GB2168192A

GB2168192A - Gas cooled nuclear reactors

Info

Publication number: GB2168192A
Application number: GB08528863A
Authority: GB
Inventors: Davide Wilkie; Micheal Walter Parkin; Robin Heward Goldthorp
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1984-12-07
Filing date: 1985-11-22
Publication date: 1986-06-11
Also published as: GB8528863D0; GB2168192B

Abstract

The graphite sleeves of a fuel stringer (10) of a gas cooled nuclear reactor are formed with a rib/groove profile (32) on their external surfaces in order to increase the resistance of flow through an annular passage between the stringer (10) and the seal bore (18) of a channel (16) during raising or lowering of the fuel stringer from or into the reactor core. The ribbing may extend around or along the sleeve or may extend helically about each sleeve in multi- start fashion and may have an asymmetric profile so that the resistance to coolant flow is less when the stringer is in situ in the reactor core than when the stringer is being raised or lowered. <IMAGE>

Description

SPECIFICATION Gas cooled nuclear reactors This invention relates to fuel assemblies or stringers for gas cooled nuclear reactors in which the fuel is in the form of fuel elements, eg. urania pellets enclosed in tubular stainless steel cladding, grouped into clusters each enclosed within a graphite sleeve, the sleeves being coupled together end-to-end to form the fuel-bearing part of the stringer.

It is known to affect on-load refuelling of gas cooled reactors of this type, ie. a spent fuel assembly is removed from the reactor followed by insertion of a replacement fuel assembly, while the reactor continues to produce power.

One problem encountered in on-load fuelling is the lateral vibration of the suspended fuel stringer during removal or insertion thereof, such vibration being induced by the high density gaseous coolant flow through the annular gap between the fuel stringer and the channel in which the stringer is received in use. A major source of such vibration has been identified as being in the reduced diameter section of the channel known as the piston seal bore.

When the stringer occupies its in-core position, piston ring seals on the stringer engage with the seal bore so as to direct coolant flow in the annular gap downwards where it joins with the upward coolant flow through the centre of the fuel stringer. When the fuel stringer is lifted away from its lowermost incore position, coolant flow through the seal bore gap occurs.

One approach to solving the problem is disclosed in published Patent Application No 2114359 in which grooves are formed in the outer peripheral surface of the graphite sleeves. Another solution is disclosed in Patent Application No 21 20448A involving modification of the reactor design by the incorporation of a perforated metal liner above the seal bore. Of these two solutions, the former is preferred because it does not entail modification of the reactor itself.

In Patent Application No 2114359, the form of grooving disclosed is suitable for tripping vortices and is therefore adequate when the flow induced vibrations are attributable predominantly to vortex shedding. However, it has been found that another instability mechanism can arise particularly when the seal bore on its downstream side (as considered in the direction of coolant flow when the seal is broken during refuelling) joins the downstream part of the channel by means of a small taper angle, eg. of the order of 6". The latter instability mechanism appears to be a fluid elastic effect as the coolant flows across the seal bore and into the downstream larger diameter channel.

The object of the present invention is to provide an improved fuel stringer design which is suitable for the fluid elastic form of instability.

According to one aspect of the present invention there is provided in or for a fuel stringer of a gas-cooled nuclear reactor, a graphite sleeve having a rib/groove profile on its external surface over at least a major part of its length (and preferably substantially its entire length) so as to form a series of narrow rib formations which are spaced apart by wider groove formations so as to increase resistance to coolant flow passing through the seal bore during refuelling.

Although this does lead to reduction in vibration, the resulting change in resistance to coolant flow gives rise to the possibility of heat up of the graphite moderator of the reactor core which, in turn, may tend to reduce the working life of the moderator.

Accordingly the present invention contemplates a graphite sleeve with a rib/groove profile on its external surface over at least a major part of its length wherein the rib/groove formations are arranged so that their length dimensions have a component of extension longitudinally of the sleeve.

In an extreme case it is envisaged that the ribs and grooves may extend generally parallel to the axis of the sleeve. By arranging the rib and groove formations so that their lengths extend predominantly axially of the sleeve, the axial resistance to coolant flow may be less than in the case where the rib and groove formations extend circumferentially. Moreover, the resistance to peripheral flow of coolant around the sleeve may be increased; this is considered to be a factor relevant to vibration reduction since a source of vibration appears to stem from transfer of coolant from one side of the sleeve to the other and vice versa.

According to another aspect of the invention a graphite sleeve with a rib/groove profile on its external surface is characterised in that the ribbing is constituted by convolutions of at least one rib which extends helically about the sleeve. Where more than one helically extending sleeve is employed the ribs are arranged in multi-start fashion. The helix angle of the ribbing is preferably between 30 and 60 with respect to a radial plane. Typically it will be of the order of 45 .

According to a further aspect of the invention a graphite sleeve having a rib/groove profile on its external surface is characterised in that the ribbing extends generally helically about the sleeve and comprises at least two sections in which the ribbing and grooving creates swirling flows in opposite senses about the sleeve.

In this aspect of the invention, said sections are preferably axially successive with respect to the longitudinal axis of the sleeve, the arrangement being such that the ribbing of axially adjacent sections extends helically in op posite senses about the sleeve. There may be two sections for convenience of manufacture and the ribbing may be formed by machining helically extending grooves in multi-start fashion, one section being machined from one end of the sleeve towards the centre of the sleeve and the second section being machined from the opposite end of the sleeve and in such a way that its ribbing extends in the opposite sense to that of the first section.

According to yet another aspect of the invention, a graphite sleeve with a rib/groove profile on its external surface has a rib/groove configuration such that the resistance to coolant flow along the exterior of the sleeve in one direction is less than in the opposite direction.

The rib/groove configuration is preferably asymmetric in cross-sectional profile so that less resistance is presented to flow in said one direction. Thus, in use, the sleeve may be arranged so that the reduced flow resistance path corresponds to the direction of coolant flow when the stringer is in situ in the reactor.

The asymmetry of the rib/groove configuration may be such that the groove formations merge smoothly with one flank of the rib formations but meet the opposite flank of the rib formations at a discontinuity. The rib formations may have one flank less steeply inclined than the opposite flank.

Preferably the ratio of rib height to annulus passage width is of the order of 0.06 to 0.13.

The perpendicular spacing between successive rib formations is preferably at least several times greater than the rib height, preferably 5 to 10 times greater and most preferably of the order of 6 to 7 times greater. The ratio of rib width to rib height is preferably of the order of 0.5 to 2.

Various embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic view, partly-sectioned, showing the normal in-core disposition of a fuel stringer; Figure 2 is a similar view to Fig. 1 but showing the fuel stringer being withdrawn or inserted into the reactor channel; Figure 3 is an enlarged, partly sectioned, diagrammatic view of the seal bore portion of the channel; Figure 4 is a fragmentary axial view of the exterior profile of a graphite sleeve in accordance with one aspect of the invention; and Figures 5-8 illustrate alternative forms of rib/groove arrangements.

Referring to Figures 1 and 2, the reactor fuel channel for the fuel stringer 10 is defined by a round-section passageway 12 in the moderator structure 14 and a guide tube 16.

The guide tube 16 includes a seal bore 18 (shown enlarged in Figure 3) and piston rings 20 on the stringer 10 make sealing engagement with the bore 18 when the stringer is in its normal in-core position as shown in Figure 1. In this situation, the piston rings block upward coolant flow through the annular gap 22 between the seal bore 18 and the stringer.

Thus, coolant flows (indicated by arrows 24) through the moderator structure are constrained to join the main coolant flow (arrow 26) through the centre of the stringer.

When, as in Figure 2, the stringer is located away from its normal in-core position, the seal is broken and the high density CO2 coolant gas can flow past the seal bore 18 as indicated by arrow 28. In these circumstances, flow induced vibration of the stringer can occur and it has been found that the predominant instability mechanism depends upon the tapering section 30 downstream of the seal bore. Where the taper angle is of the order of 30 to the vertical, the instability mechanism is largely associated with vortex shedding and where the angle is shallower, eg. 6 , it is attributable mainly to the fluid elastic effect previously mentioned. In general, both instability mechanisms may be present at both the 6" and 30 angles but a different one has been found to predominate.

As shown in the embodiment of Figure 4, each graphite sleeve 31 is formed with a rib/groove profile on its exterior surface, the rib formations 32 being narrow in axial width and being spaced axially by several times their height, preferably 6 or 7 times the height.

Such an arrangement has been found to significantly reduce stringer vibration resulting from the fluid elastic effect. The ribs will also facilitate tripping of vortices and may therefore counteract the instability mechanism associated with vortex shedding. Typically, the ratio of rib height to annular passage width (ie. between the sleeve and the seal bore is in the range 0.06 to 0.13. The rib width (at its crest) to rib height ratio is typically 0.5 to 2.

In the embodiment of Figure 5 (which is a fragmentary radial sectional view), the rib/groove profile comprises longitudinally extending rib formations 32, again narrow in width compared with the width of the groove formations.

In the embodiment of Figure 6, the rib/groove profile comprises a number of continuous narrow ribs 32 extending helically about the sleeve in multi-start fashion and separated by a perpendicular spacing several times the height of the rib formations, preferably 6 to 7 times greater.

The helical arrangement of the ribbing affords the advantages of offering less resistance to axial flow of coolant compared with circumferential ribbing and of being more amenable to manufacture since continuous ribbing/grooving is more convenient to machine than a large number of circumferential or long itudinal ribs/grooves.

The ribbing in this and the other embodiments may be interrupted adjacent the upper end of the sleeve by a circumferential groove 35 for facilitating handling of the sleeve.

In the embodiment of Figure 7, helical ribbing is also employed but in this case the ribbing is arranged in two sections 40, 42 in which the rib/groove formations of section 40 extend in the opposite direction to those of section 42 so that the coolant is caused to swirl about each sleeve in opposite senses over sections 40, 42 thereby subjecting the sleeve to oppositely acting torques which tend to cancel one another.

In the embodiment of Figure 8, each graphite sleeve 31 is formed with an asymmetric rib/groove profile on its exterior surface. The rib formations 32 may extend circumferentially about the sleeve but preferably, as in Figure 6 or 7, the rib formations extend helically about the sleeve and are constituted by convolutions of a number of continuous helically extending ribs arranged in multi-start fashion.

As will be apparent from Figure 8, axial flow of coolant in the direction B does not impinge on any discontinuities at the exterior surface of the sleeve since the rib formations 32 and groove formations 33 merge with one another along one flank of the rib formations so as to provide a smooth flow surface, said one flank being of shallow inclination. In contrast, coolant flow in the opposite direction A will encounter the discontinuities 34 present at the more steeply inclined opposite flanks of the rib formations resulting in a substantial resistance to flow in this direction (which will correspond to the coolant flow direction in the annular gap 22 when the stringer is located away from its fully installed position).

Claims

1. A graphite sleeve for a fuel stringer of a gas-cooled nuclear reactor, characterised in that said sleeve has a rib/groove profile on its external surface over at least a major part of its length so as to form a series of narrow rib formations which are spaced apart by wider groove formations.

2. A sleeve as claimed in Claim 1 in which said rib and groove formations are arranged so that their length dimensions have a component of extension longitudinally of the sleeve.

3. A sleeve as claimed in Claim 1 or 2 in which said rib formations are constituted by convolutions of at least one rib which extends helically about the sleeve.

4. A sleeve as claimed in Claim 3 in which there are two or more of said ribs arranged in multi-start fashion.

5. A sleeve as claimed in Claim 3 or 4 in which the helix angle of the ribbing is between 30 and 60".

6. A sleeve as claimed in Claim 5 in which the helix angle of the ribbing is about 45 .

7. A sleeve as claimed in any one of Claims 3 to 6 in which the helical ribbing comprises at least two sections in which the ribbing and grooving creates swirling flows of coolant in opposite senses about the sleeve.

8. A sleeve as claimed in Claim 7 in which said sections are axially successive with respect to the longitudinal axis of the sleeve.

9. A sleeve as claimed in any one of Claims 1 to 8 in which the rib/groove configuration is such that resistance to coolant flow along the exterior of the sleeve in one direction is less than in the opposite direction.

10. A sleeve as claimed in Claim 10 in which the rib/groove configuration is asymmetric is cross-sectional profile so that less resistance is presented to flow in said one direction.

11. A sleeve as claimed in any one of Claims 1 to 10 in which the perpendicular spacing between successive rib formations is at least several times the rib height.

12. A sleeve as claimed in Claim 11 in which said perpendicular spacing is between five and ten times greater than the rib height.

13. A sleeve as claimed in Claim 12 in which said perpendicular spacing is between six and seven times the rib height.

14. A sleeve as claimed in any one of Claims 1 to 13 in which the ratio of rib width to rib height is of the order of 0.5 to 2.

15. A graphite sleeve for a fuel stringer of a gas-cooled nuclear reactor, substantially as hereinbefore described with reference to, and as shown in, Figures 1 to 3 together with any one of the remaining Figures of the accompanying drawings.

16. A fuel stringer for a gas cooled nuclear reactor comprising a plurality of sleeves coupled end-to-end and each enclosing a cluster of fuel pins, characterised in that each sleeve is constructed as claimed in any one of Claims 1 to 15.

17. A gas cooled nuclear reactor comprising at least one fuel stringer as claimed in Claim 16 accommodated in a coolant-conducting channel including a seal bore which is engaged by seals on the stringer when the latter is in situ within the reactor core.

18. A reactor as claimed in Claim 17 in which, with respect to the annular passage defined between the fuel stringer and said seal bore, the ratio of rib height to annular passage width is of the order of 0.06 to 0.13.