Patent Number: 048896837
Section: summary

BACKGROUND OF THE INVENTION This invention relates to thermally responsive trigger 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 up to 700.degree. C. to release neutron absorber 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 de-gaussed and ceases to function magnetically. The attainment of the Curie point can, for example, allow 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. Bellows compression devices are also known (see for example GB-PS 1,580,322) in which an expansible material such as sodium-potassium alloy contained in a helical tube, is connected with a reservoir column outside a bellows to compress the bellows with temperature rise and the bellows then operates a trigger via linkages. Such devices include a significant number of co-related parts and hence there must be a modest probability of failure of the parts themselves or at the interconnection between the parts. Repair or replacement of failed parts in a nuclear reactor can be very difficult and costly. FEATURES AND ASPECTS OF THE INVENTION The present invention continues with the bellow concept but improves it in that it is arranged for the bellows to be closed (that is it does not have any openings into connecting tubes or bulbs), the bellows has a filling of liquid metal, and the bellows is disposed to act as a trigger device in the vicinity of its free end. The bellows and trigger are then located at the actual location at which temperature is to be sensed. With this improvement presented it is possible to incorporate it into a nuclear fast reactor in a very advantageous way. In known fast reactor shut-down arrangements a neutron absorber rod is suspended above the reactor core and is arranged to be released to fall into a receiving channel in the core on receipt of an emergency signal. However, both the core support structure and the absorbing carrying structure can distort and substantial tolerances have to be provided to ensure that the absorber rod will enter its receiving channel and pass into that channel despite distortion. In this context the present invention takes the already known sub-assembly form of construction of fast reactor cores and the already known devices called "demountable sub-assembly vehicles" used in such cores for conducting irradiation experiments. These vehicles are self-contained and designed for easy removal and insertion into the core. The fact that they are self-contained allows the avoidance or reduction of problems of distortions in and deflections between components of a large integrated structure. The fact that they are easily removable and replaceable allows a high standard of testing and performance to be maintained The trigger device of the present invention accordingly finds use in a demountable sub-assembly vehicle having therein a fuel unit, a triggerable absorber unit and coolant inlet and outlet arranged so that coolant in flow between inlet and outlet passes first over the fuel unit and then over a temperature responsive trigger in the absorber unit, which trigger includes said device to trigger the absorber unit to fall when a threshold temperature is reached.