A BWR fuel assembly is elongated along a fuel assembly axis and comprises a lower tie plate, an upper tie plate axially spaced from the lower tie plate, a bundle of fuel rods extending axially between the lower tie plate and the upper tie plate, and a tubular fuel channel extending from the lower tie plate to the upper tie plate with encasing the fuel rods. The fuel assembly comprises an interaction device mounted on the lower tie plate and configured to interact with the fuel channel. The interaction device has an inactive configuration and an active configuration.

It is possible to achieve self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during a reactor operation. According to the present invention, the lower portion of the lower tie plate 7 as a part of the fuel assembly 3, which is inserted into the fuel support 9, extends, and a stable member 21 is provided around the extension portion 20, and thereby it is possible to achieve the self-support of the fuel assembly without the upper grid plate. In addition, since an increase in a weight due to extension of the lower portion of the lower tie plate 7 can prevent the floating during the reactor operation, a floating preventing mechanism using the upper grid plate is not necessary. Hence, it is possible to achieve the self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during the reactor operation.

A neutron absorber member is configured to be inserted into a control rod guide tube of a spent fuel assembly. The neutron absorber member includes a first plate and a second plate. The first plate extends along a longitudinal axis between a first end and a second end. The first plate has a slot formed therethrough at the first end. The slot extends from the first end towards the second end. The second plate is disposed within the slot of the first plate. The first plate and the second plate are arranged to have a cross-sectional shape of a cruciform.

It is possible to achieve self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during a reactor operation. According to the present invention, the lower portion of the lower tie plate 7 as a part of the fuel assembly 3, which is inserted into the fuel support 9, extends, and a stable member 21 is provided around the extension portion 20, and thereby it is possible to achieve the self-support of the fuel assembly without the upper grid plate. In addition, since an increase in a weight due to extension of the lower portion of the lower tie plate 7 can prevent the floating during the reactor operation, a floating preventing mechanism using the upper grid plate is not necessary. Hence, it is possible to achieve the self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during the reactor operation.

A nuclear reactor includes a plurality of mechanisms (11) that drive the contact members (9) that control the reactivity, of the core. Each mechanism includes a driving member (21) including a driving part (23) forming one out of a screw or a nut, a member (27) for applying a rotary torque of the rotor (19) to the driving member (21), a driven member (29) translationally connected to one of the control members (9) and including the other out of screw and a nut; and a member (33) that is selectively mobile between a position of blocking the driving member (21) and a position of releasing the driving member (21). In each drive mechanism (11), the motor (15) is fully immersed in the primary coolant inside the vessel (3); the rotor (19) has a central passage (35), the member for applying the rotary torque (27) being situated in or near the central passage (35); the driving member (21) includes a connecting part (37) engaged in the central passage (35) and collaborating with the member for applying the rotary torque (27), the connecting part (37) being free to effect a translational movement inside the central passage (35) with respect to the rotor (19) when the or each blocking member (33) is in the releasing position.

A nuclear reactor includes a plurality of mechanisms (11) that drive the contact members (9) that control the reactivity, of the core. Each mechanism includes a driving member (21) including a driving part (23) forming one out of a screw or a nut, a member (27) for applying a rotary torque of the rotor (19) to the driving member (21), a driven member (29) translationally connected to one of the control members (9) and including the other out of screw and a nut; and a member (33) that is selectively mobile between a position of blocking the driving member (21) and a position of releasing the driving member (21). In each drive mechanism (11), the motor (15) is fully immersed in the primary coolant inside the vessel (3); the rotor (19) has a central passage (35), the member for applying the rotary torque (27) being situated in or near the central passage (35); the driving member (21) includes a connecting part (37) engaged in the central passage (35) and collaborating with the member for applying the rotary torque (27), the connecting part (37) being free to effect a translational movement inside the central passage (35) with respect to the rotor (19) when the or each blocking member (33) is in the releasing position.

A neutron absorber member is configured to be inserted into a control rod guide tube of a spent fuel assembly. The neutron absorber member includes a first plate and a second plate. The first plate extends along a longitudinal axis between a first end and a second end. The first plate has a slot formed therethrough at the first end. The slot extends from the first end towards the second end. The second plate is disposed within the slot of the first plate. The first plate and the second plate are arranged to have a cross-sectional shape of a cruciform.

A nuclear reactor control rod with SiC fiber reinforced structure comprises wing sections and a central joint section. Each of the wing sections is a flat plate spreading axially and radially, and includes storage tubes and a wing surface structural member. The storage tubes are arranged in parallel in a flat plane and contain a neutron absorbing member containing the neutron absorbing material. The wing surface structural member is formed by molding of SiC/SiC composite material as to cover surfaces of the storage tubes and formed to have an outward shape of a flat plate. The central joint section and storage tubes are made of SiC/SiC composite material. The central joint section bundles the wing sections together at center. The storage tubes are bundled together with fibers made of SiC or a textile made of SiC.

A nuclear reactor control rod with SiC fiber reinforced structure comprises wing sections and a central joint section. Each of the wing sections is a flat plate spreading axially and radially, and includes storage tubes and a wing surface structural member. The storage tubes are arranged in parallel in a flat plane and contain a neutron absorbing member containing the neutron absorbing material. The wing surface structural member is formed by molding of SiC/SiC composite material as to cover surfaces of the storage tubes and formed to have an outward shape of a flat plate. The central joint section and storage tubes are made of SiC/SiC composite material. The central joint section bundles the wing sections together at center. The storage tubes are bundled together with fibers made of SiC or a textile made of SiC.