The present disclosure relates to reducing losses in the effective delayed neutron fraction during the operation of a reactor, making it possible to provide for a high efficiency of burning out of minor actinides, and also that of increasing the leak-tight integrity of the primary circuit and the reliability of the reactor. The above-mentioned technical result is achieved in an integral molten salt fast reactor with a circulating fuel composition, comprising a vessel with inlet and outlet secondary circuit pipelines and a connection pipe for initial filling and replenishment with molten salt coolant, heat exchangers of the primary/secondary circuit, a side reflector, an upper reflector and a lower reflector, a core with a shell, and a main circulation pipe, wherein the side reflector is made of sections between which the heat exchangers of the primary/secondary circuit are arranged such that they lie flush against the shell of the core.

To reduce size and mass of a nuclear reactor system, an integrated in-vessel shield separates the role of a neutron reflector and a neutron shield. Nuclear reactor system includes a pressure vessel including an interior wall and a nuclear reactor core located within the interior wall of the pressure vessel. Nuclear reactor core includes a plurality of fuel elements and at least one moderator element. Nuclear reactor system includes a reflector located inside the pressure vessel that includes a plurality of reflector blocks laterally surrounding the plurality of fuel elements and the at least one moderator element. Nuclear reactor system includes the in-vessel shield located on the interior wall of the pressure vessel to surround the reflector blocks. In-vessel shield is formed of two or more neutron absorbing materials. The two more neutron absorbing materials include a near black neutron absorbing material and a gray neutron absorbing material.

To reduce size and mass of a nuclear reactor system, an integrated in-vessel shield separates the role of a neutron reflector and a neutron shield. Nuclear reactor system includes a pressure vessel including an interior wall and a nuclear reactor core located within the interior wall of the pressure vessel. Nuclear reactor core includes a plurality of fuel elements and at least one moderator element. Nuclear reactor system includes a reflector located inside the pressure vessel that includes a plurality of reflector blocks laterally surrounding the plurality of fuel elements and the at least one moderator element. Nuclear reactor system includes the in-vessel shield located on the interior wall of the pressure vessel to surround the reflector blocks. In-vessel shield is formed of two or more neutron absorbing materials. The two more neutron absorbing materials include a near black neutron absorbing material and a gray neutron absorbing material.

Heat can be stably extracted with easy criticality control. A nuclear reactor includes: a fuel portion being a reactor core having a nuclear fuel body; a shielding portion covering all over outer sides of the fuel portion to shield against radiations generated from the reactor core; and a thermal conduction part that conducts heat generated in the reactor core to exterior of the shield part. The nuclear fuel body contains a fissile material with an enrichment not less than 5% by weight throughout an operation period.

Heat can be stably extracted with easy criticality control. A nuclear reactor includes: a fuel portion being a reactor core having a nuclear fuel body; a shielding portion covering all over outer sides of the fuel portion to shield against radiations generated from the reactor core; and a thermal conduction part that conducts heat generated in the reactor core to exterior of the shield part. The nuclear fuel body contains a fissile material with an enrichment not less than 5% by weight throughout an operation period.

Ensuring a high output temperature while preventing leakage of radioactive substances, etc. A nuclear reactor includes a fuel unit; a shield unit that covers a circumference of the fuel unit for shielding from radioactive rays; and a heat conductive portion that penetrates the shield unit, is arranged such that the heat conductive portion extends to inside of the fuel unit and outside of the shield unit, and transfers heat of the fuel unit to the outside of the shield unit by solid heat conduction.

A reactor core includes an inner core region that extends in a vertical direction, and has a plurality of first fuel pins accommodating an inner core fuel; an outer core region that extends in the vertical direction, is arranged to surround the inner core region from an outer peripheral side, and has a plurality of second fuel pins accommodating an outer core fuel; and a sodium plenum provided above the inner core region and the outer core region, in which a dimension of the outer core fuel in the vertical direction is larger than a dimension of the inner core fuel in the vertical direction, and the position of a center of the outer core fuel in the vertical direction is higher than the position of a center of the inner core fuel in the vertical direction.

A nuclear reactor structure configuring a pebble accommodating space of a pebble bed type nuclear reactor includes a core material including graphite and a ceramic/ceramic composition material covering a surface of the core material. According to a core material processing step (A) of processing the core material including graphite into a quadrangular prism, a bottom surface of which is an approximately isosceles trapezoid, a step (B) of obtaining a base material by covering the core material with an aggregate including a ceramic fiber, and a CVD step (C) of putting the base material into a CVD reactor and forming a SiC matrix in gaps of the aggregate, thereby forming a ceramic/ceramic composite material on a surface of the core material, the nuclear reactor structure capable of enhancing durability, preventing cracking, etc. from occurring, and preventing exposure of graphite as the core material from occurring, can be provided.

A nuclear reactor structure configuring a pebble accommodating space of a pebble bed type nuclear reactor includes a core material including graphite and a ceramic/ceramic composition material covering a surface of the core material. According to a core material processing step (A) of processing the core material including graphite into a quadrangular prism, a bottom surface of which is an approximately isosceles trapezoid, a step (B) of obtaining a base material by covering the core material with an aggregate including a ceramic fiber, and a CVD step (C) of putting the base material into a CVD reactor and forming a SiC matrix in gaps of the aggregate, thereby forming a ceramic/ceramic composite material on a surface of the core material, the nuclear reactor structure capable of enhancing durability, preventing cracking, etc. from occurring, and preventing exposure of graphite as the core material from occurring, can be provided.

A system for transferring heat from a nuclear reactor comprises a nuclear reactor comprising a nuclear fuel and a reactor vessel surrounding the nuclear reactor and a heat transfer system surrounding the nuclear reactor. The heat transfer system comprises an inner wall surrounding the nuclear reactor vessel, first fins coupled to an outer surface of inner wall, an outer wall between the inner wall and a surrounding environment, and second fins coupled to an inner surface of the outer wall and extending in a volume between the outer surface of the inner wall and the inner surface of the outer wall, the outer surface of the inner wall and the first fins configured to transfer heat from the nuclear reactor core to the second fins and the inner surface of the outer wall by thermal radiation. The heat transfer system may be directly coupled to the nuclear reactor vessel, or may be coupled to an external reflector surrounding the nuclear reactor vessel. Related heat transfer systems and systems for selectively removing heat from a nuclear reactor are disclosed.