page_title: Integral Molten Salt Reactor
section_title: Cooling
hierarchical_section_title: Integral Molten Salt Reactor / Safety / Cooling
caption_attribution_description: English: Description of the IMSR passive closed cooling system, IRVACS
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: A nuclear reactor is a thermal power system—it generates heat, transports it and eventually converts it to mechanical energy in a heat engine, in this case a steam turbine. 
  Such systems require that the heat is removed, transported and converted at the same rate it is generated.
  A fundamental issue for nuclear reactors is that even when the nuclear fission process is halted, heat continues to be generated at significant levels by the radioactive decay of the fission products for days and even months. 
  This is known as decay heat and is the major safety driver behind the cooling of nuclear reactors, because this decay heat must be removed. 
  For conventional light water reactors the flow of cooling water must continue in all foreseeable circumstances, otherwise damage and melting of the (solid) fuel can result. 
  Light water reactors operate with a volatile coolant, requiring high pressure operation and depressurization in an emergency.
  The IMSR instead uses liquid fuel at low pressure. 
  IMSR does not rely on bringing coolant to the reactor or depressurizing the reactor, using instead passive cooling. 
  Heat continuously dissipates from the Core-unit. 
  During normal operation, heat loss is reduced by the moderate temperature of the reactor vessel in normal operation, combined with the stagnant air between the Core-unit and guard vessel, which only allows radiant heat transfer. Radiant heat transfer is a strong function of temperature; any increase in the temperature of the Core-unit will rapidly increase heat loss. 
  Upon shutdown of the primary salt pumps, the reactor passively drops power to a very small level. 
  It can still heat up slowly by the small but constant decay heat as previously described. Due to the large heat capacity of the graphite and the salts, this increase in temperature is slow. 
  The higher temperatures slowly increase thermal radiant heat loss, and subsequent heat loss from the guard vessel itself to the outside air. 
  Low pressure nitrogen flows by natural convection over the outside of the guard vessel, transporting heat to the metal reactor building roof. 
  This roof provides the passive heat loss required, acting as a giant radiator to the outside air. 
  As a result, heat loss is increased while decay heat naturally drops; an equilibrium is reached where temperatures peak and then drop. 
  The thermal dynamics and inertia of the entire system of the Core-unit in its containment silo is sufficient to absorb and disperse decay heat. 
  In the long term, as decay heat dissipates almost completely, and the plant is still not recovered, the reactor would increase power to the level of the heat loss to IRVACS, and stay at that low power level (and normal temperature) indefinitely.
  In the event that the low pressure nitrogen coolant leaks from the IRVACS then natural air will offer similar cooling capability. Albeit with a minor nuclear activiation of the argon in the air.
  The molten salts are excellent heat transfer fluids, with volumetric heat capacities close to water, and good thermal conductivity.
caption: The IMSR uses a passive, always-on backup cooling system for the reactor. A cooling path is provided between the outside of the guard vessel that surrounds the Core-unit. Any heatup of the Core-unit will increase heat transfer to the guard vessel, in turn increasing heat loss to the natural circulation gas. The heated gas is cooled by the reactor building metal roof, and is returned to the guard vessel to be heated again. Click on image to enlarge.
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