GB2071396A

GB2071396A - Structural shells

Info

Publication number: GB2071396A
Application number: GB8007462A
Authority: GB
Original assignee: Nuclear Power Co Ltd
Current assignee: Nuclear Power Co Ltd
Priority date: 1980-03-05
Filing date: 1980-03-05
Publication date: 1981-09-16

Abstract

A structural shell for housing a nuclear reactor fuel assembly carried from the wall of the shell is pressurised and has outwardly directed pre-formed protuberances at the load-points. The preformed protuberances are of proportions generally similar to the notional deflections that would be caused by the load in a normal membrane wall. Bending moments local to the load points are reduced. <IMAGE>

Description

SPECIFICATION Structural shells This invention reiates generally to structural shells but is primarily directed to steel shells for housing loads carried from the wall thereof.

It is known that curved thin plates or membranes are well suited for carrying distributed loads such as occur in roof structures of buildings and pressure vessels. When curved thin plates or membranes are required to carry concentrated loads, deflections local to the load points or load lines can occur thereby inducing severe bending stresses. In conventional shells, the stresses are reduced to acceptable levels by increasing the thickness of the plate or membrane local to the load points or load lines, but it is an object of the invention to provide a structural shell wherein increased wall thicknesses are avoided.

According to the invention, a structural shell carrying a concentrated load from a wall thereof is pressurised and is profiled to include protuberances formed local to the load points or load lines, the protuberances being directed away from the load.

The protuberances, which preferably correspond generally in shape and proportions to the notional deflection that would be produced in a generally similar shell without protuberances under the same load conditions, reduce the local bending moment to a value lower than would be induced in the generally similar shell and pressure in the shell provides a bending moment of opposite sign to counteract the bending moment induced by the load.

The invention finds application in a nuclear reactor containment shell for housing a nuclear fuel assembly carried from a wall of the shell, wherein the shell is pressurised with an inert gas or liquid coolant and is provided with protuberances local to the load points or lines and directed outwardly of the shell.

In a preferred nuclear reactor construction the nuclear fuel assembly is carried by a skirt from a generally spherical region of the containment shell and a circumferential protuberance local to the load line consists in two opposed frusto-conical surfaces disposed tangentially to the spherical surface.

The invention will be more readily understood from the following description of the shells of two alternative nuclear reactor constructions which embody the invention, the embodiments being described by way of example only and with reference to the accompanying diagrammatic drawings Figs 1 and 2 which are fragmentary sectional views.

In Fig 1 a vessel designated 1 represents a generally cylindrical shell open at its upper end and having a hemispherical lower end. A nuclear reactor fuel assembly is carried on a skirt 2 from the hemispherical region of the wall of the shell and is submerged in liquid alkali metal coolant.

The region of the shell local to the circumferential load line designated 'L' forms a protuberance to the profile of the shell the protuberance being directed outwardly and formed by two frusto conical sections each of which is tangential to the part spherical adjoining regions. In such a construction the load of the fuel assembly is resisted mainly by the hoop stress in the vessel along the load line, but also by the bending stress induced at the junction of the conical sections and by the membrane stiffness of the protuberance.

Consideration of the geometry shows that the membrane stiffness is greater than it would be in the part spherical membrane of a conventional shell (shown in broken line) so that the bending moment is reduced correspondingly. The pressure of the liquid metal on the conical sections provides a bending moment of opposite sense to that of the load so that the resultant bending stress is reduced.

Fig 2 shows a section of a similar shell having an open upper end and a generally hemispherical lower and wherein the lower end is fabricated from curved sections the intersection (designated 'I') of which is at a non-tangential point thereby providing a protuberance extending about the shell. The load line is arranged to coincide with the intersection of the sections.

It should be noted that for the optimum conditions a balance of the various parameters affecting stresses is required and the vessel geometry to be adopted will depend on the prevailing conditions but nevertheless both the described constructions provide an improved structural shell wherein the stresses induced by a concentrated load can be accommodated without the need for increased wall thickness local to the load line.

1. A structural shell carrying a concentrated load from a wall thereof, the shell being pressurised and profiled to include preformed protuberances local to the load points or load lines, the protuberances being directed away from the load.

2. A structural shell according to claim 1 the shell constituting a containment for a nuclear reactor fuel assembly which is carried from the wall of the shell, wherein the shell is pressurised with an inert gas or liquid coolant and is provided with preformed protuberances local to the load points or lines and directed outwardly of the shell.

3. A structural shell according to claim 2 wherein the nuclear fuel assembly is carried by a skirt from a generally spherical region of the containment shell and a preformed circumferential protuberance local to the load line consists in two opposed frusto-conical surfaces disposed

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Structural shells This invention reiates generally to structural shells but is primarily directed to steel shells for housing loads carried from the wall thereof. It is known that curved thin plates or membranes are well suited for carrying distributed loads such as occur in roof structures of buildings and pressure vessels. When curved thin plates or membranes are required to carry concentrated loads, deflections local to the load points or load lines can occur thereby inducing severe bending stresses. In conventional shells, the stresses are reduced to acceptable levels by increasing the thickness of the plate or membrane local to the load points or load lines, but it is an object of the invention to provide a structural shell wherein increased wall thicknesses are avoided. According to the invention, a structural shell carrying a concentrated load from a wall thereof is pressurised and is profiled to include protuberances formed local to the load points or load lines, the protuberances being directed away from the load. The protuberances, which preferably correspond generally in shape and proportions to the notional deflection that would be produced in a generally similar shell without protuberances under the same load conditions, reduce the local bending moment to a value lower than would be induced in the generally similar shell and pressure in the shell provides a bending moment of opposite sign to counteract the bending moment induced by the load. The invention finds application in a nuclear reactor containment shell for housing a nuclear fuel assembly carried from a wall of the shell, wherein the shell is pressurised with an inert gas or liquid coolant and is provided with protuberances local to the load points or lines and directed outwardly of the shell. In a preferred nuclear reactor construction the nuclear fuel assembly is carried by a skirt from a generally spherical region of the containment shell and a circumferential protuberance local to the load line consists in two opposed frusto-conical surfaces disposed tangentially to the spherical surface. The invention will be more readily understood from the following description of the shells of two alternative nuclear reactor constructions which embody the invention, the embodiments being described by way of example only and with reference to the accompanying diagrammatic drawings Figs 1 and 2 which are fragmentary sectional views. In Fig 1 a vessel designated 1 represents a generally cylindrical shell open at its upper end and having a hemispherical lower end. A nuclear reactor fuel assembly is carried on a skirt 2 from the hemispherical region of the wall of the shell and is submerged in liquid alkali metal coolant. The region of the shell local to the circumferential load line designated 'L' forms a protuberance to the profile of the shell the protuberance being directed outwardly and formed by two frusto conical sections each of which is tangential to the part spherical adjoining regions. In such a construction the load of the fuel assembly is resisted mainly by the hoop stress in the vessel along the load line, but also by the bending stress induced at the junction of the conical sections and by the membrane stiffness of the protuberance. Consideration of the geometry shows that the membrane stiffness is greater than it would be in the part spherical membrane of a conventional shell (shown in broken line) so that the bending moment is reduced correspondingly. The pressure of the liquid metal on the conical sections provides a bending moment of opposite sense to that of the load so that the resultant bending stress is reduced. Fig 2 shows a section of a similar shell having an open upper end and a generally hemispherical lower and wherein the lower end is fabricated from curved sections the intersection (designated 'I') of which is at a non-tangential point thereby providing a protuberance extending about the shell. The load line is arranged to coincide with the intersection of the sections. It should be noted that for the optimum conditions a balance of the various parameters affecting stresses is required and the vessel geometry to be adopted will depend on the prevailing conditions but nevertheless both the described constructions provide an improved structural shell wherein the stresses induced by a concentrated load can be accommodated without the need for increased wall thickness local to the load line. CLAIMS

3. A structural shell according to claim 2 wherein the nuclear fuel assembly is carried by a skirt from a generally spherical region of the containment shell and a preformed circumferential protuberance local to the load line consists in two opposed frusto-conical surfaces disposed tangentially to the spherical surface.

4. A structural shell substantially as hereinbefore described with reference to either Figure 1 or Figure 2 of the accompanying drawings.