The main purpose of the invention is a mitigation assembly (40) for a nuclear reactor comprising a box (41) with an upper portion (41) forming the head of the assembly housing an upper neutron shielding (UNS) device, comprising a head (105) comprising removable locking means (100) and a slug (101) installed free to move in translation relative over a given travel distance, the locking means (100) being configured such that locking/unlocking between the head (105) and the box (41) can be made by displacement of the slug (101) by means of an extraction grab with its pawls attached in the slug (101), characterised in that the lower part of the upper neutron shielding device comprises a cone-shaped sealing block (107) with the tip of the cone oriented downwards, cooperating with a cone-shaped internal surface (108) of the box (41), a sealing device (109) being formed between the two, the assembly created forming a removable sealing plug (110).

A method of operating a nuclear fission reactor. The reactor comprises a reactor core, and a coolant tank containing coolant, the reactor core comprises an array of fuel assemblies. Each fuel assembly extends generally vertically and comprises one or more fuel tubes containing fissile fuel. The fuel tubes are immersed in the coolant. The method comprises monitoring and/or modelling fuel concentrations and/or fission rates in each of the fuel assemblies; and in dependence upon results of the monitoring and/or modelling, moving fuel assemblies horizontally within the array, without lifting the fuel tubes from the coolant, in order to control fission rates in the reactor core. A nuclear reactor implementing the method, and fuel assemblies for use in the method are also disclosed.

A method of operating a nuclear fission reactor. The reactor comprises a reactor core, and a coolant tank containing coolant, the reactor core comprises an array of fuel assemblies. Each fuel assembly extends generally vertically and comprises one or more fuel tubes containing fissile fuel. The fuel tubes are immersed in the coolant. The method comprises monitoring and/or modelling fuel concentrations and/or fission rates in each of the fuel assemblies; and in dependence upon results of the monitoring and/or modelling, moving fuel assemblies horizontally within the array, without lifting the fuel tubes from the coolant, in order to control fission rates in the reactor core. A nuclear reactor implementing the method, and fuel assemblies for use in the method are also disclosed.

A device for turning a container about a turning axis is provided. The device includes a stationary holding member and two bearing members which are spaced apart along the turning axis on each side of a space for receiving a container, and which are capable of being used for bearing on the container while enabling the rotation of the container about the turning axis relative to the bearing members. The device includes a movable holding member that is mounted onto the stationary holding member so as to rotate about the turning axis, at least one bearing member being supported by the movable holding member while being mounted onto the movable holding member so as to rotate about the drive axis, the or each bearing member supported by the movable holding member not rotating about the turning axis relative to the stationary holding member when the movable holding member rotates about the turning axis relative to the stationary holding member.