GB2221079A

GB2221079A - A method and apparatus for inhibiting thermal cycling

Info

Publication number: GB2221079A
Application number: GB8817164A
Authority: GB
Inventors: Dono Kwok Wai Tong
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1988-07-19
Filing date: 1988-07-19
Publication date: 1990-01-24
Also published as: GB8817164D0

Abstract

Thermal cycling during mixing of fluid streams of different temperatures, for example liquid metal streams issuing from fuel sub-assemblies in a fast reactor, is reduced by sub-dividing each stream into a plurality of jets before the streams meet. A flow-diving means 36 may be of cruciform shape, or cruciform with a superimposed circle, to provide four jets and may have a tapered inlet to reduce drag. Dividing the stream reduces eddy size. Other shapes such as a rod, or oval or triangular section, are possible. The dividing means may be provided above the fuel sub-assembly rather than within in as shown. <IMAGE>

Description

A method and apparatus for inhibiting thermal cycling This invention relates to the reduction of thermal cycling during the mixing of fluid streams of differing temperatures and more particularly, but not exclusively, in the mixing of liquid metal streams issuing from fuel element sub-assemblies of a liquid metal cooled nuclear reactor (hereinafter called wa Fast Reactor").

In a Fast Reactor liquid metal, which is usually sodium, is caused to flow upwardly through a multiplicity of fuel element sub-assemblies in parallel relationship in the core of the Reactor, and subsequently issues as liquid metal streams above the core where mixing occurs.

Liquid metal flowing through fissile fuel element sub-assemblies and breeder fuel element sub-assemblies issues at different respective temperatures and produces a phenomenon called "Thermal Striping" on above core components and structures in the Reactor. In Thermal Striping rapid temperature cycling and fluctuations as the liquid metal at different temperatures mixes leads to repeated straining of the surface of the above core components and structures, and if sufficiently severe leads to crack initiation and crack propagation during the life of the Reactor.Thermal striping has been well documented in the literature, an example being "Thermal striping in liquid metal cooled fast breeder reactors", by C Betts, C Boorman and N Sheriff, Second International Topical Meeting on Nuclear Reactor Thermal Hydraulics, Santa-Barbara, USA, January 1983, which is incorporated by reference herein.

Extensive research and development has been devoted to overcoming the effects of Thermal Striping in Fast Reactors, but the problem still remains and causes concern.

According to one aspect of the present invention there is provided a method of reducing rapid temperature cycling when two fluid streams at different temperatures are arranged to coalesce, the method comprising sub-dividing each fluid stream into a plurality of jets before the streams meet so as to enhance mixing of the fluid streams.

According to another aspect, there is provided apparatus for carrying out the method of the invention, the apparatus comprising a nuclear fuel element subassembly having an inlet port for liquid metal coolant at one end and an outlet port for liquid metal coolant at the other end thereof, the sub-assembly having flow divider means locatable at or near to the outlet port so that, in operation, the liquid metal issuing from the outlet port is sub-divided into a plurality of jets.

Preferably, the flow divider means divides the liquid metal into at least three jets. The flow-divider means might be of cruciform shape, or of cruciform shape with a superimposed circle thereon, although other shapes might be used.

Desirably, the cross-sectional shape of the flow-divider means is adapted to minimise the pressure drop in the fluid stream flowing therethrough.

The invention further includes a Fast Reactor having a multiplicity of nuclear fuel element sub-assemblies and breeder fuel element sub-assemblies therein, each said sub-assembly having an inlet port for liquid metal at one end and an outlet port for liquid metal at the other end thereof wherein flow divider means are located at or near each said outlet port to produce a plurality of jets of the liquid metal at the said outlet port and thereby enhance mixing of the liquid metal discharged from the outlet ports.

The invention will now be further described by way of example only with reference to the accompanying drawings, in which: Figure 1 shows in medial cross-section a diagrammatic representation of part of a nuclear fuel element sub-assembly; Figure 2 shows a sectional view on the line II-II of Figure 1, and Figure 3 shows a modification of the view of Figure 2.

Referring now to Figure 1, part of a nuclear fuel element sub-assembly 10 for a Fast Reactor is shown which conventionally comprises a wrapper 12 generally of hexagonal cross-section but with hollow cylindrical ends, and a multiplicity of fuel pins 14 arranged lengthwise within the wrapper 12 at a position intermediate an outlet port 16 of the wrapper 12 and a tapered inlet port 18 of the wrapper 12. The fuel pins 14 are shown located in known manner between an upper support grid 20 and a lower support grid 22 and the fuel pins 14 are spaced apart by conventional spacer grids (not shown) so as to allow liquid metal coolant to flow between the fuel pins 14.The inlet port 18 leads to a known gag 26 of cylindrical form comprising a series of perforated plates 28 and meshes 29 located within a hollow cylinder 31 and disposed one on top of the other so as to provide a number of ducts in the form of passageways 30 (only three are shown) which extend lengthwise of the wrapper 12. A circular mixing plate 35 having a radiused lower corner is disposed above the fuel pins 14 to enhance mixing of liquid metal coolant within the wrapper 12.

In use, liquid metal (eg sodium) flows into the wrapper 12 in the direction of arrow 'A' through the inlet port 18, upwardly through the passageways 30, and then upwardly around the fuel pins 14 before it discharges from the wrapper 12 through the outlet port 16.

At the outlet port 16 a flow divider 36 of the invention is disposed and as shown in Figure 2 is of cruciform shape. The flow divider 36 is formed from strip material, for example of the same material as the wrapper 12 which might be a stainless steel or a Nimonic high nickel alloy. The flow divider 36 divides the liquid metal at the outlet port 16 into four jets, and has a tapered edge 38 to reduce the drag imposed on the liquid metal.

In use in the Fast Reactor, a multiplicity of fuel element sub-assemblies 10 would be installed in a parallel array in the core of the Reactor. Typically some of the sub-assemblies 10 would contain fuel pins 14 having fissile material whilst other sub-assemblies 10 would contain fuel pins 14 having breeder material and be located so as to surround the fissile material sub-assemblies 10. The fissile material fuel pins 14 operate at a higher temperature than the breeder material fuel pins 14, hence the liquid metal is discharged at different temperatures from the fissile material fuel pins 14 and the breeder material fuel pins 14.Because of the effect of the flow dividers 36, a plurality of jets of liquid metal issue from each sub-assembly 10 with reduced size of eddies in comparison with those in a liquid metal stream issuing from a sub-assembly 10 without such a flow divider 36. This enhances mixing of the jets issuing from adjacent sub-assemblies 10 with a consequent reduction in Thermal Striping effects above the core of the Reactor.

Although a flow divider 36 of cruciform shape has been shown other shapes may be used, for example as shown in Figure 3 in which a circular strip 40 is incorporated in a cruciform shaped flow divider 42. It will also be appreciated that the flow divider may be formed of alternative shaped material for example of rod, or of oval or of triangular cross-section, such shapes being selected to minimise the drag imposed on the flow of liquid metal in each wrapper 12.

One advantage of the invention is that the flow divider is subjected to a substantially uniform temperature within the sub-assembly 10 which obviates any effect of thermal shock on the flow divider. By placing the flow divider at or near the outlet. port 16 of the sub-assembly 10, any re-combination of the liquid metal jets from a sub-assembly 10 before they mix with liquid metal jets from adjacent sub-assemblies is minimised.

Use of the flow divider within the sub-assembly 10 is the preferred form of the invention, but flow dividers positioned above at least some sub-assemblies 10 might be possible provided they sub-divided the liquid metal before mixing of the liquid metal occurred above the core of the Reactor.

It will be understood that the invention may be incorporated in or be associated with alternative nuclear fuel element sub-assemblies, and might also have applications outside the nuclear field.

Claims

1. A method of reducing rapid temperature cycling when two fluid streams at different temperatures coalesce, the method comprising sub-dividing each fluid stream into a plurality of jets before the streams meet so as to enhance mixing of the fluid streams.

2. Apparatus for reducing rapid temperature cycling when two fluid streams at different temperature coalesce, the apparatus comprising a nuclear fuel element sub-assembly having an inlet port for liquid metal coolant at one end and an outlet port for liquid metal coolant at the other end thereof, the sub-assembly having flow divider means locatable at or near to the outlet port so that, in operation, the liquid metal issuing from the outlet port is sub-divided into a plurality of jets.

3. Apparatus as claimed in claim 2, in which the flow divider means divides the liquid metal into at least three jets.

4. Apparatus as claimed in claim 1 or claim 3, in which the flow-divider means is of cruciform shape.

5. Apparatus as claimed in claim 2 or claim 3, in which the flow-divider means is of cruciform shape with a superimposed circle.

6. Apparatus for reducing rapid temperture cycling substantially as hereinbefore described with reference to and as shown in Figs 1 and 2, or Fig 3, of the accompanying drawings.

7. A method as claimed in claim 1 and substantially as hereinbefore described.

8. A fast reactor having a multiplicity of nuclear fuel element sub-assemblies and breeder fuel element sub-assemblies therein, each said sub-assembly having an inlet port for liquid metal at one erd and an outlet port for liquid metal at the other end thereof, wherein flow divider means are located at or near each said outlet port to produce a plurality of jets of the liquid metal at the said outlet port and thereby enhance mixing of the liquid metal discharged from the outlet ports.