Patent Number: 039416526
Section: description

The localising apparatus diagrammatically illustrated in FIG. 1 comprises a source 10 of gas (for instance, argon) at a pressure P.sub.1 slightly higher than the pressure P.sub.2 of the coolant (liquid sodium) at the outlet of assemblies 12. A distributor 14 enables any of tubes 16, each connected to an emulsion pump associated with one of the assemblies 12, to be connected to the source 10. Clearly, the term "assembly" covers both a group of fuel pencils in the same envelope, and a single fuel element. The emulsion coming from that pump which is supplied with gas rises into a collecting and degasifying tank 18 via a vertical tube 20 (one tube 20 is provided for each assembly 12). The level of the tank 18 above the free surface 22 of the sodium is such that a separation is established therein between the liquid sodium, which returns to the mass of sodium in the core via the tubes 20 to which emulsion is not supplied, and the gas. The return of the sodium from the degasifying tank via the tubes 20 not supplied with emulsion might have the disadvantage of polluting such tubes with sodium charged with fission products by their passage through the tube corresponding to the faulty assembly, so that the signal might be slightly difficult to detect. One possible solution to this is illustrated in FIG. 3, which shows how the sodium is returned from the degasifying tank 18 to the reactor via a special tube 20' provided for this purpose and disposed at a lower level than the other tubes. The gas is sucked in by a pump 24 which sends it to an analysing installation 26. For the sake of clarity, FIG. 1 shows in solid arrows the flow paths followed by the gas, the emulsion and the coolant on its way back from the tank 18, broken arrows showing the normal flow of the sodium coolant. The emulsion pump operates on the following principle: the gas injected at the base of a vertical tube 20 at pressure P.sub.1 produces an emulsion whose mean density is lower than that of the liquid coolant. The emulsion rises in the tube 20 and reaches a level higher than that of the surface. The gas flow required for pump operation being low in relation to the total liquid flow for each assembly, supply tubes 16 of small section are enough. Clearly, the volume of the tank 18 is selected to keep the time required for checking an assembly within reasonable limits. The degasifying tank can have a volume of the order of 30 liters for an analysing installation 26 adapted to deliver a signal when it receives a total flow of the order of 0.1 - 0.2 liters per second coming from an assembly 12. FIG. 2 shows an emulsion pump which can be used in the installation illustrated in FIG. 1, in which the pump is associated with an assembly 12. The outlet of the assembly 12 is placed in line with an upwardly flared passage 30 of generally frusto-conical shape with which the core cover plate 32 is formed. The pump diffusor is formed by a perforate cone 34 inserted in a bore 38 in a plate 36 which on the one hand acts as a support for the tubes 20 and on the other co-operates with the cones 34 to bound gas distribution chambers 40 into which the tubes 16 discharge. An identical sodium flow does not pass through all the assemblies 12 of a nuclear reactor, since the dynamic pressures diminish from the centre to the periphery by a factor often of the order of 2. To allow for this, the cone 34 is continued by a bottom tip 42 whose end is obturated and whose side wall is formed with slots or apertures 44 of a size adequate to prevent them being clogged by impurities. By this method the liquid admitted to the cone 34 will be at the same pressure in all tubes, such pressure being the height of the sodium in the reactor vessel and therefore all the emulsion pumps will be identical for any given reactor. Clearly, the tips 42 must penetrate into the passages 30 deeply enough for each diffuser to collect only sodium coming from the assembly 12 to be checked. The flared shape of the passages 30 compensates for the reduction in section due to the introduction of the tip 42 and prevents any increase in speed which might result from such reduction. Disposed in the tank 18 of flat shape, opposite each tube 20 and at a level 28, are anti-splash plates 46 (FIG. 2) adapted to encourage the separation of the gas from the liquid and limit its level locally. The gas pump 24 and the analysing installation 26 disposed outside the reactor screening enclosure 48 are connected to the tank via a conduit 50. The following numerical data, given by way of example, are those of an apparatus adapted for use with a fast neutron 250 MWe reactor in which the outlet temperature of the assemblies under normal operating conditions is 833.degree.K: Inside diameter of tube 20: 25 mm Inside diameter of tip 42: 16 mm Pressure P.sub.1 of the gas in the emulsifier: 1.175 bar sodium flow in the pump: 0.28 l/s Gas flow in the emulsifier at pressure P.sub.1 and normal opera- ting temperature: 0.45 l/s Gas flow at outlet of tube 20: 0.53 l/s Speed of the emulsion in center of tube 20: 2 m/s proportion of gas by volume at the center of the tube 20: 0.5 Delivery of the pump: 0.45 Emersion ratio: x/H = 1.8 Clearly, the emersion ratio must be so selected that the level of the emulsion does not drop below the tank 18. The value of 1.8 shown above takes account of this fact, whose importance is clear if it is remembered that for this particular reactor the outlet temperature of the assemblies on stoppage (corresponding to minimum level 28) is 453.degree.K, while such temperature is 833.degree.K during operation. The main advantages afforded by the invention can be gathered from the foregoing description: the apparatus is very simply constructed and there is very little risk of breakdown (pumps without moving members, switching distributor disposed in the gas circuit); sensitivity is increased by the intimate mixing of the gas and liquid during the formation of the emulsion. The use of pumps with tips enables standard diffusers to be used throughout the installation.