GB2113840A

GB2113840A - Thermally responsive trigger device

Info

Publication number: GB2113840A
Application number: GB08202436A
Authority: GB
Inventors: James Ford; John Webb
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1982-01-28
Filing date: 1982-01-28
Publication date: 1983-08-10
Also published as: GB2113840B

Abstract

The device comprises a sodium- filled bellows (10). The bellows has freedom to expand with temperature rise to a threshold temperature at which point further expansion of the bellows operates a trigger (17) which is preferably re-cockable. The trigger may support one or more control rods (23) in a nuclear reactor. A compact form of the invention has a bellows expandable to move a closure (13) through an unrestrained portion of travel within a cup (16) which is surrounded by a partial cup-shape spring trigger (17) until the bottom of the cup is engaged and further movement depresses the spring trigger to reduce the rim diameter of its wall parts, and thus allow e.g. a control unit 23 to fall into a reactor core. The device can be set to operate at a given temperature by inserting a shallower or deeper cup (16). The device is resettable. <IMAGE>

Description

SPECIFICATION Thermally responsive trigger device This invention relates to thermally responsive trigger devices.

The invention is concerned with such devices suitable for use in liquid metal (conveniently identified as sodium) cooled systems such as sodium-cooled fast reactors where a trigger may typically be required at temperatures of the order of 7000C to release control units.

"Hot" magnetic triggers are known. Such devices rely upon the fact that all magnetic materials have a Curie point which is a temperature at which the material becomes degaussed and ceases to function magnetically. The attainment of the Curie point can, for example, aliow the release of a component which had previously been magnetically held or retained. For such triggers to be acceptable a suitable magnetic material has to be selected to give the trigger function at the correct temperature and at the same time the selected material must be acceptable in the context of its use, and not deteriorate or introduce risks. Such devices are not adjustable and they also operate under stress.

Fusible links are also known but these can give rise to problems similar to those referred to above in relation to magnetic triggers.

The present invention provides a thermally responsive trigger device comprising a sodium filled bellows capable, with temperature rise, of axial elongation with nominal freedom followed by axial elongation whilst acting on a trigger to operate that trigger.

Preferably the trigger is re-cockable.

In one way of carrying the invention into effect a sodium-filled stainless-steel bellows is held at one end and has its other end constrained laterally but free to move axially in a cup. The cup is supported on a spring trigger so that, when the end of the bellows reaches the base of the cup, the cup is depressed to operate the trigger.

The invention will now be described further with reference to the accompanying drawing in which: Figure 1 shows an arrangement according to the invention in section; Figure 2 shows an element of the arrangement of Figure 1 in a perspective view; Figure 3 shows another element of the arrangement of Figure 1 in perspective view; and Figure 4 shows in diagrammatic plan view the arrangement of Figure 1 used to control the release of control elements in a fast reactor.

In Figure 1 there is shown a bellows 10 having a sodium filling 11. The bellows has a top end closure 12 and a bottom end closure 13. The closure 12 is held in one part of a structural frame 14 by a screw 1 5 and the closure 13 is constrained laterally but free to move axially in a cup 1 6. The cup is supported on a spring trigger 1 7 (Figure 2) and it has a stem 18 movable in a guide aperture 1 9 in another part of the structural frame 14.

In Figure 2 the spring trigger 1 7 is shown having two divergent curved wall parts 20 and a curved base 21 with a hole 22 to accommodate the stem 1 8 of the cup 16.

The upper edges of the trigger 1 7 can support control elements 23 by engaging under lips 24 on the rods. This is illustrated in Figure 1.

In Figure 3 the cup 1 6 is shown in perspective view. The cup has flats 25 to accommodate movement of the wall parts 20 of the trigger.

In Figure 4 a sub-assembly vehicle 30 is shown (such a vehicle is described in more detail in the context of use with a sodium cooled fast reactor in our copending application designating the inventors as J Webb and J Ford and filed on the same day as the present application). The vehicle 30 consists of a hollow body part 31 in which there are located six units 32. These may be either control units (like elements 23) or fuel units, or voids, the combination being selectable. In Figure 4 there are shown four control units 23 and two fuel units 33. The units 23 are latched on the trigger 1 7 and are supported at a higher level than the fuel units and are, in this way, above the reactor core.

In operation, the bellows 10 is situated so as to be responsive to sodium coolant which has flowed upwardly over the fuel units 33 (such flow is indicated by arrows 34). As the coolant temperature rises with the change from zero power in the reactor to operating power so the temperature of the bellows rises and the bellows expands axially. This expansion is unconstrained as the end closure 13 moves freely into the cup 1 6. Should the temperature of the bellows now rise further (because, for example, there has been an unplanned restraint in the coolant flow) then the closure 1 3 of the bellows acts on the base of the cup 1 6 and depresses the cup.At the same time the curved base 21 of the trigger is depressed and this causes the curved wall parts 20 to take up a less divergent orientation until the upper edges of the curved wall parts leave the lips 24 of the control units 23 and the units are free to fall by gravity into the reactor core.

The device described above can be designed to use readily available and relatively inexpensive materials compatible with the hostile environment in which the operation has to be performed and of well-tried performance, namely stainless steel and sodium. The temperature at which the trigger releases can be made adjustable by only relatively simple mechanical operations such as inserting a shallower or deeper cup. The trigger is recockable by the simple operation of raising a released control unit 23 until it relatches on the upper edge of the trigger 1 7.

The device described above also has the merit that, apart from the trigger 1 7, it operates unstressed except at the point of operation and even at that point the stress is very low.

Claims

1. A thermally responsive trigger device comprising a liquid metal (eg sodium) filled bellows.

2. A device as claimed in claim 1 having nominal freedom to elongate axially with temperature rise to a threshold and thereafter to elongate axially whilst acting on a trigger to operate that trigger.

3. A device as claimed in claim 2 in which the trigger is recockable.

4. A device as claimed in claim 2 or 3 in which the bellows (10) is held at one end (12) and has its other end (13) constrained laterally but free to move axially in a cup (1 6), the cup being supported on a spring trigger (17} so that, when the bellows reaches the base of the cup, the cup is depressed to operate the trigger.

5. A device as claimed in claim 4 in which the spring trigger has a partial cup shape (Figure 2) to offer an inwardly, movable rim, around which releasable members (23) can be latched.

6. A thermally responsive trigger as claimed in any preceding claim supporting nuclear reactor control rods against gravity and triggerable to release the control rod to fall by gravity.

7. A device as claimed in claim 3 substantially as hereinbefore described with reference to the drawings.

7. A trigger as claimed in claim 6 in which the trigger can be recocked by raising the fallen control rods.

8. A device as claimed in claim 4 substantially as hereinbefore described with reference to the drawings.

New claims filed on 14 Jan 1983 Superseded claims 1-8 New or amended ciaims:-- CLAIMS

1. A thermally responsive trigger device including a bellows having a fixed end and a free end and which has means causing it to change its length with temperature change sensed at a measuring location characterised in that the bellows (10) has a filling of liquid metal (11), is closed, and is disposed at the temperature measuring location together with a trigger (1 7) responsive to the movement of the free end of the bellows when a temperature threshold is exceeded at said bellows.

2. A trigger device as claimed in claim 1 in which the trigger (17) is recockable.

3. A device as claimed in claim 1 or 2 in which the bellows (10) is held at one end (12) and has its other end (13) constrained laterally but free to move axially in a cup (1 6), the cup being supported on a spring trigger (1 7) so that, when the bellows reaches the base of the cup, the cup is depressed to operate the trigger.

4. A device as claimed in claim 3 in which the spring tirgger has a partial cup shape (Figure 2) to offer an inwardly, moveable rim, around which releasable members (23) can be latched.

5. A thermally responsive trigger device as claimed in any preceding claim supporting nuclear reactor control rods against gravity and triggerable to release the control rod to fall by gravity.

6. A trigger as claimed in claim 5 in which the trigger can be recocked by raising the fallen control rods.