User:James D DE/sandbox
Type of nuclear reactor fueled or cooled by molten material / From Wikipedia, the free encyclopedia
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt (mixture). MSRs' operation at or close to atmospheric pressure, rather than the 75-150 times atmospheric pressure of typical light-water reactors (LWRs), improves safety by eliminating a source of explosion risk and reduces the size and cost of the reactor's containment structures. Chemically, molten salts bind nearly all fission products, including caesium and iodine, preventing their release in an accident. MSRs' operating temperatures, higher than in a traditional LWR, offer improved electricity-generation efficiency and, in some cases, process-heat or hydrogen production opportunities. Salts' high boiling point of up to 1500°C and fuel salts' negative temperature coefficient of reactivity contribute to passive safety. The main design challenges relate to materials, which must withstand high temperatures, irradiation and the corrosivity of hot salts, and, in circulating-fuel-salt designs, managing the behaviour of the chemically-complex fuel salt as it circulates. Early estimates of MSR costs suggest they could be comparable or cheaper than LWRs.[3]
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Static fuel salt reactor (cutaway diagram), in this case the Moltex SSR-W. Note the LWR-like fuel assemblies.[1] |
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Molten salt coolant only reactor diagram, in this case using TRISO fuel pebbles.[2] |
While many design variants have been proposed, there are three main categories regarding the role of molten salt. Note that the most-discussed type of MSR (thorium-fuelled breeders, e.g. LFTR) are a subtype of the "circulating fuel salt" category, but exist alongside quite different alternatives.
Category | Examples |
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Molten salt fuel - circulating | ARE • MSRE • DMSR • MSFR • LFTR • IMSR • AWB, CMSR • EVOL • DFR • TMSR-500 |
Molten salt fuel - static | SSR |
Molten salt coolant only | TMSR • FHR |
Of these, the "original" 1950s/1960s designs (ARE/MSRE) used circulating fuel salt. A static fuel salt design has been in development since 2014. Designs using molten salt as coolant only were introduced in 2002.
In a circulating fuel salt design, the fuel salt passes through a volume where it achieves criticality, either due to the presence of a moderator (for thermal-neutron designs) or due to the dimensions of the volume (for fast-neutron designs) and then past heat exchangers which transfer the fission heat to a coolant circuit. Both pool-type and loop-type designs have been proposed. Some degree of salt purification/reprocessing is typically foreseen. In some designs, breeding can take place within the fuel salt (see for example LFTR).
In a static fuel salt design, the fuel salt is held in traditional metal fuel pins which are immersed in a pool of non-fissile coolant salt. This coolant salt circulates between the fuel pins and the heat exchangers. No circulation or purification/reprocessing of the fuel salt is foreseen, eliminating significant sources of complexity; the "price" of this simplification is having to fabricate the fuel pins/assemblies.
In a coolant salt only design, solid fuel is used (e.g. TRISO pebbles) and the (non-fissile) molten salt is the heat transfer medium to transfer fission heat to the heat exchangers.
MSRs can belong to most categories of nuclear reactors:
- They can be fast, thermal or epithermal.[4] Thermal reactors employ a moderator (usually graphite) to slow the neutrons down.
- They can be burners or breeders. A common misunderstanding is that circulating fuel salt designs are necessarily breeders, although this was a focus of much ORNL work. Some such designs include the fertile material (e.g. thorium) in the fuel salt, rather than as a separate blanket.
- They can use a variety of fuels (low-enriched uranium, thorium, depleted uranium, waste products)[5] and coolants (molten salt, liquid metal or gas). In particular the use of liquid rather than solid fuel may make it easier to use varying and impure mixes of actinides from nuclear waste without the constraints that solid fuel could impose (precise dimensions of fuel pellets, homogeneous distribution of actinides in fuel, precise tolerances on fuel behaviour under irradiation).
- They can be monolithic or modular, large or small
The lack of operating experience beyond three small R&D reactors, as well as the need for new fuel fabrication and waste disposal processes, increases the R&D/licensing effort required to bring these otherwise promising technologies to market.