page_title: Integral Molten Salt Reactor
section_title: Design
hierarchical_section_title: Integral Molten Salt Reactor / Design
caption_attribution_description: English: IMSR Core-unit, primary containment and silo.
context_page_description: The Integral Molten Salt Reactor is designed for the small modular reactor market. It employs molten salt reactor technology which is being developed by the Canadian company Terrestrial Energy.
  It is based closely on the denatured molten salt reactor, a reactor design from Oak Ridge National Laboratory. It also incorporates elements found in the SmAHTR, a later design from the same laboratory.
  The IMSR belongs to the DMSR class of molten salt reactors and hence is a "burner" reactor that employs a liquid fuel rather than a conventional solid fuel; this liquid contains the nuclear fuel and also serves as primary coolant.
  In 2016, Terrestrial Energy engaged in a pre-licensing design review for the IMSR with the Canadian Nuclear Safety Commission and entered the second phase of this process in October 2018 after successfully completing the first stage in late 2017.
  The company claims it will have its first commercial IMSRs licensed and operating in the 2020s.
context_section_description: The Integral Molten Salt Reactor is so called because it integrates into a compact, sealed and replaceable nuclear reactor unit, called the IMSR Core-unit. The core-unit comes in a single size designed to deliver 400 megawatts of thermal heat which can be used for multiple applications. If used to generate electricity then the notional capacity is 190 megawatts electrical. 
  The unit include all the primary components of the nuclear reactor that operate on the liquid molten fluoride salt fuel: moderator, primary heat exchangers, pumps and shutdown rods.
  The Core-unit forms the heart of the IMSR system. In the Core-unit, the fuel salt is circulated between the graphite core and heat exchangers. 
  The Core-unit itself is placed inside a surrounding vessel called the guard vessel. The entire Core-unit module can be lifted out for replacement. The guard vessel that surrounds the Core-unit acts as a containment vessel. In turn, a shielded silo surrounds the guard vessel.
  The IMSR belongs to the denatured molten salt reactor (DMSR) class of molten salt reactors (MSR). 
  It is designed to have all the safety features associated with the Molten Salt class of reactors including low pressure operation (the reactor and primary coolant is operated near normal atmospheric pressure), the inability to lose primary coolant (the fuel is the coolant), the inability to suffer a meltdown accident (the fuel operates in an already molten state) and the robust chemical binding of the fission products within the primary coolant salt (reduced pathway for accidental release of fission products).
  The design uses standard assay low-enriched uranium fuel, with less than 5% U²³⁵ with a simple converter (also known as a "burner") fuel cycle objective (as do most operating power reactors today). The proposed fuel is in the form of uranium tetrafluoride (UF₄) blended with carrier salts. These salts are also fluorides, such as lithium fluoride (LiF), sodium fluoride (NaF) and/or beryllium fluoride (BeF₂). These carrier salts increase the heat capacity of the fuel and lower the fuel's melting point. 
  The fuel salt blend also acts as the primary coolant for the reactor.
  The IMSR is a thermal-neutron reactor moderated by vertical graphite tubular elements. 
  The molten salt fuel-coolant mixture flows upward through these tubular elements where it goes critical. 
  After heating up in this moderated core the liquid fuel flows upward through a central common chimney and is then pulled downward by pumps through heat exchanges positioned inside the reactor vessel. 
  The liquid fuel then flow down the outer edge of the reactor core to repeat the cycle. 
  All the primary components, heat exchangers, pumps etc. are positioned inside the reactor vessel. 
  The reactor’s integrated architecture avoids the use of external piping for the fuel that could leak or break.
  The piping external to the reactor vessel contain two additional salt loops in series: a secondary, nonradioactive coolant salt, followed by another (third) coolant salt. 
  These salt loops act as additional barriers to any radionuclides and improve the system's heat capacity. It also allows easier integration with the heat sink end of the plant; either process heat or power applications using standard industrial grade steam turbine plants are envisioned by Terrestrial Energy.
  The IMSR Core-unit is designed to be completely replaced after a 7-year period of operation. This ensures that a sufficient operational lifetime of materials used in the IMSR reactor core can be achieved. During  operation, small fresh fuel/salt batches are periodically added to the reactor system. This online refueling process does not require the mechanical refueling machinery required for solid fuel reactor systems.
  Many of these design features are based on two previous molten salt designs from Oak Ridge National Laboratory (ORNL) – the ORNL denatured molten salt reactor (DMSR) from 1980 and the solid fuel/liquid salt cooled, small modular advanced high temperature reactor (SmAHTR), a 2010 design. The DMSR, as carried into the IMSR design
caption: IMSR Core-unit, primary containment and silo. This cut-away view shows the internals of the IMSR Core-unit, the containment and the surrounding structural silo. The Core-unit is a sealed reactor vessel that contains the graphite moderator (shown in white), primary fuel salt, and primary heat exchangers and pumps (shown in blue).                   Core-unit features[6] Shape:cylindrical shell Inner diameter:3500 mm Wall thickness:50 mm Inside Height:7000 mm Transport weight:170 tonne
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