Failed fuel location detector

An apparatus for detecting the location of a failed fuel assembly containing a failed fuel pin among a number of fuel assemblies constituting a core of a nuclear reactor having means for flowing coolant into and out of the fuel assemblies, the fuel assemblies being divided into a plurality of groups respectively consisting of first distinct pluralities of fuel assemblies, every fuel pin in the fuel assemblies belonging to the same group being tagged with a distinct gas of the same kind, said apparatus comprising:

• means for monitoring the presence of fission product gases released from the failed fuel assembly into a cover gas inside a reactor vessel;
• means for detecting the kind of tag gas released from the failed fuel assembly into the cover gas, thereby to identify the group of fuel assemblies which includes the failed fuel assembly;
• a plurality of intermediate mixing means for gathering and mixing together coolants delivered thereto, each of said plurality of intermediate mixing means corresponding to a respective subset of fuel assemblies among a plurality of subsets of fuel assemblies, the plurality of subsets consisting of respective second pluralities of fuel assemblies defined such that there is a one-to-one correspondence between each fuel assembly and each combination of one subset and one group;
• sampling means for delivering samples of coolant flowing out of each of the fuel assemblies to the respective ones of the intermediate mixing means corresponding to the subsets defined to include the fuel assemblies;
• a selector valve means for sequentially collecting the mixed coolants from said plurality of intermediate mixing means;
• and
• means for detecting delayed neutrons in the mixed coolants collected by said selector valve means.