Reduction in rate of radiation exposure to excore nuclear reactor components

A thermal neutron reactor including a reactor core having a plurality of individually replaceable fuel assemblies, a cylindrical pressure vessel enclosing the reactor core, said pressure vessel having a circumferential weld at an elevation opposite a portion of the reactor core, said weld requiring a rate of radiation exposure less than that of the pressure vessel material and serving to define axial segments of certain selected peripheral fuel assemblies, that are in close proximity to said weld and span the same elevation as the weld so as to provide the greatest potential radiation source to the weld, said axial segments having a length on the order of about 6 inches to about several feet, each said selected peripheral fuel assembly comprising a lattice of vertical elements providing gaps therebetween for coolant fluid flow, and the axial segment of each selected peripheral fuel assembly defining a shield zone including portions of the vertical elements formed from material of lower fissile concentration than that which is typical for the entire core, and remaining portions of each selected peripheral assembly defining a fuel zone including portions of the vertical elements containing a higher concentration of fissile material than in the shield zone, whereby the selected peripheral fuel assemblies serve as composite assemblies that reduce the rate of radiation exposure to said weld while also contributing to production of power by the core by providing fissile material for fission reactions.

In the operation of a nuclear reactor of the type including a reactor core having a plurality of individually replaceable standard fuel assemblies containing fissile material, a method of controlling the rate of radiation exposure to a localized area of an excore component of the reactor, said method comprising:

• identifying peripheral portions of the core that are in close proximity to the localized area and provide the greatest potential radiation source to the localized area;
• identifying each fuel assembly location that includes a partial volume comprising some part of the mentioned peripheral portions of the core;
• and
• while the reactor is in a fuel loading mode, fueling the reactor at each identified fuel assembly location with a composite assembly in which the mentioned partial volume comprises between about 4% to about 50% of the total assembly volume and includes material of lower fissile concentration than that which is typical for the entire core, and in which the remaining volume thereof includes material of higher fissile concentration than that in the partial volume, whereby each composite assembly serves to reduce the rate of radiation exposure to the localized area while also contributing to production of power by the core by providing fissile material for fission reactions.

A composite fuel assembly useful as a component of a nuclear reactor core and being characterized by its utility in controlling the rate of radiation exposure to a localized excore area while also contributing to production of power by the core by providing fissile material for fission reactions, said composite fuel assembly comprising:

• support means defining exterior dimensions of said assembly that are standard to a particular type of nuclear reactor core and render the assembly individually insertable and replaceable at a peripheral location in the core;
• a shield zone comprising material of lower fissile concentration than that which is typical for the entire core into which the assembly will be inserted, said shield zone material being supported by said support means and occupying a first partial volume of said assembly, the size and location of said first partial volume corresponding to that portion of the assembly which, when the assembly is inserted into its respective core, is in close proximity to the mentioned localized excore area and provides the greatest potential radiation source to the localized excore area;
• a fuel zone comprising material of higher fissile concentration than the shield zone material, said fuel zone material being supported by said support means and occupying a second partial volume of said fuel assembly;
• and
• said shield zone occupying between about 4% to about 50% of the total shield zone/fuel zone volume.

A composite fuel assembly of the type used as a component of a nuclear reactor core and being characterized by its utility in controlling the rate of radiation exposure to a localized excore area while also contributing to production of power by the core by providing fissile material for fission reactions, said composite fuel assembly comprising:

• a plurality of vertically disposed, cylindrical cladding tubes;
• support means for holding the cladding tubes in a substantially parallel, closely packed arrangement;
• fuel pellets of enriched uranium and shield pellets of non-fissionable shield material encased by said cladding tubes;
• said shield pellets being located in at least one axial segment of the cladding tubes, the size and location of said axial segment having a length being on the order of about 6 inches to about several feet and corresponding to that portion of the composite fuel assembly which, when the assembly is inserted into its respective core, is in close proximity to the mentioned localized excore area and provides the greatest potential radiation source to the localized excore area;
• and said fuel pellets residing in remaining portions of the cladding tubes.