IMSR

The Integral Molten Salt Reactor (IMSR) is a small modular reactor (SMR) that employs molten salt reactor technology. Its design is based closely on the DMSR design from Oak Ridge National Laboratory, TN, USA and incorporates elements found in the SmAHTR, a later design from the same laboratory. The IMSR is being developed by Terrestrial Energy Inc. (TEI),^[1] headquartered in Mississauga, Canada. The IMSR belongs to the DMSR class of MSR and hence is “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.

The IMSR facility in cutaway bird's view. New modules are brought in by road and are then lifted into the reactor cavity by bridge crane. Also shown are secondary heat exchangers and manifolds and the power generating part of the plant. Click on image to enlarge.

Design

Cut-away view of the IMSR core unit, showing the core unit placed inside a surrounding solid buffer salt tank, itself sitting in a below grade silo. The IMSR reactor module forms the core of the IMSR facility. The reactor module contains integral heat exchangers, pumps, a control rod, and the reactor vessel itself. The entire module can be lifted out for replacement. Wrapped around the buffer salt tank are cooling pipes that form the passive cooling jacket. Click on image to enlarge.

The IMSR “integrates” into a compact, sealed and replaceable nuclear reactor unit (the IMSR Core-unit) all the primary components of the nuclear reactor that operate on the liquid molten fluoride salt fuel: moderator, primary heat exchangers, pumps and control rods.

The IMSR belongs to the DMSR class of MSR. It therefore employs a uranium dominant fuel with a simple converter (also known as a “burner”) fuel cycle objective. This is unlike the majority of other molten salt reactor designs that employ the thorium fuel cycle, which requires the more complex breeder objective. Therefore the design uses the well-known uranium fuel cycle and low enriched uranium fuel, as most of today's operating power reactors do. The IMSR fuel itself is in the form of uranium tetrafluoride (UF4). This fuel is blended with carrier salts, which 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 (coolant) and lower the melting point of the uranium fluoride fuel.

This liquid fuel-coolant mixture is pumped through a critical nuclear reactor core that is moderated by graphite elements, making this a thermal neutron reactor. After heating up in the core, pumps force the liquid fuel through heat exchangers positioned inside the reactor vessel. The reactor’s “integrated” architecture (all the primary components, heat exchangers etc. are positioned inside the reactor vessel) avoids the use of external piping that could leak or break. The piping external to the reactor vessel contains a secondary, nonradioactive coolant salt. This salt acts as an additional containment barrier and heat sink, and transfers its energy to either a standard industrial grade steam turbine plant, which generates electricity or to a process heat application, or to a combination of the two.

The IMSR Core-unit is designed to be fully replaceable in normal operation as described below. During the power operations period, small fresh fuel salt batches are periodically melted and added to the reactor system. This online refueling process does not require the mechanical refueling machinery required for solid fuel reactor systems.

These design features are based heavily on two previous molten salt designs from Oak Ridge National Laboratory (ORNL) – the ORNL denatured molten salt reactor (DMSR)^[2] from 1980 and the solid fuelled but liquid salt cooled, small modular advanced high temperature reactor (SmAHTR), a 2010 design. The DMSR, as carried into the IMSR design, proposed to use molten salt fuel and graphite moderator in a simplified converter design using LEU (in combination with thorium, which may be used in the IMSR), with periodic additions of LEU fuel. Most previous proposals for molten salt reactors all bred more fuel than needed to operate, so were called breeders. Converter or “burner” reactors like the IMSR and DMSR can also utilize plutonium from existing spent fuel as their makeup fuel source. The more recent SmAHTR proposal was for a small, modular, molten salt cooled but solid TRISO fuelled reactor.^[3]

Terrestrial Energy is currently working on 3 different unit sizes, 80 megawatts-thermal (MWth), 300 MWth and 600 MWth, generating 33, 141, and 291 electrical megawatts (MWe) of electricity respectively using standard industrial grade steam turbines. As standard industrial grade steam turbines are used, cogeneration, or combined heat and power, is also possible.

TEI’s goal is to have the IMSR licensed and ready for commercial rollout by early next decade.

Operations

A key feature of the proposed IMSR design is the replaceable core-unit. Rather than opening the reactor vessel, replacing the graphite and other components that make up the nuclear reactor core, and then closing the reactor vessel again, the IMSR core-unit is instead replaced as a unit. This includes the pumps, pump motors, control rod and heat exchangers, all of which are either inside the vessel or directly attached to it. To facilitate a replacement, there are two reactor silos in the reactor building, one with an operating IMSR core-unit and one idle or in cool-down. After 7 years of operation, the IMSR core-unit is shut down and allowed to cool in place. At the same time, a new IMSR core-unit is activated in the previously unused second reactor silo. This entails connection to the secondary (coolant) salt piping, placement of the containment head and biological shield and loading with fresh fuel salt. The containment head provides double containment (the first being the sealed reactor vessel itself). The new IMSR core-unit can now start its 7 years of power operations while the previously operated unit in the silo next to it is cooling down to allow short lived radionuclides to decay. After that cool-down period, the spent IMSR core-unit is lifted out and replaced, allowing the cycle to revert in another 7 years to the first silo for power operations. This works because the IMSR, being liquid fueled, employs online refueling. During the power operations period, small fresh fuel salt batches are periodically melted and added to the reactor system. As the reactor is liquid fueled this process does not require mechanical refueling machinery. Due to replaceable core-units and online refueling, the IMSR reactor vessel is never opened, thereby ensuring a clean operating environment. The IMSR facility accumulates sealed spent IMSR core-units and spent fuel salt tanks in onsite, below grade silos. This operational mode also reduces uncertainties with respect to long term service life of materials and equipment, replacing them by design rather than allowing ageing related issues such as creep or corrosion to accumulate.

Safety

Nuclear power reactors have three fundamental safety requirements:

* Control

* Cool

* Contain

Perhaps the most obvious is that nuclear reactors require that control over the critical nuclear chain reaction must be maintained. As such, the design must provide for exact control over the reactivity of the reactor core, and must ensure reliable shut-down when needed. Under routine operations, the IMSR relies on intrinsic stability for reactivity control. This behavior is known as negative power feedback - the reactor is self-stabilizing in power output and temperature, and is characterized as a load-following reactor. As backup, the IMSR employs a flow-driven control rod, which sinks into the core if pumped flow is lost. A second backup is provided in the form of a meltable can placed inside the IMSR Core-unit and filled with a very strong neutron absorber which will melt and permanently shut down an IMSR Core-unit if overheating occurs.

A nuclear reactor is a thermal power system - that is it generates heat, transports it, and eventually converts this heat to motion in a heat engine, in this case a steam turbine. Such systems require that the heat generation matches the heat removal from the system. A fundamental issue of nuclear reactors in this respect, is the fact that even when the nuclear fission process is halted, considerable heat continues to be generated at significant levels by the radioactive decay of the fission products for days and even months post shutdown. This is known as decay heat and is the major safety driver behind cooling of nuclear reactors because this decay heat cannot be shut off. For conventional light water reactors the presence of this decay heat means that in all foreseeable circumstances, flow of cooling water must continue 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. The IMSR does not rely on bringing coolant to the reactor or depressurizing the reactor. The IMSR relies upon a unique passive cooling system. Heat is constantly being lost from the IMSR core-unit. During normal operation, the heat loss from the IMSR core-unit is reduced by the use of meltable insulation, in the form of a normally solid buffer salt. The buffer salt is placed in an annular tank enveloping the reactor on all sides except the top. Upon shutdown of the primary salt pumps, the reactor passively shuts itself down, but can still heat up slowly by the small but constant decay heat as previously described. This heatup will melt the buffer salt, thereby initially absorbing decay heat through the latent heat of fusion, then providing convection cooling through the now liquid buffer salt. On the outside of the annular buffer salt tank, a series of water filled cooling pipes are laid out. This is the so-called cooling jacket. With the buffer salt melted there is much less thermal resistance, and the buffer salt becomes a natural convection coolant that transports heat to the jacket. This causes the cooling jacket water to evaporate. There is sufficient water in the jacket for more than 7 days of evaporative cooling. Beyond that period, heat losses to the air and ground match decay heat generation, avoiding the need to replenish the cooling water supply. Overall the thermal dynamics and inertia of the entire system of the IMSR Core-unit in its containment silo is sufficient to absorb and disperse decay heat.

The IMSR is a type of molten salt reactor. All molten salt reactors have a number of features that contribute to containment safety. These mostly have to do with the properties of the salt itself. The salts are chemically inert. They do not burn or generate combustible material. The salt also has a low volatility and this allows a very low operating pressure of the vessel and cooling loops. In other words, the salts have extremely high boiling points, in the ballpark of 1400 °C. This provides a very large margin to the normal operating temperature of some 600 °C to 700 °C. This makes it possible to operate at low pressures without risk of coolant/fuel boiling. In addition, the high chemical stability of the salt precludes energetic chemical reactions such as hydrogen gas generation and detonation and sodium combustion, that can challenge the design and operations of other reactor types. In technical terms there is a lack of stored energy and potential energy, chemical or physical. The fluoride salt itself locks many fission products up as chemically stable, non volatile fluorides, such as cesium fluoride. Similarly, other high risk fission products such as iodine, dissolve into the fuel salt and stay there as bound up iodide salt.^[4] See liquid fluoride thorium reactor and molten salt reactor for more information.

In addition to the containment provided by the salt properties, the IMSR has multiple physical containment barriers. The IMSR core-unit is a fully sealed, integral reactor unit. This makes leaks very unlikely. The IMSR core-unit is surrounded by the buffer salt tank, itself a fully sealed unit surrounded by structural steel and concrete. The IMSR core-unit is covered up from the top by a steel containment head which is itself covered by thick round steel plates. The plates serve as radiation shield, but also provide protection against external hazards such as explosions or aircraft crash penetration. The reactor building provides an additional layer of protection against such external hazards, as well as a controlled, filtered confinement area.

Most molten salt reactors use a gravity drain tank as an emergency storage reservoir for the molten fuel salt. The IMSR deliberately avoids this drain tank with its own set of attendant safety problems, as IMSR reactor control or emergency cooling relies on the other methods already described. This simplifies the design and eliminates the drain line and the risks from low level penetrations of the vessel. The result is a more compact, robust design with fewer parts and few failure scenarios.

References

Further reading

http://www.forbes.com/sites/peterdetwiler/2014/09/22/molten-salt-nuclear-reactors-part-of-americas-long-termenergy-future/

http://ansnuclearcafe.org/2014/09/30/business-focused-approach-to-molten-salt-reactors/

http://terrestrialenergy.com/wp-content/uploads/2015/01/ANS-NN-2014-12-ws.pdf