Mitigation assembly for nuclear reactor comprising a removable sealing plug

Mitigation assembly for a nuclear reactor, comprising a box with a longitudinal axis designed to be inserted vertically in the diagrid of the reactor, the box comprising a central portion in which corium circulates and an upper portion forming the head of the assembly housing an upper neutron shielding device, that comprises an upper neutron shielding head supporting neutron absorbers, and comprising removable locking means with the box and a slug forming part of the upper neutron shielding head, the slug being free to move in translation relative to the remaining part of the upper neutron shielding head over a given travel distance, the locking means being configured such that locking and unlocking between the upper neutron shielding head and the box can be made by displacement of the slug along the longitudinal axis by means of a grab for extraction of the upper neutron shielding device using the pawls of the grab attached in the slug,

• wherein the upper portion of the mitigation assembly also comprises a cone-shaped sealing block with the tip of the cone oriented towards the bottom of the box, cooperating with a cone-shaped internal surface of the box, a sealing device being formed between the sealing block and the internal surface of the box, the assembly comprising the upper neutron shielding head and the sealing block forming a removable sealing plug of the mitigation assembly.