GB2091932A

GB2091932A - Nuclear reactor having fuel assemblies with controllable coolant flow therethrough

Info

Publication number: GB2091932A
Application number: GB8138560A
Authority: GB
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1981-01-23
Filing date: 1981-12-22
Publication date: 1982-08-04
Also published as: GB2091932B; DE3201960A1; JPS57142593A; FR2498801A1; FR2498801B1

Abstract

A nuclear reactor which includes in a vessel, fuel assemblies supported on a core support structure and having nozzles 5 extending into openings in the support structure 6. The nozzles and support structure have cooperating passages (2, 3, 7, 8, 9) which are in alignment in predetermined angular positions of the fuel assemblies to permit control of coolant flow through the assemblies. <IMAGE>

Description

SPECIFICATION Nuclear reactor having fuel assemblies with controllable coolant flow therethrough Nuclear reactors have a number of fuel and blanket assemblies arranged to form a reactor's core. The reactor core is cooled by the passage of a cooling medium through the assemblies. In several reactor designs, it has been found desirable to provide each individual assembly with an orifice, usuaily located at the assembly inlet.

Such orifices perform several functions. In reactor designs in which the coolant flow is pump impelled, the orifice may be sized such that the pressure drop across the core is predominantly the pressure drop attributable to the orifice, such that the total pressure drop and total core coolant flow are only weakly variable with core conditions such as power level. The relative orifice sizes of assemblies can be varied to appropriately distribute coolant flow between assemblies of relatively varied power levels.

One current orifice design is a system in which the orifice is fixed in a casting which supports four fuel assemblies. The orifice in this system can be changed only by removal and replacement of the entire casting, a major task. Another current design utilizes the inlet nozzle of the fuel assembly itself as the orifice.

In this design, the orifice can be changed only by the replacement of the fuel assembly. In no known existing reactor design is the orifice easily replaceable or easily variable in size.

But significant advantage can be obtained by the use of a variable-size orifice in individual assemblies. The power generation distribution between fuel assemblies of a reactor core varies with core life, power level, control rod insertion pattern, and a number of other plant parameters. The operator of the plant could monitor the core power distribution and change the orificing pattern accordingly, diverting relatively more flow to the high temperature fuel assemblies and thereby reducing the peak core assembly temperature. This can be expected to increase the usable life of fuel assemblies.

Blanket assemblies contain a fertile species of nuclei which absorb neutrons from the nuclear reaction in fuel assemblies and become fissile nuclei suitable for an energy producing fission reaction. In early core life, these blanket assemblies should require only relatively little cooling flow, but will need more as the buildup of the fissile species progresses. Such blanket assemblies could also be equipped with a variable size orifice; small in size at the beginning of core life, bigger later.

It is therefore the principal object of the present invention to provide fuel and blanket assemblies with a variable-size orifice which will allow occasional variation of effective orifice size over core life.

With this object in view, the present invention resides in a nuclear reactor comprising a core disposed in a vessel and consisting of fuel assemblies supported on a core support structure and means for passing coolant through said support structure and said assemblies for removing the heat generated therein during operation of the reactor, said fuel assemblies having protruding coolant nozzles received in openings in said support structure and having- radial coolant flow passages, characterized in that said nozzles and the walls defining said nozzle receiving openings have a number of coolant flow passages so arranged that predetermined ones of said passages are closed or open depending on the angular position of said nozzles in said nozzle receiving openings.

The effective orifice size is adjustable by the rotation of the assembly with respect to its support module.

The assembly may be round or polygonal in cross-section, but in any case has some provision for a definite number of fixed alignment positions with respect to the module and some means for the locking or securing of this orientation during operation. The arrangement of holes in the module and nozzle provides for a selection of effective orifice sizes based on selection of the alignment position to be utilized during the ensuing operational time interval.

The arrangement according to this invention allows selection of flow as appropriate to a particular assembly and could be utilized to establish non-symmetric core flow patterns when desired.

The nozzle and module may be located at either end of the assembly or at an intermediate position.

The operation required to alter the orifice size is a simple and speedy maneuver. The assembly may be lifted a slight distance, perhaps a distance needed to clear whatever means of locking or securing the assembly is employed, rotated the desired number of degrees, and reseated.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, in which: Figure 1 is a sectioned elevation of an assembly and module; Figures 2, 3 and 4 are cross-sectional views taken as indicated from Fig. 1; Figure 5 is a rolled-out view of a section of the module, showing the arrangement of holes therein; and Figure 6 is a rolled-out view of a section of the assembly showing the arrangement of holes therein.

The following description relates to an ap plication of the invention in which it is desired to provide a six-sided assembly with three orifice effective sizes. The assembly is designed to be supported in the reactor core by the six-sided channel formed by the six surrounding assemblies and can be seated in this channel in any of six angular orientations. The rotation from one orientation to the next can be accomplished by lifting the assembly out of the core, rotating it by 60 , and replacing the assembly.

Referring to Fig. 1, the drawing shows the preferred embodiment for the above-described application. The variable size orifice system is installed at the inlet bottom end of the assembly. The assembly has a cylindrical assembly nozzle 5 which extends into the module assembly 6. The module assembly 6 has at one end a module fixed orifice 1 through which all coolant which traverses the assembly must pass. The module assembly 6 has a module inner wall 1 3 and a module outer wall 12, forming therebetween a jacket 10 which distributes coolant flow to all holes in the module inner wall 1 3. These holes are grouped axially in sets: module upper flow hole set 7, module lower flow hole set 8, and unblockable flow hole set 9.The assembly nozzle 5 has an upper flow hole set 2 located in axial juxtaposition with the module upper flow hole set 7 when the assembly is fully seated in its operational position. The assembly nozzle 5 also has a nozzle lower flow hole set 3 in similar axial juxtaposition with the module lower flow hole set 8. In the proximity of the unblockable flow hole set 9, the assembly nozzle 5 has an assembly reduced diameter region 1 7.

The radial arrangement and numbers of holes in the hole sets are such that each of the six possible orientations of the assembly causes some number of holes in the module assembly 6 to be blocked by non-alignment with a hole in the assembly nozzle 5 and some number of holes in the module assembly 6 to be unblocked by reason of alignment with holes in the assembly nozzle 5.

In the preferred embodiment, the unblockable flow hole set 9 cannot be blocked due to the assembly reduced diameter region 1 7.

The coolant flow path after passage through the module fixed orifice 1 is upward through jacket 10, through unblockable flow hole set 9, through an assembly lower fixed orifice 11, through an assembly upper fixed orifice 4, and thereafter upward through the assembly.

The total effective orifice of the assembly is a hydraulic combination of the orifices in the assembly nozzle and the module: fixed orifice 1, the assembly lower fixed orifice 11, and the assembly upper fixed orifice 4.

The jacket 10 also distributes coolant flow to module upper flow hole set 7 and module lower flow hole set 8. These hole sets, if unblocked, provide flow paths around the assembly upper fixed orifice 4 and the assembly lower fixed orifice 11. Whether these holes are blocked or unblocked depends on the assembly rotational orientation.

The holes in the fixed orifices are numerous and small in size, the design being chosen to prevent cavitation and to establish the largest part of the assembly pressure drop as due to the orifices. The flow holes greatly exceed the orifice holes in size and impose only a very small pressure drop. The opening of a flow path around a fixed orifice by alignment of flow holes therefore serves to greatly reduce the amount of flow through the orifice and reduces the contribution made by that orifice to the thereby altered overall effective assembly orifice size. In the above, it is shown that different orifice effective sizes based on assembly orientation can be planned and designed into the arrangement of holes. The following specifically describes the hole arrangement of the preferred embodiment.

Fig. 5 is a section of the module inner wall 13, taken as indicated from Fig. 1, which has been rolled flat.

Fig. 6 is a section of the assembly nozzle 5, taken as indicated from Fig. 1, which has been rolled flat.

Figs. 5 and 6 disclose the radial positions and numbers of holes in the various hole sets to achieve the goals of the preferred embodiment. The hole arrangment in Figs. 5 and 6 provides for three different combinations of flow paths through the assembly, as explained below.

The holes 18 of the unblockable flow hole set 9, as stated above, are always unblocked.

The assembly, and consequently the nozzle lower flow hole set 3, may be aligned with the module such that the nozzle middle holes 21 at 120 , 180", 300 and 360 of the nozzle flow hole set 3 are all blocked by the absence of any holes at 180 , 240, 360 and 60 in module lower flow hole set 8. In that alignment the module middle holes 19 at 120' and 300 of module flow hole set 8 will be blocked by the absence of holes at 60 and 240 on the nozzle. Nozzle upper flow hole set 2 holes and module upper flow hole set 7 holes are also blocked.

Alternatively, the nozzle may be rotated 60 such that the nozzle middle holes 21 at 120 and 300 of nozzle lower flow hole set 3 are aligned with the module middle holes 1 9 at 120 and 300 of module lower flow hole set 8. This unblocks two holes of nozzle lower flow hole set 3. That same alignment also unblocks both holes of nozzle upper flow hole set 2 as these are now aligned with the two holes in module upper flow hole set 7.

Another rotation of 60 in the same direction will block nozzle middle holes 21 at 120 and 300 of nozzle lower flow hole set 3 but unblock the holes of nozzle lower flow hole set 3 at 180 and 160 such that no effective change has occurred in flow area at nozzle lower flow hole set 3 by this rotation. But that rotation has blocked nozzle upper flow hole set 2.

From the above description and a study of Figs. 5 and 6, it can be seen that this embodiment has three flow configurations: (1) flow through unblockable flow hole set 9 only; (2) flow through unblockable flow hole set 9 and flow additionally through two holes of the nozzle lower flow hole set 3 and additionally through the two holes of nozzle upper flow hole set 2 and; (3) flow through unblockable flow hole set 9 and additionally through two holes of nozzle lower flow hole set 3 only.

The flow configuration (1) above has all assembly flow passing through all fixed orifices: module fixed orifice 1, assembly lower orifice 11, and the assembly upper fixed orifice 4. This flow configuration is therefore the most tightly orificed configuration of the three.

The flow configuration (2) above allows most coolant flow to bypass the assembly lower fixed orifice 11 and the assembly upper fixed orifice 4 by passage through the unblocked holes of nozzle upper flow hole set 2.

This configuration is therefore the most loosely orificed configuration of the three.

The flow configuration (3) above allows most coolant flow to bypass the assembly lower fixed orifice 11, but this flow is still directed through the assembly upper fixed orifice 4. This configuration has therefore an effective orifice size which is intermediate between the other two.

The flow configuration (2) above is depicted by the flow arrows in Fig. 1, Figs. 2, 3 and 4 show the alignment of holes consistent with flow configuration (2).

In Fig. 2, the section demonstrates that the assembly reduced diameter region 1 7 is not adapted to block the hoses 1 8 of the unblockable flow hole set 9.

In the higher section depicted in Fig. 3, flow is shown through the unblocked holes at 120 and 300".These unblocked holes are identical to the nozzle middle holes 21 at 120 and 300 in Fig. 6, and module middle holes 19 at 120 and 300 in Fig. 5. Fig. 3 also shows blocked holes 1 5 and 1 6 at 180 and 260". These are the nozzle middle holes 21 at 180 and 360 in Fig. 6.

Fig. 4 shows the flow through the unblocked holes at 180 and 360 . These are identical to the nozzle upper holes 22 in Fig.

6 and the module upper holes 20 in Fig. 5.

While the above analysis began with a particular initial orientation of the assembly nozzle 6 in the module assembly 6, and proceeded with rotation in one direction, the results are independent of these choices. The three flow configurations above are the only possible configurations for flow in this preferred embodiment.

The operator can select an effective orifice size by the appropriate orientation of the assembly nozzle 5 with the module assembly 6.

This selection can be altered at any time when the assembly nozzle 5 can be rotated.

The preferred embodiment described above is a particular adaptation of the invention which meets the aforestated needs for three different effective orifice sizes for a six-sided assembly. The details of any embodiment will vary widely depending upon the application.

Since numerous changes may be made in the above-described disclosure without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the drawings be interpreted as illustrative rather than limiting.

Claims

1. A nuclear reactor comprising a core disposed in a vessel and consisting of fuel assemblies supported on a core support structure (6) and means for passing coolant through said support structure and said assemblies for removing the heat generated therein during operation of the reactor, said fuel assemblies having protruding coolant nozzles (5) received in openings in said support structure (6) and having radial coolant flow passages (18, 20, 21), characterized in that said nozzles (5) and the walls (13) defining said nozzle receiving openings have a number of coolant flow passages (2, 7; 3, 8; and 9) so arranged that predetermined ones of said passages are closed or open depending on the angular position of said nozzles in said nozzle receiving openings.

2. A reactor as claimed in claim 1, characterized in that first and second sets (2, 7; 3, 8) of coolant flow passages are formed in axially spaced relationship in the walls (13) defining the nozzle receiving opening and said coolant nozzle (5) and said nozzle (5) has flow restricting orifices (4) arranged between said first and second sets of coolant flow passages, said fuel assembly having at least a first angular position in which only said second set (3, 8) of passages is open while said first set (2, 7) is closed such that all coolant is forced to flow through said flow restricting orifices (4) and a second angular position in which both said first and second sets of passages are open, said first set of passages being downstream of said flow restricting orifices (4).

3. A reactor as claimed in claim 2, characterized in that third coolant flow passages (18) are arranged axially sdpaced from and upstream of said second set (3, 8) with additional flow restricting passages (11) being disposed between said second set (3, 8) and said third coolant flow passages (18), said fuel assembly having a third angular position in which said first and second sets of flow passages are closed, said third flow passages (18) being open in any angular position of said fuel assembly.

4. A reactor as claimed in any of claims 1 to 3, characterized in that said fuel assemblies are blanket assemblies which include fertile material which is converted into heat producing fuel during operation of the nuclear reactor.