Patent Number: 
Section: claims

1. A pressurised water nuclear reactor vessel comprising:an outer casing which comprises at least one cylindrical shell having a circular cross-section, with a vertical centre axis, and a dished bottom head which closes a lower end of the shell,a primary cooling fluid, in the casing,a core comprising a plurality of nuclear fuel assemblies, arranged inside the casing,a core support plate which is substantially perpendicular relative to the centre axis and which is arranged inside the casing beneath the core, a vessel bottom head space being delimited between the support plate and the dished bottom head, the support plate being perforated with holes for circulation of the primary cooling fluid which place the vessel bottom head space in communication with the core,means for channelling the primary cooling fluid from the inlet(s) as far as the vessel bottom head space,a calming device which is arranged beneath the support plate in the vessel bottom head space and which is capable of calming at least a portion of the primary cooling fluid conveyed by the channelling means before this fluid passes through the support plate, the calming device having, in directions perpendicular relative to the centre axis, dimensions of between 30% and 60% of the inner diameter of the shell,wherein the calming device comprises at least one calming plate which is substantially perpendicular relative to the centre axis, and a plurality of calming holes, the calming holes being provided in the calming plate and being capable of calming the primary cooling fluid by passing it through the calming holes and the calming plate and the calming holes have, along the centre axis, a height of between 15% and 40% of the maximum height of the vessel bottom head space, taken along this axis. 2. The vessel according to claim 1, wherein the calming plate is unitary. 3. The vessel according to claim 2, wherein the calming plate is a machined forged component. 4. The vessel according to claim 1, wherein the calming holes extend through the calming plate parallel with the centre axis and are distributed in the form of a square-mesh grid. 5. The vessel according to claim 4, wherein the calming holes have a square cross-section perpendicularly relative to the centre axis. 6. The vessel according to claim 5, wherein the calming holes are delimited at least by a first set of mutually parallel blades which belong to the calming plate, and by a second set of mutually parallel blades which belong to the calming plate, the blades of the first set being perpendicular relative to the blades of the second set. 7. The vessel according to claim 6, wherein the first and second set of mutually parallel blades are straight and each extend through the entire calming plate in a direction which is perpendicular relative to the centre axis. 8. The vessel according to claim 7, wherein the fuel assemblies are arranged in the core in accordance with a square-mesh grid, each square mesh within the square-mesh grid having an equal and predetermined side measurement, the mutually parallel blades of the first set being arranged in accordance with a whole multiple constant pitch of half of the predetermined side measurement, the mutually parallel blades of the second set also being arranged in accordance with a whole multiple constant pitch of half of the predetermined side measurement. 9. The vessel according to claim 6, wherein each of the mutually parallel blades in the first set and in the second set has a thickness of between 5 and 50 millimetres. 10. The vessel according to claim 6, wherein the calming device comprises columns for fixing the calming plate to the core support plate, these columns being rigidly fixed to nodes of the calming plate which are provided at the intersection of blades of the first and second sets assemblies. 11. The vessel according to claim 5, wherein the calming plate has, along the centre axis, a height of between 0.3 and 3.5 times the largest dimension of the cross-section of the calming holes, taken perpendicularly relative to the centre axis. 12. The vessel according to claim 1, wherein the calming device comprises a cylindrical portion which surrounds the calming plate and which is fixedly joined to the an outer edge of the calming plate. 13. The vessel according to claim 12, wherein the cylindrical portion extends along the centre axis beyond the calming plate towards the core support plate, over a height of between 25% and 100% of the height of the calming plate. 14. The vessel according to claim 1, wherein the calming holes are distributed over an entire surface cross section of the calming plate that is perpendicular to the central axis.