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