Behaviour of Natural Circulation Flow in Uljin Unit 1 and 2 Plant

Since the TMI-2 accident, occurrence of severe accident natural circulation inside the RCS during in vessel core melt progressions before the reactor vessel breach had been emphasized and analyses were performed to clarify its thermal-hydraulic characteristics. In general, the natural circulation occurs due to the density difference, which for a single phase flow, is typically generated by temperature difference. Three natural circulation flows can be formed during severe accident; in-vessel, hot-leg countercurrent flow and flow through the coolant loops. Each of these flows may be present during high pressure transients such as station blackout (SBO) and total loss of feedwater (TLOFW). This natural circulation redistributes the heat energy generated in the core into the RCS structures. This energy redistribution will slow the heatup of the core, which in turn may affect the damage progression or the extent of the core damage. Slowing the core damage would allow more time for systems to be recovered to mitigate or terminate the accident. However, the energy removed from the core will be deposited on the other structures. This deposition of energy on the structure heats up the RCS and steam generator pipes which renders the occurrence of either the RCS creep rupture or the steam generator tube rupture faster. The question of which one occurs first seems still in debate due to the uncertainties inherent in the calculation. Because the steam generator tube rupture provides a direct path of fission product release to the environment, an accident management of stopping the natural circulation is also conceivable and should be taken into account in developing the accident management guideline. In this paper we have analysed the behaviour of the natural circulation taking into account the accident management action during the SBO accident and found the way to stop the natural circulation. The analysis was performed using MELCOR 1.8.5 code for Uljin unit 1 and 2 nuclear power plant