The integral molten salt reactor (IMSR) is a nuclear power plant design targeted at developing a commercial product for the small modular reactor (SMR) market. It employs molten salt reactor technology which is being developed by the U.S. company Terrestrial Energy.

The IMSR is based closely on the denatured molten salt reactor (DMSR), a reactor design from Oak Ridge National Laboratory. In addition, it incorporates some elements found in the small modular advanced high temperature reactor (SmAHTR), a later design from the same laboratory. The IMSR belongs to the DMSR class of molten salt reactors (MSR) and hence is a "burner" reactor that employs a liquid fuel rather than a conventional solid fuel. This liquid contains the nuclear fuel as well as serving as the primary coolant.

In 2016, Terrestrial Energy engaged in a pre-licensing vendor design review for the IMSR with the Canadian Nuclear Safety Commission and successfully completed the first stage in late 2017. The company completed the second stage of the CNSC vendor design review in 2023, the first high-temperature advanced reactor to do so. The company is conducting pre-licensing engagements with the U.S. Nuclear Regulatory Commissions about the IMSR.

Terrestrial Energy and The Texas A&M University System announced plans in February 2025 to site an IMSR plant at the Texas A&M-RELLIS campus about 9 miles west of Texas A&M University in College Station. The company claims it will have its first commercial IMSRs licensed and operating in the early 2030s.

The integral molten salt reactor (IMSR) 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 442 megawatts of thermal heat. If used to generate electricity then the notional capacity is 195 megawatts electrical. The unit includes 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. The IMSR purposely avoids the use of either enriched lithium or beryllium as both are costly, of limited commercial supply, and lead to high levels of radioactive tritium production.

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.