Transient analysis of break below the grid in Tehran research reactor using the newly enhanced COBRA-EN code

Recent availability of high performance computers and computational methods along with continuing increase in operational experience necessitate revising some operational constrains and conservative safety margins. Implementation of advanced methods constitute a real necessity in the safety and design analysis and allow obtaining more realistic simulation of the processes taking place during the normal operations and transients.In this paper, COBRA-EN thermal–hydraulic code is extended and upgraded to include analysis of plate type fuels in research reactors. Furthermore, the convection heat transfer coefficient in the code is modified and a simple model of natural convection is added and a procedure to take hot channel data or other required information in each time step is included for post-processing of the results. To evaluate the capability of the upgraded code three cases of break below the grid accident in Tehran research reactor (TRR) core which contains plate type fuels are simulated. The safety analysis report of TRR confirms the results, convergence rate and reliability of developed code.

► The case of loss of flow and flow inversion in a pool type reactor are investigated.
► The COBRA-EN code is upgraded to model plate type fuel elements.
► It is observed that the performance of the upgraded code is adequate.
► Natural circulation is an important attribute in transient analysis of pool type reactors.