A study on premature occurrence of critical heat flux

The natural circulation flow characteristics are different from that of a forced circulation system. The flow in the former is dependent on the driving head caused by the density difference between hot and cold legs of the system. Many a times, the natural circulation flow is not fully developed and secondary flow may exist. So, under oscillatory conditions of such a system, the mechanisms of occurrence of the boiling crisis is more complicated and the flow oscillations can cause an early occurrence of the boiling crisis as compared to that of steady circulation even for the same geometric and operating conditions. AHWR is a thorium based vertical, pressure tube type, heavy water moderated, boiling light water cooled natural circulation reactor. AHWR fuel cluster is a cylindrical assembly of 120 mm diameter. The cluster has 54 fuel pins arranged in 3 concentric rings around a central rod. AHWR fuel cluster is simulated by bundle average method. In the current paper first analysis for CHF under oscillatory flow has been carried out for tubular T/S having ID of 9.1 mm and heated length of 500 mm using COPCOS and same was compared with RELAP 5/MOD 3.2. Effect of oscillatory flow on CHF in AHWR is also analysed in the paper