Patent Number: 
Section: claims

1. A fuel assembly mechanical flow restriction apparatus for detecting failure of a nuclear fuel rod in a nuclear fuel assembly situated in a reactor core of a boiling water reactor, the reactor core comprising a plurality of nuclear fuel assemblies comprising parallel fuel rods supported at an upper end by an upper tie plate and an outer channel surrounding the fuel rods for the passage of reactor coolant from a lower end to the upper end of the fuel assembly, the outer channel having upper edges, the upper end of the fuel assembly passing through and being supported by a reactor core top guide structure, the fuel assembly mechanical flow restriction apparatus comprising: a testing hood enclosing the said flow restrictor comprising a top plate and side plates to form a structure with an open bottom forming an internal volume for positioning over the tops of at least one of the nuclear fuel assemblies and for receiving gases escaping from a failed fuel rod within the fuel assembly, the side plates for resting on the reactor core top guide structure;  a flow restrictor positioned within the testing hood and over at least one of the nuclear fuel assemblies, the flow restrictor comprising a sealing plate for positioning on the upper edges of the outer channel of the fuel assembly for mechanically blocking fuel assembly coolant flow exiting the upper end of the fuel assembly;  a probe assembly having at least one probe head for sampling the coolant water within the fuel assembly for detecting failure of a nuclear fuel rod in the nuclear fuel assembly;  means for pneumatically causing the sealing plate of the flow restrictor to be positioned on the upper edges of the outer channel of the fuel assembly for mechanically blocking fuel assembly coolant flow from exiting the upper end of the fuel assembly; and  means for pneumatically causing the probe head to be immersed in the fuel assembly reactor coolant water within the outer channel. 2. A fuel assembly mechanical flow restriction apparatus for detecting failure of a nuclear fuel rod in a nuclear fuel assembly situated in a reactor core of a boiling water reactor, the reactor core comprising a plurality of nuclear fuel assemblies, each of the plurality of nuclear fuel assemblies comprising parallel fuel rods supported at an upper end by an upper tie plate and an outer channel surrounding the fuel rods for the passage of reactor coolant from a lower end to the upper end, the outer channel having upper edges, the upper end passing through and being supported by a reactor core top guide structure, the fuel assembly mechanical flow restriction apparatus comprising: a testing hood enclosing the said flow restrictor that forms a structure with an open bottom for positioning over the top of a group of fuel assemblies from the plurality of fuel-assemblies and for receiving gases escaping from the group of fuel assemblies, and  a flow restrictor positioned within the testing hood, the flow restrictor comprising a sealing plate for positioning on the upper edges of the outer channel of a fuel assembly from the group of fuel assemblies, the sealing plate substantially and mechanically blocking fuel assembly coolant flow exiting the upper end of the fuel assembly. 3. The fuel assembly mechanical flow restriction apparatus of  claim 2  wherein a testing apparatus is coupled to the flow restrictor, the testing apparatus for detecting failure of a nuclear fuel rod in the fuel assembly over which the flow restrictor is placed. claim 2 4. A process for detecting failure of a nuclear fuel rod in a nuclear fuel assembly, the process comprising: placing a testing hood enclosing a flow restrictor over the top of a plurality of fuel assemblies;  substantially and mechanically blocking coolant flow in a fuel assembly from the plurality of fuel assemblies from within the testing hood; and  extracting a sample of water from the fuel assembly. 5. The process of  claim 4  including, after the substantially and mechanically blocking step, the step of waiting for the coolant temperature in the fuel assembly to exceed a threshold value before extracting the sample of water. claim 4 6. A process for detecting failure of a nuclear fuel rod in a nuclear fuel assembly, the process comprising: placing a testing hood enclosing a flow restrictor over the top of a plurality of fuel assemblies;  substantially and mechanically blocking coolant flow in a fuel assembly from the plurality of fuel assemblies from within the testing hood; and  extracting a sample of gas from the plurality of fuel assemblies. 7. The process of  claim 6  including, after the substantially and mechanically blocking step, the step of waiting for the coolant temperature in the fuel assembly to exceed a predetermined value before extracting the sample of gas. claim 6