Patent Number: 039416526
Section: summary

BACKGROUND OF THE INVENTION The invention relates to failed fuel detection and more particularly a apparatus for detecting and locating failures of the fuel cans of liquid-cooled nuclear reactors, for example nuclear reactors cooled by an upward flow of molten sodium. Since fuel failures allow fission products to escape into the coolant, they must be detected and located as quickly as possible. In liquid-cooled reactors having no pressure tubes, such determination of the location of fuel failures is complicated by the mixing of the liquid coming from the various fuel sub-assemblies at their outlets. One solution for this kind of reactor is to take at the outlet of each fuel sub-assembly a small portion of the liquid flow which has passed therethrough, mix the sample with a gas having a large neutron capture cross-section, separate the gas and monitor it. Since the gas becomes radioactive in contact with the contaminated liquid, a sub-assembly having a failed fuel is shown up by the activity of the gas coming from the corresponding sample. A single mechanicaly sampling pump might be used (or if the liquid is conductive an electro-magnetic pump) with a selector valve which connects the inlet of the pump to the outlets of all the sub-assemblies in the core in succession. This method has serious drawbacks, since it is difficult to design and manufacture a liquid sample selector valve which correctly operates in the conditions to which it is subjected for long periods of time. Moreover, if the pump is mechanical its moving parts are fragile. SUMMARY OF THE INVENTION The invention is intended to obviate such disadvantages. The invention also relates to an apparatus for locating fuel can failures in nuclear reactors cooled by an upward flow of a liquid coolant, the apparatus comprising a source providing a gas at a pressure slightly higher than the pressure of the liquid at the outlet of nuclear fuel assemblies to be monitored in the reactor, emulsifiers each associated with an assembly and disposed thereabove, a selector means for supplying gas to each of the emulsifiers successively from the source, collecting means connected to all said emulsifiers and in which the gas separates from the liquid coolant, and gas-analysing means connected to the collecting means. A single analysing installation and a single selector valve can be used to monitor all the assemblies of a reactor, or alternatively a number of independent systems can be provided each of which is associated with some of the assemblies. It can be seen that the invention obviates the disadvantages of the apparatus described hereinbefore. The extreme simplicity of construction of the emulsifying device enables one to be used for each assembly, thus obviating any distributor in the liquid circuit (the distributor incorporated in the circuit of gas inactive at ambient temperature operates under much less severe conditions, is much less susceptible to breakdown and is as a rule placed outside the reactor enclosure). The gas is intimately mixed with the liquid during the expansion producing pumping, so that the analysing installation can be less sensitive and therefore less expensive.