الفهرس 
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Abstract
The partial core flow failures that arise from blockage in the core or its inlet structure, cause localized flow starvation in one or a few fuel assemblies. The flow blockages over a localized region of a reactor core may be caused by debris swept against the core inlet structure, or by the failure of the fuel or other component in the core. A flow blockage differs from many other reactor accidents because of their localized nature. Usually one or at most of a few fuel assemblies are initially affected. This often makes the prompt detection of the accident difficulty since the localized changes in power density and coolant conditions may not have sufficient effect on the core flux levels, coolant pressure DROP, or average coolant outlet temperature to cause the protection system to shut the reactor down. Analysis of none, one, two and three blocked coolant channels in a hot channel is carried out by the two codes; Modified CAST and RELAP5. The validation is carried out by the previous two codes besides analytical model and experimental data for none blockage case and results seem in a good agreement with other codes and experiments. A steady state, Cartesian two dimensional, laminar flow, and single phase computational procedure is applied to predict velocity, temperature, and neutron fluxes field in a hot channel of ETRR2 in case of blockage and without blockage in natural circulation mode at 400 KW. Continuity, momentum equations are coupled with the energy equation and the traditional twogroup neutron diffusion equations. Discretization method is the conservative controlvolume formulation method with colocated (nonstaggered) variable arrangement. The resulting finite volume equations are not linear, so it is relaxed. The Strongly Implicit Method (SIM) is implemented in this Modified CAST model to solve the linear equation sets. Two group diffusion equations are implemented in neutronic model. Two group constants were constructed from WIMSD4 code. RELAP5 is a one dimensional flow model, transient, two phase, laminarturbulent flow which is also used to solve this incident of blockage of coolant channel for hot channel in the whole core. The results of the two codes are in close for none, one and two blocked coolant channels. But the results for three blockages seem in large difference because the onset of nucleate boiling occurs. RELAP5 code treats this phenomenon of two phase flow while in Modified CAST mode; two phase flow solution was not included.