It is established that typically in a nuclear reactor has its channel integrity depending on the coolability of the moderator as the prime heat sink as well as normal working or operating conditions. Hence a thorough understanding of the behavior of the heat transfer involving the moderator is important in the safe design of the reactor. CFD investigations are carried out to study the thermal dissipation estimation in a typical Calandria using a 3–dimensional RANS code. Internal flow computations and experimental studies are carried out for a calandria embedded with a matrix of tubes working together as a reactor. Numerical investigations are carried on the Calandria reactor vessel considering 480 fuel channels, with tangential inlets (14⁰), and outlets located at the bottom with 30⁰ angle to study the flow pattern and the associated temperature distribution. Computations are made for simulations of flow and convective heat transfer for assigned near–to working conditions with different moderator injection rates and reacting heat fluxes in the setup. The results of computation provide an estimate of the tolerance bands for safe working limits for the heat dissipation at different working conditions by virtue of prediction of the hot spots in the calandria. The isothermal CFD results are validated by a set of experiments on a specially designed scaled model conducted over a range of flows and simulation parameters. The results are shown include comparison of the computational and experimental work in this regard. The CFD analysis with heat transfer is carried out using a industry standard CFD tool CFX, using 0.48 million nodes for numerical analysis. The work assumes significance for the design considerations of the reactors and for detailed and critical parametric studies for optimization for the geometry considered.