Effects of Slow Power Transients on CHF

Abstract

Slow power transients can occur during fuel shuffling, abnormal flux tilts or slow loss of regulations in CANDU reactors. Depending on the severity of the transients and flow conditions, the fuel elements may experience a rapid dryout. Critical heat flux (CHF) prediction methods, based on steady-state conditions, are frequently used in the analysis of these transients. This paper examines the validity of these methods in assessing the effects of transients on CHF. Due to the scarcity of transient data relevant to CANDU conditions, an experiment was undertaken at the Chalk River Laboratories to investigate the effects of slow power transients on dryout power, quenching, hysteresis (during dryout and rewetting) and two-phase pressure drop. The tests were carried out in a 2.5-m long Inconel-600 tube of 5.45-mm inside diameter mounted vertically in the MR-1 high-pressure water-loop at Chalk River. The test section, having a hydraulic diameter equal to that of the smallest subchannel in a CANDU 37-element bundle, had uniform axial flux distribution and was heated by a transformer/rectifier system capable of generating 300 kW. The conditions in a CANDU reactor were simulated in the MR-1 test facility by setting the flow parameters appropriately. Surface-temperature and pressure-drop data were obtained during linear power variations, in the vicinity of CHF, at rates corresponding to 1-, 2-, 4- and 8- min cycles. Data analysis revealed that the transient dryout power, on average, was within +/- 10% of the steady-state power. Transient time-to-dryout was about 30% greater (averaged over all the runs) than the calculated time-to-dryout. For the l-min power cycle, the ASSERT code predicted reasonably well the transient dryout power and the time-to-dryout.