GB2104712A

GB2104712A - Nuclear reactors

Info

Publication number: GB2104712A
Application number: GB08224558A
Authority: GB
Inventors: John Earl Matheson
Original assignee: Babcock and Wilcox Co
Current assignee: Babcock and Wilcox Co
Priority date: 1981-08-26
Filing date: 1982-08-26
Publication date: 1983-03-09
Also published as: JPS648800B2; KR840001373A; FR2512256A1; JPS5842993A; KR850001388B1; DE3228971A1

Abstract

A nuclear reactor has an upper (15) and a lower grid plate. Protrusions (16) project from the upper grid plate (15). Fuel assemblies having end fittings (10) fit between the grid plates. An arrangement is provided for accepting axial forces generated during the operation of the nuclear reactor by the flow of the cooling medium and thermal expansion and irradiation-induced growth of the fuel assembly, which comprises rods (11). Each fuel assembly rests on the lower grid plate and its upper end is elastically supported against the upper grid plate (15) by the above-mentioned arrangement. The arrangement comprises four (for example) torsion springs (13) each having a torsion tube (26) and a torsion bar (25) nested within the torsion tube and connected at one end thereto. The other end of the torsion bar (25) is connected to an associated one of four lever arms (21, 22, 23, 24). The torsion tube (26) is rigidly connected to the other end fitting (10) and the springs (13) are disposed such that the lever arms (21, 22, 23, 24) are biassed against the protrusions (16). <IMAGE>

Description

SPECIFICATION Nuclear reactors The present invention relates to nuclear reactors and is more particularly concerned with holddown spring arrangements for securing fuel assemblies in the reactor.

Typically, a nuclear reactor for the generation of electrical power inciudes a core of fissionable material to heat a coolant flowing upwardly therethrough. The fissionable material is enclosed in elongate fuel rods assembled in a square array commonly called a fuel assembly and held in spaced parallel relationship by a number of spacer grids distributed at intervals along the length of the assembly. The fuel asemblies are held in an array by core grid plates at the top and bottom and are provided with upper and lower end fittings for mating with the grid plates.

Typically, holddown spring means is provided between the upper end fitting and the upper core grid plate to provide sufficient holddown force acting against hydraulic lift forces in the core. The spring means also allows for axial dimensional growth of the fuel assembly due to either differential thermal expansion or irradiationinduced material change. The problems of the design, therefore, lie in the ability to provide sufficient holddown force against lift while allowing sufficient room for growth. Sufficient material strength and stiffness must be available within constrained volumes. The stiffness/volume efficiency of a spring becomes very important when used for nuclear fuel holddown.

There are two types of spring currently in use as holddown devices or arrangements for fuel assemblies. The helical type spring is the most common, with the other type being the leaf spring arrangement. U.S. Patent No. 4 072 562 discloses a single torsion bar type design.

However, this design has never been utilised in a nuclear reactor.

Helical springs attempt to solve the problem of stiffness/volume efficiency by placing the spring at the centre of the upper end fitting of the assembly where the most available open volume exists. Drawbacks to this approach include the fact that the springs have to be carefully positioned to allow for access of control rods and that the springs are fully exposed to the coolant flow, thereby subjecting the spring to the dynamic stresses of flow-induced vibration.

Leaf spring designs are utilised by one manufacturer of nuclear fuel. However, since these designs tend to be stroke limited, it is necessary to join the fuel assembly structure with members that are less susceptible to irradiationinduced growth. Moreover, these leaf springs often have to be ganged to achieve sufficient holddown.

The torsion bar spring is a highly efficient device. For the same weight it can store four times the energy of an equally stressed leaf spring. However, a difficulty with using torsion bars is that sufficient dimension is not available in the fuel assembly to obtain sufficient flexure of the bar. Typically, fuel assemblies are only 200 mm (eight inches) square. Thus, the torsion arm is limited in size to something less than 200 mm (eight inches). The diameter of a bar which will provide sufficient holddown cannot be stroked enough to accommodate growth without failing if the length of the bar is limited to less than 200 mm (eight inches).

According to the present invention there is provided a nuclear reactor having upper and lower grid plates, a protrusion projecting from the upper grid plate, a fuel assembly having an end fitting and positioned between the upper and lower grid plates, and an arrangement for accepting axial forces, said arrangement comprising at least one torsion spring having a torsion tube and a torsion bar generally concentrically disposed within the torsion tube and connected at one end thereto and connected at the other end to a lever arm disposed generally perpendicular thereto, the torsion tube being rigidly connected to the end fitting, and the torsion spring being disposed such that the lever arm is biassed against the protrusion.

An embodiment of the present invention described hereinbelow overcomes or at least alleviates the disadvantage of the prior art devices to a large extent by providing a double-back torsion type holddown spring comprising a lever arm acting on a torsion bar which in turn acts upon a torque or torsion tube. This device is assembled in a "rod-in-shell" fashion. the lever arm is attached to the torsion bar which is attached through an interfacing connector to the torque tube. This interfacing connector may be an integral part of the torque tube or a separate part.

The entire device reacts through a restraint device on the lever arm end of the torque tube.

At least two major problems are solved or alleviated by the embodiment of the present invention described below. The first relates to previous torsion bar designs. Single bar torsion springs are limited in flexibility by the maximum installed length of the spring available within the upper end fitting structure. With the present arrangement, this is overcome by adding a concentric torque tube thus effectively doubling the torsion spring length within the same dimension. The resulting additional stroke also overcomes some of the limitations of leaf springs.

The second problem is that of flow-induced vibration associated with helical springs. Helical springs act like a cylinder to cross flow and vibrate due either to shedding of vortices or turbulent buffeting. This vibration fatigues the spring material and produces wear at coil interfaces. The double back torsion bar spring may be largely shrouded from the flow and will virtually eliminate or at least reduce flow-induced vibration concerns.

An advantage of the below-described embodiment of the invention is the elimination of the need to use helical springs for holddown where a longer stroke (i.e. greater effective length) is required than leaf springs can provide.

Torsion springs have none of the disadvantages of helical springs cited earlier. A further advantage is that the torsion springs can be fabricated without special spring winding equipment. They can generally be fabricated in any well equipped machine shop. In addition, the use of torsion type springs allows other modifications to the upper end fitting structure not possible with helical springs. Reduction of pressure drop can be achieved and it is possible to adopt the end fitting for full reconstitution in situ.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a partially cutaway elevational view of a fuel assembly upper end fitting employing holddown spring means and installed in a nuclear reactor according to a preferred embodiment of the invention; Figure 2 is a top plan view of the end fitting of Figure 1; Figure 3 is a perspective view of a double back torsion bar spring used in the preferred embodiment of the present invention; Figure 4 is a view taken along a line 4-4 of Figure 3; Figure 5 is an end view of the torsion bar of Figure 3; Figure 6 is a view like Figure 5 showing part of an alternative embodiment; and Figure 7 is an elevational view of an alternative form of the torsion bar of the arrangement of Figure 1.

Figure 1 shows a nuclear reactor fuel assembly including fuel rods 11 having longitudinal axes 12 held in spaced parallel relationship by spacer grids such as 14 and an upper end fitting 10. The fuel assembly is disposed in the core of a nuclear reactor between a lower core grid plate (not shown) and an upper core grid plate 1 5. The plate 1 5 is apertured to accept the end fitting 10 and has a protrusion 1 6 aligned to depress holddown spring lever arms 21, 22, 23 and 24 associated with respective springs 1 3. The relative positions of the lever arms 21, 22, 23 and 24 may be seen from Figure 2, which is a top plan view of the end fitting 10.

Referring back to Figure 1, one of the doubleback torsion springs 1 3 is shown disposed in a bore 1 7 of the end fitting 10. The spring 1 3 includes a torsion tube 26 and a torsion bar 25 disposed within the tube 26 and affixed thereto at one end thereof by a connector 27. The other end of the tube 26 is provided with a bushing 30 rigidly connected thereto and having splines 29.

The bushing 30 has a bore 38 for accepting the bar 25 extending therethrough. Formed in the open end of the bore 17 are spline slots 31 for receiving the splines 29 of the bushing 30. The lever arm 22 extends perpendicularly to the bar 25 such that depression thereof by the protrusion 16 will twist the bar 25 and tube 26. A block 37 is provided to accept the axial force exerted on the arm 22 and transmit the axial force to the fuel assembly.

Figure 3 shows the spring 1 3 associated with the arm 21. An elbow 39 is provided to reach the arm 21 over the bore 17 of the adjacent spring 1 3. The torsion bar 25 disposed within the torsion tube 26 is shown connected thereto by the connector 27. Figure 4 shows a section taken through the connector 27, which has an aperture 33 for mating with an end 28 of the bar 25. The aperture 33 and the arm 38 are shaped to preclude rotation therebetween. Figure 5 shows that in the preferred embodiment the aperture 33 and the arm 38 are square. Figure 6 shows an alternative embodiment in which the connector 27 has a grooved aperture 36 to receive splines 32 of the bar 28. Figure 4 shows welds 40 rigidly affixing the connector 27 to the torsion tube 26.

In operation, the depression of each lever arm 21, 22, 23 and 24 is resisted by the torsional spring force generated by the twisting of the torsion bar 25 and torsion tube 26 rigidly affixed thereto by the connector 27 and rigidly affixed to the bushing 30 held from twisting by the splines 29 received in the spline slots 31 of the end fitting 10. The axial force is transferred to the fuel assembly via the blocks 37. The four opposed springs 13 exert twisting moments on the fuel assembly which are mutually opposite, thus resulting in a zero twisting moment.

Figure 7 illustrates a further alternative embodiment which employes a retainer ring (not shown) disposed in the upper portion of the end fitting 10 and having arms 35 protruding through slots 34. The protrusion 1 6 bears upon the arm 35, which in turn bears upon a lever arm 20 at an inclined surface 19 thereof. As the arm 35 depressed the arm 20 to the position shown in phantom, the arm 35 slides along the surface 19, effectively increasing the stroke length of the arm 35 relative to a straight arm. This allows a stiffer spring material to be used.

Claims

1. A nuclear reactor having upper and lower grid plates, a protrusion projecting from the upper grid plate, a fuel assembly having an end fitting and positioned between the upper end lower grid plates, and an arrangement for accepting axial forces, said arrangement comprising at least one torsion spring having a torsion tube and a torsion bar generally concentrically disposed within the torsion tube and connected at one end thereto and connected at the other end to a lever arm disposed generally perpendicular thereto, the torsion tube being rigidly connected to the end fitting, and the torsion spring being disposed such that the lever arm is biassed against the protrusion.

2. A nuclear reactor according to claim 1, wherein said arrangement comprises four said torsion springs each positioned with the torsion tube and torsion rod aligned parallel with a corresponding edge of the end fitting.

3. A nuclear reactor according to claim 1 or claim 2, comprising a connector welded to one end of the torsion tube and apertured to accept said one end of the torsion bar, said one end of the torsion bar and the connector aperture being shaped to preclude rotation therebetween.

4. A nuclear reactor according to claim 3, wherein the torsion tube is disposed within a receiving bore in the end fitting.

5. A nuclear reactor according to claim 4, wherein a bushing is affixed to the non-connector end of the torsion tube, the bushing having at least one spline protrusion and the receiving bore being slotted to receive said at least one spline protrusion.

6. A nuclear reactor according to claim 5, wherein the end fitting is provided with a nurnber of longitudinally extending slots having a corresponding number of retaining ring arms extending therethrough, the ring arms being positioned to bear on the lever arms and the lever arms being provided with an inclined surface on which the ring arm can bear.

7. A nuclear reactor substantially as herein described with reference to Figures 1 to 5, or Figures 1 to 5 as modified by Figure 6 or Figure 7, of the accompanying drawings.