A nuclear reactor is configured with an intermediate coolant loop for transferring thermal energy from the reactor core for a useful purpose. The intermediate coolant loop includes a bypass flowpath with an air heat exchanger for dumping reactor heat during startup and/or shutdown. A fluidic diode along the bypass flowpath asymmetrically restricts flow across the bypass flowpath, inhibiting flow in a first flow direction during a full power operating condition and allowing a relatively uninhibited flow in a second direction during a startup and/or shut down low power operating condition.

A nuclear reactor is configured with a primary coolant loop for transferring heat away from the nuclear reactor core. In a shutdown event, the primary coolant pump may stop pumping primary coolant through the reactor core, resulting in decay heat buildup within the reactor core. An inertial energy coast down system can store kinetic energy while the nuclear reactor is operating and then release the stored kinetic energy to cause the primary coolant to continue to flow through the nuclear reactor core to remove decay heat. The inertial energy coast down system may include an impeller and a flywheel having a mass. During normal reactor operation, the flowing primary coolant spins up the impeller and flywheel, and upon a shutdown event where the primary coolant pump stops pumping, the flywheel and impeller can cause the primary coolant to continue to flow during a coast down of the flywheel and impeller.

A printed circuit heat exchanger for use in a reactor includes a core formed from a stack of plates diffusion bonded together. The core has: a top face, a bottom face disposed opposite the top face, a first side face extending between the top face and the bottom face, and a second side face disposed opposite the first side face. The printed circuit heat exchanger includes: a plurality of primary channels defined in the core, each of the primary channels extending from a primary inlet defined in the first side face to a primary outlet defined in the second side face; and a plurality of secondary channels defined in the core, each of the secondary channels extending among at least some of the primary channels from a secondary inlet defined in the top face to a secondary outlet defined in the top face.

The method includes configuring a nuclear reactor to at least partially mitigate liquid metal coolant backflow in the nuclear reactor in response to an at least partial failure of a primary electromagnetic pump (EMP) within a reactor pressure vessel of the nuclear reactor, the nuclear reactor being liquid metal-cooled, the primary EMP configured to circulate liquid metal coolant through at least a reactor core of the nuclear reactor, the configuring including, installing a backflow EMP within the reactor pressure vessel, such that when selectively activated, the backflow EMP at least partially mitigates liquid metal coolant backflow through the primary EMP.

The method includes configuring a nuclear reactor to at least partially mitigate liquid metal coolant backflow in the nuclear reactor in response to an at least partial failure of a primary electromagnetic pump (EMP) within a reactor pressure vessel of the nuclear reactor, the nuclear reactor being liquid metal-cooled, the primary EMP configured to circulate liquid metal coolant through at least a reactor core of the nuclear reactor, the configuring including, installing a backflow EMP within the reactor pressure vessel, such that when selectively activated, the backflow EMP at least partially mitigates liquid metal coolant backflow through the primary EMP.

Methods and systems for in-vessel natural circulation alkali metal reactor systems, purification systems, and associated methods are disclosed. A nuclear reactor vessel system includes an inner vessel that defines an inner volume sized to at least partially enclose a reactor. The reactor includes a plurality of nuclear fuel elements at least partially enclosed within a cladding, the reactor being cooled by a liquid metal coolant in a primary coolant loop. A pool of immersing fluid occupies a volume inside the inner vessel. The reactor vessel system includes an outer vessel sized to wholly or substantially enclose the inner vessel. A nuclear reactor power system includes a reactor core including an active fuel region; and a rotatable drum including at least one of a neutron absorbing material, a neutron leakage enhancing material, or a neutron reflecting material, the rotatable drum positioned external to the active fuel region of the reactor core.

Methods and systems for in-vessel natural circulation alkali metal reactor systems, purification systems, and associated methods are disclosed. A nuclear reactor vessel system includes an inner vessel that defines an inner volume sized to at least partially enclose a reactor. The reactor includes a plurality of nuclear fuel elements at least partially enclosed within a cladding, the reactor being cooled by a liquid metal coolant in a primary coolant loop. A pool of immersing fluid occupies a volume inside the inner vessel. The reactor vessel system includes an outer vessel sized to wholly or substantially enclose the inner vessel. A nuclear reactor power system includes a reactor core including an active fuel region; and a rotatable drum including at least one of a neutron absorbing material, a neutron leakage enhancing material, or a neutron reflecting material, the rotatable drum positioned external to the active fuel region of the reactor core.

A liquid metal-cooled nuclear reactor includes, within a reactor pressure vessel, a primary electromagnetic pump (EMP) circulating liquid metal coolant through the reactor core and a backflow EMP. The nuclear reactor may be configured to at least partially mitigate liquid metal coolant backflow in response to a primary EMP failure. The backflow EMP is coupled in series with the primary EMP within the reactor pressure vessel. The backflow EMP may be selectively activated in response to failure of the primary EMP to mitigate liquid metal backflow through the primary EMP. The primary EMP and backflow EMP may receive power from separate power sources. Multiple backflow EMPs may be coupled in parallel to the primary EMP via parallel liquid metal coolant lines. A nuclear reactor may include multiple primary EMPs and multiple sets of backflow EMPs, where each separate set of backflow EMPs is coupled to a separate primary EMP.

A liquid metal-cooled nuclear reactor includes, within a reactor pressure vessel, a primary electromagnetic pump (EMP) circulating liquid metal coolant through the reactor core and a backflow EMP. The nuclear reactor may be configured to at least partially mitigate liquid metal coolant backflow in response to a primary EMP failure. The backflow EMP is coupled in series with the primary EMP within the reactor pressure vessel. The backflow EMP may be selectively activated in response to failure of the primary EMP to mitigate liquid metal backflow through the primary EMP. The primary EMP and backflow EMP may receive power from separate power sources. Multiple backflow EMPs may be coupled in parallel to the primary EMP via parallel liquid metal coolant lines. A nuclear reactor may include multiple primary EMPs and multiple sets of backflow EMPs, where each separate set of backflow EMPs is coupled to a separate primary EMP.

The method of launching natural circulation of the liquid metal coolant in the heat sink circuit of the fast neutron nuclear reactor without connection to the main heat source (first circuit heat sink) and without using pumping equipment, but only as a result of electric heating of the downing and lifting sections of the heat sink circuit to the specified temperatures and, as a consequence, the occurring difference in the densities of the coolant on the lifting and downing sections of the heat sink circuit.