A combinatorial heterogeneous-homogeneous reactor configuration in which an array or groups of homogeneous fuel assemblies are interlinked together in a heterogeneous lattice. The present invention removes the limitation of a homogeneous reactor by providing a reactor concept that utilizes the inherent advantages of homogeneous fuel elements but in a heterogeneous fuel lattice arrangement that limits the
power density of any one homogeneous fuel element and yet forms a reactor arrangement that is capable of producing any
product demand of interest. The present invention provides a method for producing medical isotopes by the use of a modular reactor core comprised of homogeneous fuel assemblies arranged in a regular rectangular or triangular
pitch lattice. The aqueous fuel solution is contained within individual fuel assemblies that are right circular cylinders clad in
corrosion-resistant alloys such as stainless steel, zircalloy, zircalloy alloys, or other
metal alloys that are resistant to corrosive fissile environments but preserve
neutron economy. The fuel assemblies are supported below by a core plate that is tied directly to the lower reactor support structure. The bottom of each
assembly opens into a common plenum area which provides a hydrodynamic communication /
coupling path between the individual assemblies in the lattice. The fuel assemblies are supported above by an upper plate that is welded to each
assembly tube. The top of each
assembly opens to a common upper plenum which provides a means of thermodynamic pressure
equalization among the four assemblies in the reactor core lattice.