GB2247770A

GB2247770A - Nuclear reactors

Info

Publication number: GB2247770A
Application number: GB9117383A
Authority: GB
Inventors: Donald Broadley; Mirek Dostal; David John Hartley
Original assignee: NNC Ltd
Current assignee: NNC Ltd
Priority date: 1990-08-13
Filing date: 1991-08-12
Publication date: 1992-03-11
Also published as: GB9017682D0; JPH04233496A; GB9117383D0

Abstract

In control rod insertion apparatus for a nuclear reactor, one or more thermal expansion elements (54) are inserted between the control rod drive (47) and the control rod, so that in the event of an excessive increase in reactor coolant temperature the control rod is moved into the core independently of operation of the drive. Excessive expansion of the elements may be arranged to cause unlatching of a control rod holder (59) from the drive, so that the control rod is free to fall into the core, but if fall of the rod into the core is impeded, the elements then push the rod into the core. Alternatively (figure 1), expansion of the elements may simply move the rod relative to the drive into the core. Various forms of expansion element are described (figures 2 to 7). <IMAGE>

Description

Nuclear Reactors This invention relates to nuclear reactors, and particularly to apparatus for automatically increasing the extent of insertion of control rods into the core of a nuclear reactor in response to an excessive rise in core coolant temperature.

The invention may be applied particularly, but not exclusively, to liquid metal cooled fast breeder reactors.

In the event of a fault occurring in a reactor, the safety equipment wil normally move the control rods down into the core to shut down the reactor. However, it is conceivable that the trip system which initiates such control rod insertion could fail, resulting in an excessive rise in coolant temperature. Furthermore, if the coolant flow becomes interrupted or reduced without the trip system operating, boiling of the coolant could occur. Overheating in the reactor caused by loss of the heat sink could bring about structural failure in the reactor.

It is an object of the present invention to provide apparatus which in response to an excessive increase in coolant temperature increases the extent of insertion of a control rod into the core, without reliance on any external control rod drive power supply or any control signal.

According to the invention there is provided apparatus for inserting one or more control rods into a nuclear reactor core, comprising control rod drive means; and, coupled between the drive means and the control rod, at least one thermal expansion element which extends in length in response to increase in reactor coolant temperature and which is thereby operative, if the reactor coolant temperature becomes excessive, to move the control rod into the core independently of operation of the drive means.

The or each thermal expansion element may comprise a bimetallic element. Alternatively, the element may comprise an enclosure filled with a quantity of material the volume of which increases appreciably in response to said increase in coolant temperature, the enclosure including means to allow increase in the length of the enclosure in response to increase in said volume.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which, Figure 1 is a schematic sectional view of one form of control rod insertion apparatus in accordance with the invention, Figures 2 to 6 are schematic sectional views of alternative forms of bimetallic actuator elements for use in the apparatus of Figure 1, Figure 7 is a schematic sectional view of a further alternative form of actuator element, and Figure 8 is a schematic sectional view of a second form of control rod insertion apparatus in accordance with the invention.

Referring to Figure 1 of the drawings, apparatus 1 for inserting a control rod (not shown) downwardly into a reactor core comprises a vertical control rod drive line 2 which is coupled at its upper end to a drive mechanism (not shown) for moving the drive line 2, and thereby the control rod, using electrical, electromagnetic or hydraulic power.

A thermal expansion device 3 and a compression spring arrangement 4 are incorporated axially end-to-end into the drive line 2. The expansion device 3 and the spring arrangement 4 are contained within an outer guide tube 5 having inwardly-directed flanges 6 and 7 at its upper and lower ends 8 and 9, respectively.

The upper end of the drive line 2 comprises an inner guide tube 10 having attached thereto an annular collar 11. The inner guide tube passes through the flange 6 and extends into the outer guide tube 5. Within the outer guide tube 5 the inner guide tube 10 has an outwardly-directed flange 12 at its lower end 13. A spring assembly 14 comprising, for example, a column of individual annular disc springs, is fitted between the flanges 6 and 12 and around the inner guide tube. The spring assembly is under some compression, so that the flange 6 and the collar 11 are normally held in contact.

A coolant flow tube 15 attached to the flange 7 extends upwardly from that flange, such that the upper end of the tube 15 lies within the inner guide tube 10.

The thermal expansion device 3 comprises a stack 16 of bimetallic ring elements which are held concentric by the inner surface of the outer guide tube 5. The upper end 18 of the stack 16, at normal coolant temperatures, lies well below the end flange 12 of the inner guide tube 10, so that the device acts as a lost-motion device, as explalned below. Slots 19 or circular apertures 20 are provided through the wall of the outer guide tube 5 to allow hot sodium coolant to make intimate contact with the bimetallic elements 16. The outlet flow of hot coolant from the core is directed to the expansion device via a reactor shroud tube 21 above the core structure. The device is preferably located close to the top of the reactor core in order to shorten the coolant flow path to the device and thereby minimise the response time of the device.

Coolant at a lower temperature received from the control rods is kept away from the expansion device 3 by passing it upwards through the coolant flow tube 15 and thence out through the inner guide tube 10.

In operation of the device, at ambient temperature the bimetallic ring elements, each of which is formed from two layers of dissimilar metals bonded together, will be substantially flat. The upper end 18 of the stack 16 will lie clear of the end flange 12, as shown, and the flange 6 will abut the collar 11 of the inner guide tube. The position of the control rod is therefore dependent solely upon the operation of the conventional control rod drive mechanism, and is not affected by the thermal expansion device.

As the coolant temperature rises above ambient, the bimetallic elements will distort due to the bimetallic effect, thereby increasing the overall height of the stack 16. As the coolant temperature increases further, the height of the stack will continue to increase, but this will not initially have any effect on the position of the control rod, as the effective overall length of the device will not have changed. In fact, the device acts, in this condition, as a lost-motion device. When the coolant temperature rises to a predetermined level, which is preferably the nominal operating temperature of the reactor, the top of the stack will have reached the lower face of the flange 12.

If, under fault conditions, the coolant temperature continues to rise, the stack 16 of bimetallic discs expands further, and the force exerted thereby against the flange 12 of the inner guide tube 10 and the flange 7 of the outer guide tube 5 causes the outer guide tube to move downwards, sliding over the inner guide tube, and further compressing the spring assembly 14. The control rod coupled to the drive line 2 is therefore moved downwards into the core, by a distance which is dependent upon the coolant temperature.

The stack 16 of flat bimetallic discs may be replaced by any of a number of alternative thermal expansion elements, such as those shown in Figures 2-7.

Figure 2 shows part of an assembly 25 comprising alternate annular bobbin-shaped members 26 of a first material having outwardly-directed flanges and members 27 of a second material having inwardly-directed flanges. The adjacent flanges are bonded together to form a succession of bimetallic elements 28. The figure shows only a few of each of the members 26 and 27, but in practice there will be sufficient to extend to the same length as the stack 16 of Figure 1.

Figure 3 shows a similar arrangement to Figure 2, but in this case the thermal expansion element comprises a generally cylindrical member 29 having alternate large and small diameter regions 30 and 31, respectively. Bobbin-shaped members 32 of a different material are inserted into the reduced-area regions 31 and are bonded to the adjacent surfaces of the member 29.

Figure 4 shows a section of a bimetallic helix 33 formed by bonding two strips 34,35 of different materials into a bimetallic strip and forming the composite strip into a helix. Such element would extend axially along its whole length when heated by the cool ant.

Figure 5 shows an arrangement of alternate cylinders 36-40 of materials of high and low thermal expansion which will expand axially at different rates.

In figure 6 a bellows device 41 is formed of cylinders 42,43 of dissimilar materials. The cylinders are interconnected only at their ends.

Figure 7 shows a thermal expansion element 44 comprising a sealed container 45 with a bellows configuration 46 formed in Its wall. The container is filed with a material, such as sodium, which has a high volumetric expansion. When heated, the material expands appreciably, causing extension of the container.

In each of the above cases any number of the elements may be stacked up to make the required overall length.

It will be seen that the thermal expansion device does not interfere with normal operation of the control rod drive system or of any other part of the reactor. Due to the lost motion effect built into the device, the device operates only if the coolant temperature becomes excessive, e.g. under fault conditions. It then operates to Insert the control rods further into the core without the use of any external power supply or control signal.

The spring assembly 14 is designed to operate continuously under compressive loading throughout the entire coolant temperature range, thereby ensuring that this structural component has the necessary spring stiffness values associated with a desired high natural frequency, which alleviates problems of induced resonance due to the effects of coolant flow induced vibration and seismic events.

A soft spring (not shown) may be included in the gap above the stack of thermal expansion elements to prevent vibratlon of the stack while the elements are below the predetermined temperature at which they close the gap.

Figure 8 shows an alternative form of control rod inserting apparatus in accordance with the invention. In this case, a hollow drive line 47, which acts in the same manner as the drive line 2 of Figure 1, is encircled by a guide tube 48 which extends between upper and lower flanges 49 and 50, respectively, of the drive line 47. The flange 50 is provided on a cylindrical member 51 which encircles, and is integral with, the lower portion of the drive line.

A support tube 52, coaxial with the guide tube 48 and the drive line 47, has an upper inwardly-directed flange 53. A column 54 of bimetallic rings, similar to the stack 16 of Figure 1, is fitted around the guide tube 48 and between the flanges 49 and 53. As in Figure 1, the column 54, when cool, does not fill the space between the flanges 49 and 53, so that lost motion is provided for during heating up of the coolant to its normal operating temperature.

The support tube 52 has a lower inwardly-directed flange 68 having, at its inner edge, a cylindrical portion 55 which extends vertically upwards, and a thicker cylindrical portion 56 which extends vertically downwards. The cylindrical portion 55 is a sliding fit in a recess 57 between the walls of the member 51 and the line 47.

A cylindrical housing 58 butts against the underside of the flange 68. The housing contains a rod holder 59 which holds the upper end of a control rod (not shown). The rod holder has an outwardly-directed flange 60 and an upwardly-extending sylRndrRcal portion 61. The housing 58 has a lower inwardly-directed flange 67. A spring arrangement 62, retained in the housing 58, is in compression between the flanges 68 and 60. A latch device 63 interconnects the rod holder 59 and the drive line 47. The latch comprises a ring of balls 64 which are seated in a groove 65 in the inner surface of the portion 61 of the rod holder 59 and are normally locked in that position by the inner surface of the cylindrical portion 56 of the support tube 52, so that the rod holder is retained in the position shown in the figure.

Hot sodium reactor coolant is fed into a shroud tube 66 which encloses the apparatus, and passes around the outside of the bimetallic ring column 54 and also through the guide tube 48 so that the rings are heated and expand in accordance with the coolant temperature. Coolant at a lower temperature from the control rod passes upwards through the bore of the drive line 47.

In operation of the apparatus at ambient temperature, the control rod is located in the core at a height determined by operation of the conventional control rod drive mechanism (not shown). As the coolant temperature rises above ambient, the bimetallic elements will distort due to the bimetallic effect, thereby increasing the overall height of the column 54. As the coolant temperature increases further, the height of the column will continue to increase, but this will not initially have any effect on the position of the control rod, as the effective overall length of the device will not have changed.

When the coolant temperature rises to a predetermined level, which is preferably the nominal operating temperature of the reactor, the top of the column will have reached the lower face of the flange 49.

If, under fault conditions, the coolant temperature continues to rise, the column 54 of bimetallic discs expands further, and the force exerted thereby against the flange 49 of the drive line 47 and the flange 53 of the support tube 52 causes the support tube to move downwards by a short distance, compressing the spring arrangement 62.

The balls 64 are then allowed to retract into a larger internal dfameter portion of the support tube 52, allowing the rod holder 59 to drop under gravity. The housing 58, with the spring arrangement 62 therein, falls with the rod holder 59. The support tube 52 Is also no longer supported and therefore drops until its upper flange 53 makes contact with the lower flange 50 of the drive line 47. The control rod is therefore fully inserted into the core, provided that the rod is free to move under the force of gravity. If, on the other hand, the rod is prevented, by, for example, friction forces and/or distortion of the rod, from dropping freely Into the core, the temperature will continue to rise, and the bimetallic rings will exert further downward force on the support tube 52. The bottom edge of the portion 56 of the support tube will make contact with a shoulder 69 of the rod holder 59, thereby forcing the rod further into the core.

As described above in relation to the Figure 1 embodiment, the column 54 of bimetallic rings may be replaced by other suitable thermal expansion elements, such as any of those described with reference to Figures 2-7. The spring arrangement 62 may, for example, be similar to the assembly 14 of Figure 1.

Claims

1. Apparatus for inserting one or more control rods into a nuclear reactor core, comprising control rod drive means; and, coupled between the drive means and the control rod, at least one thermal expansion element which extends in length in response to increase in reactor coolant temperature and which is thereby operative, if the reactor coolant temperature becomes excessive, to move the control rod Into the core independently of operation of the drive means.

2. Apparatus as claimed in Claim 1, comprising means operative in response to a predetermined increase in length of the thermal expansion element or elements to allow the control rod to drop into the core.

3. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each thermal expansion element comprises a bimetallic element.

4. Apparatus as claimed in Claim 3, wherein the or each expansion element comprises a disc formed of two layers of dissimilar materials.

5. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each expansion element comprises a plurality of alternate members of different materials having respective pairs of inwardly and outwardly-facing flanges, the flanges of adjacent members bering bonded together.

6. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each expansion element comprises a generally-cylindrical member having alternate regions of relatively larger and smaller diameters, and a respective bobbin-shaped member of a different material from said cylindrical member inserted into each smaller diameter region and bonded thereto.

7. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each expansion element comprises a helix formed of two strips of dissimilar materials.

8. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each expansion element comprises a bellows arrangement formed from two concentric cylinders of dissimilar materials.

9. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each expansion element comprises a plurality of interconnected coaxial cylinders of alternately different materials.

10. Apparatus as claimed in Claim 1 or Claim 2, wherein the or each thermal expansion element comprises an enclosure filled with a quantity of material the volume of which increases appreciably in response to said increase in coolant temperature, the enclosure including means to allow increase in vile length of the enclosure in response to increase in said volume.

11. Apparatus as claimed in Claim 10, wherein the means to allow increase in the length of the enclosure comprises a bellows configuration in the enclosure wall.

12. Apparatus as claimed in Claim 10 or Claim 11, wherein the material filling the enclosure is sodium.

13. Apparatus as claimed in any preceding claim, wherein a lost motion effect is incorporated whereby action of the or each thermal expansion element is ineffective until the coolant temperature exceeds a predetermined level.

14. Apparatus as claimed in any preceding claim, wherein the or each thermal expansion element is contained within a housing which is moved against spring pressure by expansion of the or each element to cause insertion of the control rod.

15. Apparatus for inserting one or more control rods substantially as hereinbefore described with reference to the accompanying drawings.