TRANSIENT THERMAL HYDRAULIC MODELING FOR OFF SITE POWER LOSS IN NUCLEAR REACTORS

Abstract

The international transient codes such as RETRAN, RELAP, etc., are mainly developed for nuclear power plants and it is difficult to adopt it in order to simulate a research reactor like the Egypt's second research reactor (ETRR-2) and its components. Therefore, a transient thermal-hydraulic model has been developed to simulate the steady-state operation and its transient behavior of ETRR-2 after loss of offsite power supply. The model consists of five interactively coupled sub-models for: (a) coolant, (b) clad, (c) fuel, (d) core chimney and (e) natural circulation flow. The model divides the active core into a specified axial regions and the fuel plate into a specified radial zones, then a nodal calculation is performed for both average and hot channels with a chopped cosine shaped heat generation flux. It also predicts the onset of nucleate boiling, onset of flow instability and critical heat flux phenomena. The developed model is verified under both the steady and transient states. In the steady• state conditions, its results were compared with the results of both PARET and TERMIC codes and the comparison shows a very good agreement. In the transient• state, the model results for core calculation show good agreement with PARET results for the ETRR-2. Also, the developed model was used also to simulate a 10 MW generic reactor of an IAEA benchmark problem and it shows a good agreement with benchmark calculations. The model is used to simulate the ETRR-2 behavior under loss of offsite power for typical core configuration of 29 fuel elements and minimum core configuration of 24 fuel elements in the reactor core. The simulation is performed under two cases; namely: (a) two flap valves open and (b) one flap valve fails to open. The simulation is made with a total power peaking factor of 3 asswning no engineering uncertainties. It was found that even under these unrealistic conditions, the pump coast-down is not enough to remove the decay heat in the core following full power (22 MW) operation without boiling and that the boiling phenomenon continues for long periods if one flap valve fails. The model predictions in both the steady and transient states were analyzed and discussed. When the uncertainty factors are accounted for, the safety factors do not fulfill the design goals for steady-state full power operation and bulk