Brief review of the design, specific features of thermal hydraulics of reactor cores and circulation loops of pool-type research reactors is given. Principal distinguishing features of research reactors as compared with industrial power reactor units are outlined. Design of reactor units is examined using the example of two research reactors VVR-M (Gatchina) and VVR-ts (Obninsk). Direction of coolant circulation constitutes the feature of research reactor installations which is of key importance. In contrast to power reactor units, propagation of coolant in research reactors is arranged in downwards direction, i.e. from core top to bottom. In connection with the above, particular design features of reactor support grids are discussed in the present study. A set of data is presented on the values of preset values of alarm and emergency protection triggering thresholds. The issue of modernization of the reactor core implemented by developing the family of fuel assemblies (FAs) of the new type is discussed separately using the example of modernization of the VVR-M reactor. It is demonstrated that by changing the FA design it is possible to significantly increase the neutron flux density and per unit power of reactor units. Tables containing main technical characteristics of different FAs for nuclear reactors of the IRT type are presented. Certain circuit engineering solutions for coolant circulation loops and characteristic design of research loops aimed at the solution of different research tasks are discussed.