Integral Molten Salt Reactor

It employs molten salt reactor technology which is being developed by the Canadian company Terrestrial Energy.

In addition, it incorporates some elements found in the small modular advanced high temperature reactor (SmAHTR), a later design from the same laboratory.

In 2016, Terrestrial Energy engaged in a pre-licensing design review for the IMSR with the Canadian Nuclear Safety Commission[2][3] and entered the second phase of this process in October 2018 after successfully completing the first stage in late 2017.

These salt loops act as additional barriers to any radionuclides, as well as improving the system's heat capacity.

Most previous proposals for molten salt reactors all bred more fuel than needed to operate, so were called breeders.

The more recent SmAHTR proposal was for a small, modular, molten salt cooled but solid TRISO fuelled reactor.

After 7 years of operation, the Core-unit is shut down and cools in place to allow short-lived radionuclides to decay.

This operational mode reduces uncertainties with respect to long service life of materials and equipment, replacing them by design rather than allowing age-related issues such as creep or corrosion to accumulate.

While operating, small fresh fuel salt batches are periodically added to the reactor system.

As the reactor uses circulating liquid fuel this process does not require complex mechanical refueling machinery.

As such, the design must provide for exact control over the reaction rate of the core, and must enable reliable shut-down when needed.

Upon shutdown of the primary salt pumps, the reactor passively drops power to a very small level.

Low pressure nitrogen flows by natural convection over the outside of the guard vessel, transporting heat to the metal reactor building roof.

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 the internal reactor vessel auxiliary cooling system (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.

This makes it possible to operate at low pressures without risk of coolant/fuel boiling (an issue with water cooled reactors).

"[14] There is some uncertainty as to whether this is a measurement error, as the concentrations are small and other fission products also had similar accounting problems.

The plates serve as radiation shield and 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-air confinement area.

The IMSR design is simpler and eliminates the bottom drain line and accompanying risks from low level vessel penetrations.

Molten salts have high volumetric heat capacity, a low vapor pressure and no hydrogen generation potential, so there is no need for large-volume, high-pressure vessels for the reactor and containment or other equipment areas.

Similarly, molten salt heat exchangers used are more compact than the large steam generators employed in PWRs.

Large, high pressure components require heavy weldings and forgings that have limited availability.

Various non-electric applications exist that have a large market demand for energy: steam reforming, paper and pulp production, chemicals and plastics, etc.

Water-cooled conventional reactors are unsuitable to most of these markets due to the low operating temperature of around 300 °C, and too large in size to match single point industrial heat needs.

The IMSR's smaller size and higher operating temperature (around 700 °C in the reactor, up to 600 °C delivered) could potentially open up new markets in these process heat applications.

In 2016, Terrestrial Energy engaged in a pre-licensing design review for the IMSR with the Canadian Nuclear Safety Commission (CNSC).

[2][3] It successfully completed the first stage of this process in late 2017,[4] and entered the second phase of the design review in October 2018.

As part of the MOC, the agencies undertook in May 2022 a joint review of Terrestrial Energy’s Postulated Initiating Events (PIE) analysis and methodology for the IMSR® This work is foundational for further regulatory safety reviews and the regulatory program to prepare license applications required to operate IMSR® plants in Canada and the United States.