Patent Number: 
Section: claims

1. A mitigation assembly for a nuclear reactor, comprising a box with a longitudinal axis, the box comprising a central portion in which corium circulates and an upper portion housing an upper neutron shielding device, wherein the upper neutron shielding device comprises an upper neutron shielding head supporting neutron absorbers, wherein the upper neutron shielding device comprises removable locking means configured to cooperate with the box, wherein the upper neutron shielding device comprises a slug being free to move in translation over a given travel distance, the locking means being configured to lock and unlock the upper neutron shielding head with the box by displacement of the slug along the longitudinal axis with a grab for extraction of the upper neutron shielding device with pawls of the grab attached in the slug,wherein the upper portion of the box 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 seal being formed between the sealing block and the internal surface of the box. 2. The assembly according to claim 1, wherein the upper portion of the box also comprises an intermediate sealing block located between the upper neutron shielding head and the cone-shaped sealing bock. 3. The assembly according to claim 2, wherein the cone-shaped sealing block and/or the intermediate sealing block are made of metal or contain neutron absorbers. 4. The assembly according to claim 1, wherein the space between the cone-shaped sealing block and the internal cone-shaped surface of the box is zero. 5. The assembly according to claim 1, wherein the tip of the cone of said cone-shaped sealing block has a rounded shape. 6. The assembly according to claim 1, wherein the locking means are composed of other pawls installed free to pivot in a vertical plane. 7. The assembly according to claim 6 wherein the box comprises an internal groove in which the pawls of the locking means can be inserted to form an upper stop for the upper neutron shielding device. 8. The assembly according to claim 1, wherein the slug comprises an internal groove in which the pawls of the grab can be attached. 9. The assembly according to claim 1, wherein the upper neutron shielding device comprises one or several hollow columns passing through the slug. 10. A nuclear reactor, comprising at least one mitigation assembly according to claim 1.