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In water‐cooled nuclear reactors, the maximum power which can be extracted from the core is limited by critical heat flux (CHF). CHF in the high‐quality region is known as dryout. In advanced nuclear reactors, the coolant flow occurs solely by virtue of natural circulation; however, instabilities may occur during off‐normal operations. This may lead to premature dryout due to lower coolant flow rates seen by the heater during such oscillations. This paper describes the experimental investigation on the effect of flow oscillations on the CHF with the time period of 120 s, which is observed typically in the large‐scale natural circulation system. Based on observations made with respect to temperature transient, the continuous dryout is preceded by the transient dryout for higher flow oscillations. But as flow fluctuation decreases, the transient dryout phenomenon is found to disappear. The applicability of the look‐up table to predict CHF under oscillatory flow conditions using suitable correction factors (CFs) for premature dryout has been evaluated. CFs for the CHF under oscillations suggested by previous authors have been compared. The maximum possible degradation in CHF value suggested by previous authors has been found to agree with the present experimental data. Percentage fluctuation in heat transfer coefficient (HTC) at fully developed annular flow conditions has been evaluated, and it is found that fluctuation in HTC is in phase with the fluctuation in flow. [ABSTRACT FROM AUTHOR]

The primary objective of core catchers is to minimize the potential risk of core meltdown by stabilization and cooling of molten corium within the reactor containment for a prolonged period by strategically cooling it. Quenching of the high‐temperature melt by top flooding strongly depends on the timing of injection of the cooling water, that is, immediate flooding or delayed flooding. To understand this aspect, we conducted experiments in a simulated core catcher of an Indian advanced nuclear reactor. In the experiments, sodium borosilicate glass, as a mixture of corium and sacrificial material simulant, was melted at about 1200°C and cooled by water from the side of a simulated core catcher and then flooding at the top of the melt pool. Two experiments were conducted; the first one was done with immediate flooding at the top of the melt pool; the second was performed with delayed top flooding of 45 min gap. The results show that immediate flooding quenches the melt pool rapidly compared to delayed flooding. A stable crust formation at the top of the melt pool and wall adhesion is observed in the delayed flooding. It was also found that no ingression of the water in the molten pool occurred with delayed flooding. On the other hand, with immediate flooding, water was found to ingress into the melt pool due to gap formations between melt and core catcher sidewall, causing the melt to break up through eruption by ingressed water. This phenomenon resulted in the formation of sand‐type debris and in faster melt cooling. [ABSTRACT FROM AUTHOR]