PATENT SPECIFICATION (11) 1502 427

(21) (22) Filed 3 June 1974

(23) rt (44) (51) w (52) G6C QFUVUXA

(72) Inventor ROBERT VICTOR MACBETH

(54) CHANNELS

(71) We, UNITED KINGDOM ATOMIC rods may be arranged in a ENERGY AUTHORITY, LONDON, a British bundle or group shrouded along its long- Authority, do hereby declare the invention, itudinal sides by a pressure tube or wrap- for which we pray that a patent may be per or shroud forming the nuclear reactor 5 granted to us, and the method by which it coolant channel employing a two phase 50 is to be performed, to be particularly de- liquid, such as light or , as scribed in and by the following coolant, One or more of the lattice posi- statement:— tions other than those in the outer row This invention relates to nuclear reactor may be occupied by an auxiliary coolant 10 coolant channels for certain types of nu- tube designed to inject coolant into the 55 clear reactors, wherein a plurality of elong- channel generally transverse of the main ated heat emitting members are supported coolant flow direction. Preferably the lat- within a channel through which a heat re- tice positions are in concentric annular ceiving liquid passes and as a result under- rows about a central lattice position oc- 15 goes boiling. Boiling in this context in- cupied by a central auxiliary cooling pipe. 60 cludes sub-cooled boiling, such as is al- The actual number of lattice positions in lowed in pressurised water cooled nuclear the inner rows may be an even number or reactors, as well as bulk boiling as occurs an odd number but preferably there is an in both boiling water nuclear reactors and odd number in every row, for example 20 in steam generating nuclear reactors. The, there may be 21 rods in the outer row, 15 65 invention aims to improve heat transfer be- lattice positions in an intermediate row and tween the heat emitting members and the 9 lattice positions in an inner row. A still fluid. further inner row of 3 lattice positions may According to the invention there is be provided. A single auxiliary coolant 25 provided a nuclear reactor coolant channel pipe may be located on the channel axis. 70 in which a liquid coolant flows and under- Any one or more lattice positions in the goes boiling, the channel having a channel intermediate rows may be occupied by an wall extending about a vertical axis, an auxiliary cooling tube or so called sparge inlet for liquid at the lower end and an pipe. It is considered expedient to follow 30 outlet for two phase liquid/vapour at the the practice to so space the rods that the 75 upper end, a plurality of similar nuclear minimum distance between the outer surface reactor fuel rods supported in the channel of the rods in the outermost row and the parallel to one another and to the channel inner surface of the channel wall is mar- axis in more than two annular rows and ginally less than the minimum distance be- 35 positioned by axially spaced grids, the fuel tween the outer surfaces of adjacent fuel 80 rod portions between the grids defining rods. Contributory to the gaining of ad- unobstructed passageways axially and vantages of the invention there may be transverse said axis, the said grids provided nuclear fuel rod support structure positioning an odd number of fuel rods in affording a plurality of lattice points in 40 the row of rods adjacent to the channel concentric rows, the structure being such 85 wall and in spaced relation to the channel that an odd number of lattice points occur wall. It is considered that the odd number in the outermost row, the remaining lattice of members in the outer row will avoid the points being distributed substantially evenly onset of a flow pattern in the coolant within the space bounded by said outer 45 which appears to limit the heat flux. The row. A variety of sequences in the num- 90 2 1502427 2

bers of lattice points in consecutive cir- used for demonstrating one non-binding hy- cular, or polygonal, rows are proposed e.g. pothesis which may explain the ad- a sequence of six: 27, 21; 15; 9; 3; gives vantageous operation of the invention. lattice points where every row is odd; a In both Figure 1 and Figure 2 of the 5 sequence of five: 25; 20; 15; 10; 5; where drawing the reference 1 indicates a cylin- 70 alternate rows are odd or an irregular drical wall of a coolant channel 2 which is sequence 19; 16; 7. Other sequences are open ended and defines a forced con- possible within the rule that at least the vection flowpath for coolant. Within the outer row has an odd number of lattice channel of Fig. 1 are supported at the lat- 10 points and these occupied by heat emitting tice positions indicated 36 heat emitting 75 rods. In the above sequences there may be rods 3 and within the channel of Figure 2, a central lattice position occupied by an 30 similar rods 3. The rods 3 are sup- auxiliary cooling tube. ported in parallel spaced apart positions as The wrapper or shroud need not be con- shown. Those in Figure 1 are arranged 15 tinuous. A number of such fuel rod bund- about a central pipe 4 to give flow sym- 80 les with shrouds make up a nuclear reactor metry with an even number of rods 3 in core. the outer row 5 adjacent the channel wall The invention and several embodiments 1. In Fig. 2 the rods are arranged so that thereof as applied to heat exchanger or nu- flow symmetry is discouraged and this is 20 clear fuel assemblies will now be described done by spacing an odd number of rods in 85 with reference to the accompanying draw- the outer row 5. The ends of each channel ings in which 2 were separately connected into a circuit Figure 1 shows in diagrammatic plan through which Freon (RTM) was pumped. view the relative lattice positions for a The rods, which were electrical conductors, 25 cluster of 36 heat emitting rods arranged in were heated by electrical resistance heating 90 accordance with the prior art and bounded and an inlet sub-cooling of 23.26 Joules by a cylindrical channel wall; per gram of the Freon was permitted. The Figure 2 shows a view similar to Figure dry out power of the rods was measured 1 showing lattice positions for a cluster of and plotted against mass flow rate (kg/sec) 30 30 heat emitting rods arranged in accord- of Freon as abscissa and it was found that 95 ance with the invention, the rods being dis- the arrangement of Fig. 1 gave a con- posed in concentric rows with an annular sistently lower dry out power than rods outer row of 15 rods adjacent the cylin- arranged as shown in Fig. 2. To visually drical channel wall; determine the effects of the two arrange- 35 Figure 3 shows a graph illustrating some ments on the flow pattern of coolant in JQO comparative trials to assess dryout when forced convection two phase flow, a further the rods arranged at lattice positions as in investigation was made using a coolant Figure 1 and Figure 2 are cooled by forced channel having a transparent panel for vi- convection cooling; sual inspection. The overall channel was 40 Figures 4 and 5 are tracings made from necessarily smaller diameter due to the 105 photographs taken through a transparent pressure involved and contained a single window in a coolant channel wall to show annular row of heat-emitting rods in Freon. in Figure 4 the flow conditions set up with Figure 4 shows the bubble pattern which an outer annular row containing an even was photographed through the transparent 45 number of rods and in Figure 5 the flow panel in the channel wall containing the 110 conditions set up with an outer annular even numbered row and shows a spiral row containing an odd number of rods; bubble pattern with large floppy vapour Figure 6 shows diagrammatically an end bubbles. Figure 5 is the corresponding view view of a cluster of a nuclear fuel rods with an odd number of rods in the annular 50 within a pressure tube for a nuclear re- row adjacent the channel wall. A stiff flow 115 actor of the steam generating, heavy water of uniformly sized bubbles is observed. moderated type, the rods being based on Apart from the difference in the number of the lattice configuration show in Figure 2; rods in the row adjacent the channel wall, Figure 7 shows diagrammatically an end every attempt was made to maintain iden- 55 view of two adjacent channel walls bound- tical conditions during each of these in- 120 ing flow channels containing lattice points vestigations. in polygonal rows suitable for use in a so Those familiar with the art will already called boiling or pressurised water nuclear be aware of the validity of Freon model- reactor; ling for obtaining, via empirical scaling 60 Figure 8 shows diagrammatically an end laws, heat transfer data applicable to for- 125 view of a channel wall bounding a flow ced convection two phase flow. Con- channel containing 48 uniformly distributed firmation of this is to be found in ASME lattice points in concentric rows, each row Publication 70-HT-20 which contains a containing an odd number of lattice points; paper "The Use of Freon 12 to model con- 65 Figures 9a, 9b, 9c and 9d are diagrams vection burn out in Water" (G. F. Stevens 130 2 1502427 3

and R. V. Macbeth). This paper is also of positions 35 are grouped about the channel interest because it discusses the modelling axis X. A central sparge pipe for auxiliary technique applied to a 36 rod bundle of coolant may occupy lattice positions 35 or heat emitting nuclear fuel rods correspond- one or more positions 35 may be occupied 5 ing to that referred to above as belonging by nuclear fuel rods. Of course, fuel rods 70 to the prior art. will occupy most of the lattice positions in In pursuance of the foregoing, the in- rows 33 and 34 but some of these posi- ventor has pointed to a parameter in nu- tions may be occupied by auxiliary coolant clear fuel bundle design which may be va- supply pipes instead of fuel rods. 10 tied quite simply with the reasonable ex- For the sake of completeness a sector of 75 pectation of improved dry out per- the assembly in Figure 8 is shown with formance. This is the number of fuel rods part of the fuel rod support grid. The grid which occupy the outer row of the bundle in 36 is composed of tubular ferrules 37 close adjacency to the channel wall, the linked by metal strips 38 to one another 15 latter being a pressure tube, a wrapper or and to an outer band 39. Each ferrule 37 80 a shroud. locates a fuel rod 37a by means of resilient I. A nuclear fuel assembly as shown in supports at its correct lattice position. Figure 6 comprising thirty nuclear fuel The satisfactory results obtained by rods 10 supported, axes parallel, by grids using the invented assembly one to 20 not shown within a cylindrical pressure look for a theory or a hypothesis which 85 tube 11. The grids have spaces to define will explain the remarkable results. So far 15 lattice positions in the outer annular the inventor's work in this direction is in- row 12 and these are occupied by nuclear complete and although he does not wish to fuel rods; the remaining lattice positions be bound in any way by this or any

25 are based on a pentagonal pattern about a particular theory, he wishes to offer the 90 central tube 13. The ends of the pressure following hypothesis. tube are connected into a forced con- Consider for example a typical prior art vection coolant channel in which boiling of cluster of nuclear fuel rods in Figure 9a water generates steam in contact with the having twelve heat emitting rods 40 on lat- 30 rods. tice positions in the outer row 41 adjacent 95 II. A nuclear fuel assembly as shown the tubular shroud 39, six rods in the in- in Figure 7 is suitable for use on a boiling termediate row 42 and one rod or sparge water nuclear reactor, or in a pressurised pipe 43 at the centre of the cluster. Now water nuclear reactor operated with a de- the coolant area around the rods in the 100 35 gree of sub-cooled boiling. The assembly outer row may be divided as shown in comprises an external tubular square sec- Figure 9b as including areas A and areas tion wrapper 20 of thin zirconium alloy B. The vapour formed in regions such as B sheet which is carried by a grid (not must tend to move out of these regions in shown). The interior of the wrapper 20 is order to follow its natural tendency to 40 divided diagonally by a plate 21 so for- achieve a more even distribution. Although 105 ming two flow channels 22 and 23. Grids one might expect the resultant vapour (not shown) provide in each channel 22 or bubbles to divide equally and to pass into 23 an outer annular row of twenty-one lat- adjacent regions A' and A", there are tice positions, an intermediate row of grounds for questioning this assumption 45 twelve positions and an inner row of 3 and for preferring to believe that the bubb- HO positions. Nuclear fuel rods 24 occupy at les will all move one way first to A' and then least the lattice points in the outer annular after a short period pass wholly into A". row which lie on a polygon and at any This oscillatory behaviour will be super- selected ones of the other lattice points imposed on the flow direction of the bulk of 50 there may be positioned absorber rods, the coolant parallel to the axes of the fuel 115 poison rods or auxiliary cooling sparge rods. Support for this hypothesis may be tubes instead of fuel rods. The corners 27 gained from observing the behaviour of gas of the wrappers may be chamfered as bubbles injected into the base 51 of reduced shown to give common core space for con- diameter of U-tube 50 and partially filled 55 trol members etc. as desired. with liquid. See Fig. 9c. Before gas injection 120 III. A nuclear fuel assembly as shown both vertical limbs 52, 53 will contain in Figure 8 having unique oddness. The liquid to the same height. When gas is in- channel wall 30 defines a channel 31 with jected into the base of the tube at roughly forty-eight lattice positions distributed its mid-point, the gas bubbles will be ob- served passing first up to the liquid surface 125 60 uniformly across the flow area. These are m one limb and then to reverse and pass arranged in concentric annular rows with wholly into the other limb similarly reach- 21 positions in the outermost annular row ing the liquid surface. This action will re- 32 adjacent the channel wall 30; 15 posi- peat at a definite period of about a few tions in the adjacent row 33, and 9 posi- seconds. The reason why this oscillation 130 65 iions in the row 34. A further three lattice 4 1502 427 4

occurs is easily deduced by inspection. which a liquid coolant flows and undergoes When the gas first enters the U-tube a boiling, the channel having a channel wall slightly greater amount of gas goes in one extending about a vertical axis, an inlet for direction than in the other. Buoyancy liquid at the lower end and an outlet for 5 forces created by the gas very quickly two phase liquid/vapour at the upper end, 70 build up to drive water in the U-tube in a plurality of similar nuclear reactor fuel this preferred direction until the buoyancy rods supported in the channel parallel to forces in the aerated U-tube limb are just one another and to the channel axis in counter balanced by the difference in more than two annular rows and 10 gravitational head created. Inertia effects positioned by axially spaced grids, the fuel 75 cause the system to go slightly beyond this rod portions between the grids defining equilibrium position however which leads unobstructed passageways axially and to a flow reversal and gas injected then transverse said axis, the said grids swings over to the other limb of the U- positioning an odd number of fuel rods in 15 tube. So the oscillation is established. Con- the row of rods adjacent to the channel 80 sideration of this oscillation in flow cur- wall and in spaced relation to the channel rents from a bubble generating region wall. within a liquid suggests comparison with 2. A coolant channel as claimed in the three adjacent sub-channels A' B A" claim 1 in which the channel wall is a 20 which in 3-dimensional form may be cylinder and the nuclear fuel rods are sup- 85 analogous to a plurality of U-flow paths ported within the cylinder in a number of with the two limbs of the U-tube re- co-axial annular rows, there being an odd presenting the sub-channels A', A", and number of fuel rods in each row. the connecting base of the U-tube re- 3. A coolant channel as claimed in 25 presenting the sub-channel B. claim 2 in which there are 21 rods in the 90 A similar situation may potentially exist outermost row 15 in the next second row in any adjoining group of sub-channels A, and 9 rods in the third inner row. B at any position throughout the rod 4. A coolant channel as claimed in claim cluster. It only requires the oscillations to 3 in which a fourth, innermost row has three 30 synchronise with each other to produce a rods. 95 pattern shown in Figure 9a and 9d which 5. a coolant channel as claimed in any illustrate as far as is possible in two of claims 2, 3 or 4 in which the rods are dimensions the flow patterns which would centred around a sparge pipe, for supply of suggest themselves in a fuel rod cluster auxiliary coolant to the channel. 35 having an even number of rods in the per- 6. A coolant channel as claimed in 100 ipheral, annular row. The arrows in Figure claim 1 in which there are alternate odd 9a show the synchronisation of the flow and even numbers of nuclear fuel rods in directions of transverse motion when alternate rows. viewed in plan whilst Figure 9d shows the 7. A coolant channel as claimed in 40 appearance of travelling sine wave motion claim 6 in which the numbers of fuel rods 105 produced as the coolant flows generally in consecutive annular rows are 5-10-15 or axially along the periphery of the cluster. 3-12-21. These flow lines 44 indicate a degree of 8. A coolant channel as claimed in coolant stagnation and even local flow re- claim 2 in which the minimum distance be- 45 versals. tween the cylinder and the surface of the 110 The superposition of flow directions on fuel rod in the outermost row is less than an odd-cluster will demonstrate inability to the minimum distance between adjacent achieve synchronism around the periphery fuel rods in the channel. of the cluster and hence transverse oscil- 9. A coolant channel as claimed in 50 lations are muted. claim 1 in which on any cross-section nor- 115 Bearing in mind the photographic mal to the axial length of the rods the evidence illustrated in Figures 4 and 5, the rods are uniformly spaced over the coolant conclusion that a rod bundle assembly with flow area. an odd number of rods in the outer row 10. A coolant channel as claimed in 55 offers the advantage of higher dryout claim 1 containing an assembly of nuclear 120 powers and lower hydraulic resistance in fuel rods spaced at a number of positions water cooled reactors in which boiling on a regular lattice with some lattice posi- occurs appears to be if not inescapable at tions occupied by auxiliary cooling con- least salutary. duits in rows other than the outer row. 60 In this specification "outermost row" 11. A coolant channel as claimed in 125 means the sum total of the closely spaced claim 1 containing an assembly of nuclear fuel rods which occupy positions on the fuel rods in which there are an odd num- periphery of rod cluster. ber of lattice positions in the outer row oc- WHAT WE CLAIM IS: cupied by nuclear fuel rods and alternately 65 1. A nuclear reactor coolant channel in even and odd numbers of lattice positions 130 5 1502 427 5

on the other rows; all the lattice positions as claimed in claim 10 substantially as de- in the other rows being occupied, the scribed with reference to Figures 2, 6, 7 or majority by nuclear fuel rods and a minor- 8 of the accompanying drawings. 15 ity by auxiliary coolant supply pipes 14. In a nuclear reactor coolant channel 5 arranged to inject coolant amongst the nu- as claimed in claim 1 nuclear fuel rod sup- clear fuel rods. port structure comprising a grid sub- 12. A nuclear reactor coolant channel stantially as described with reference to as claimed in claim 1 and substantially as Figure 8 of the accompanying drawings. 20 described herein with reference to Figures 10 2, 6, 7 or 8 of the accompanying drawings. M. T. HUNT 13. A nuclear reactor coolant channel Chartered Patent Agent containing an assembly of nuclear fuel rods Agent for the Applicant.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd., Berwick-upon-Tweed, 1978. Published at the Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained. 502 885 COMPLETE SPECIFICATION C PUCCTC This drawingming iiss aa reproduction of 0 3nccl') the OriginalIglnal on ca reduced scale Sheet 1

FIG. 1. FIG. 2.

j?-

FIG.7. FIG. 6. 2Z ^ ff rfJ s. 502 885 COMPLETE SPECIFICATION

5 SHEETS drawinB >s 0 reproduction of the Original on a. reduced scale Sheet 2

FIG. 3. rk 1

I

/ 2 J 4 502 885 COMPLETE SPECIFICATION 5 SHEETS drawing is a reproduction of the Original on a reduced scale Sheet 3 502 885 COMPLETE SPECIFICATION

; CLJCCTC ^is drawing is a reproduction of the Original on a reduced scale Sheet 4

FIG.8. 502 885 COMPLETE SPECIFICATION c This drawingawing is aa reproduction of 3 bHtfc I a t/,e Originaliginal onon ca reduced scale Sheet 5