A control rod drive mechanism (CRDM) includes a lifting rod supporting a control rod and a holding mechanism comprising an electromagnetic circuit with magnetic poles drawn together when the electromagnetic circuit is energized to hold the lifting rod. The hold is released upon de-energizing the electromagnetic circuit. A translation mechanism linearly translates the lifting rod held by the holding mechanism. The holding mechanism may include a non-magnetic spacer between the magnetic poles that defines a gap between the drawn together magnetic poles. The translation mechanism may include latches configured to engage an upper end of the lifting rod, and the holding mechanism draws the magnetic poles together to hold the latches engaged with the upper end of the lifting rod. A four-bar cam assembly may be used to cam the latches closed in response to a vertical actuation force applied to the cam bars.

A control rod drive mechanism (CRDM) includes a lifting rod supporting a control rod and a holding mechanism comprising an electromagnetic circuit with magnetic poles drawn together when the electromagnetic circuit is energized to hold the lifting rod. The hold is released upon de-energizing the electromagnetic circuit. A translation mechanism linearly translates the lifting rod held by the holding mechanism. The holding mechanism may include a non-magnetic spacer between the magnetic poles that defines a gap between the drawn together magnetic poles. The translation mechanism may include latches configured to engage an upper end of the lifting rod, and the holding mechanism draws the magnetic poles together to hold the latches engaged with the upper end of the lifting rod. A four-bar cam assembly may be used to cam the latches closed in response to a vertical actuation force applied to the cam bars.

A stationary control rod that controls overall nuclear reactivity and axial reactivity distribution of a fuel assembly, such that power level and axial power distribution within the fuel assembly is controlled without the need for movable control rods and associated hardware. The device uses magnetic fields to control the concentration and distribution of a magneto-rheological fluid containing a material with a very high neutron capture cross section, contained in one or more enclosed thimbles placed within existing thimbles in a fuel assembly. The magnetic fields are generated from electricity produced from interactions of the radiation particles within the core, or supplied using electrical cables that attach to fuel assembly top nozzles. The electricity drives a device that encloses associated wire coil assemblies that surround different axial regions of a tube that contains the magneto-rheological fluid.

A control rod guide frame comprises a self supporting stack of two or more columnar elements defining a central passage. The columnar elements may include mating features that mate at abutments between adjacent columnar elements of the stack. The control rod guide frame is suitably used in conjunction with a control rod drive mechanism (CRDM) operatively connected with at least one control rod, and a nuclear reactor core, in which the CRDM moves the at least one control rod into and out of the nuclear reactor core under guidance of the control rod guide frame. In another embodiment, a control rod guide frame comprises a stack of two or more columnar elements defining a central passage having a constant cross-section as a function of position along the central passage. In another embodiment, a control rod guide frame comprises an extruded columnar element providing continuous control rod guidance.

A control rod guide frame comprises a self supporting stack of two or more columnar elements defining a central passage. The columnar elements may include mating features that mate at abutments between adjacent columnar elements of the stack. The control rod guide frame is suitably used in conjunction with a control rod drive mechanism (CRDM) operatively connected with at least one control rod, and a nuclear reactor core, in which the CRDM moves the at least one control rod into and out of the nuclear reactor core under guidance of the control rod guide frame. In another embodiment, a control rod guide frame comprises a stack of two or more columnar elements defining a central passage having a constant cross-section as a function of position along the central passage. In another embodiment, a control rod guide frame comprises an extruded columnar element providing continuous control rod guidance.

A method is provided for operating a nuclear reactor. The method includes operating the nuclear reactor for at least one plutonium equilibrium cycle during which the core contains plutonium-equilibrium nuclear fuel assemblies; subsequently, operating the reactor for transition cycles, at least some of the plutonium-equilibrium nuclear fuel assemblies being progressively replaced with transition nuclear fuel assemblies and then with uranium-equilibrium nuclear fuel assemblies; and then operating the nuclear reactor for at least one uranium equilibrium cycle.

The present disclosure is generally related to methods, devices and systems for improving the performances of a Rod Cluster Control Assembly (RCCA) and/or a Control Element Assembly (CEA) to mitigate clad strain, especially in the high fluence region, during normal operation conditions and accident conditions. One method may include incorporating a device such as a powder collection and blockage device between the ceramic upper and ceramic lower absorber materials of the RCCA and/or CEA. Another method may include increasing the plenum volume by incorporating an axial hole into the top end plug extension. Another method may include increasing the plenum volume by incorporating an axial hole into the bottom end plug and optionally incorporating radial grooves in the bottom of the lower absorber material to provide a flow channel for gas expansion or generation to ensure that the lower absorber does not block the opening in the bottom end plug.

The present disclosure is generally related to methods, devices and systems for improving the performances of a Rod Cluster Control Assembly (RCCA) and/or a Control Element Assembly (CEA) to mitigate clad strain, especially in the high fluence region, during normal operation conditions and accident conditions. One method may include incorporating a device such as a powder collection and blockage device between the ceramic upper and ceramic lower absorber materials of the RCCA and/or CEA. Another method may include increasing the plenum volume by incorporating an axial hole into the top end plug extension. Another method may include increasing the plenum volume by incorporating an axial hole into the bottom end plug and optionally incorporating radial grooves in the bottom of the lower absorber material to provide a flow channel for gas expansion or generation to ensure that the lower absorber does not block the opening in the bottom end plug.

A nuclear reactor having an upper internals control rod assembly guide tube formed from upper and lower sections that are connected along a central axial region of the guide tube at an intermediate coupling. An extended control rod axial support is provided for at least some of the control rods over a finite distance within at least one of the interiors of the lower guide tube section or the upper guide tube section.

A nuclear reactor having an upper internals control rod assembly guide tube formed from upper and lower sections that are connected along a central axial region of the guide tube at an intermediate coupling. An extended control rod axial support is provided for at least some of the control rods over a finite distance within at least one of the interiors of the lower guide tube section or the upper guide tube section.