Reactivity initiated accidents (RIAs) are a potential concern for nuclear reactor safety. In RIA scenarios, following the insertion of positive reactivity, e.g. by an unanticipated extraction of the control rods, the reactor power may increase exponentially. The period of the exponential rise, τ, depends on the amount of positive reactivity inserted, as well as the fuel composition. During such an event, boiling of the water coolant can provide not only an effective way of heat removal, but also a stabilizing, negative reactivity feedback. However, the reactor power could reach extremely high levels and lead to a boiling crisis, e.g. by departure from nucleate boiling (DNB), in turn leading to fuel damage. The aim of the current work is to improve the understanding of transient DNB phenomena. This goal was achieved by running experiments on a specially designed flow boiling platform, which includes high speed video (HSV) and high speed infrared (HSIR) diagnostics. Specifically, the IR radiation recorded by the HSIR camera was analyzed with dedicated post processing algorithms that enable measurements of the time-dependent temperature and heat flux distributions on the boiling surface. Experiments were performed on a flat heater in upward flow conditions at atmospheric pressure. This work explores the effects of flow velocity, liquid subcooling and exponential power escalation period on critical heat flux (CHF). The results show that, for our flow conditions, the CHF value does not depend on the escalation period for periods longer than 100 ms, and is essentially the same as in steady-state boiling. For shorter periods, CHF increases as the escalation period decreases, and the effect of flow velocity becomes less important at short periods. Larger subcooling was shown to increase the CHF at all conditions. For extreme cases of 50 K and 75 K of subcooling the entire heating surface was covered by tiny bubbles. Those bubbles had a very short (less than 50 μs) lifetime and were quenched right after the nucleation. Such behavior prevented bubbles from coalescing and resulted in a very efficient heat transfer mechanism. CHF was observed at much higher values compared to steady boiling conditions, when nucleation site density and bubble size were large enough for the bubble to start coalescing. An interesting effect was observed for very short periods (5, 10 and 20 ms) and low subcoolings (10 K). At those conditions the boiling surface experiences CHF during the growth of the first generation of bubbles. Therefore, the points of ONB and CHF are almost coincident, with CHF delayed only by the time required for adjacent bubbles to coalesce and the microlayer underneath them to evaporate. Thesis Supervisor: Matteo Bucci Title: Assistant Professor of Nuclear Science and Engineering

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Transient pool-boiling heat transfer

Transient flow boiling CHF under exponentially escalating heat inputs

The effect of pressure transients on the onset of intermittent dryout in upward annular flow was experimentally investigated in order to resolve the conflict between the observations drawn from two major data sets in the literature. A delay in time to the onset of dryout at the test section exit relative to the time predicted based on steady-state data was observed in the R-12 experiments of Celata et al [1, 2]. Steady-state prediction methods were sufficient to predict the upstream progression of a pre-existing dryout front in the water experiments of Lyons and Swinnerton [3]. Steady state and pressure transient dryout experiments were performed using water with outlet pressures of 2–6 MPa and mass fluxes of 1000–2500 kg m-2 s-1 in an electrically heated 1.32 m long 4.6 mm ID vertical Inconel 600 tube with depressurisation rates of up to 1.0 MPa s-1. Transient experiments were performed with a small margin to dryout and with post-dryout initial conditions in order to test the hypothesis that these initial conditions influenced the onset of dryout during transients. The results of a comparison between the steady dryout data and two dryout prediction methods—the Biasi et al correlation [4] and the 2006 CHF look-up table [5]—were used to develop correction factor correlations to reduce systematic error when these methods were used to predict the transient time to dryout. The transient prediction performance of these two methods for all of the transient data is summarized in the table below. There was no statistically significant variation between the preand post-dryout initial conditions. Based on this result it was concluded that the initial conditions did not affect the observed time to dryout. The mean wall temperature exhibited a discontinuous decrease as the heat flux

/pdf/measurement-and-prediction-of-the-onset-of-intermittent-1goqomo2hm.pdf

Measurement and Prediction of the Onset of Intermittent Dryout During Blowdown Transients for Upward Annular Flow

Pool boiling heat transfer and critical heat flux (CHF) were experimentally studied in subcooled temperatures ranging from 0oC to 20oC and under transient power conditions. A chromealuminum- iron alloy wire was used as the heating element. The heating rate in the test section was increased linearly depending on time by applying voltage control for 1s to 1000s. The transient boiling heat transfer coefficient (TBHTC), transient wire superheat temperature, transient heat flux and transient CHF were also obtained. The results showed that in the case of all subcooled temperatures and periods, the TBHTC increased in the nucleate boiling region because of the growth, separation, motion and turbulence of the bubbles. The TBHTC also decreased in the transition from nucleate boiling to film boiling because some part of the wire covered by temporary thin vapor film. The TBHTC again increased in film boiling due to the increment of radiation heat transfer. The TBHTC decreased in the second part of the film boiling due to the heat flux and the vapor film thickness around the wire had increased. Relative to the saturation condition, the timely average of the wire superheat temperature for subcooled temperatures of 10oC and 20oC , respectively, decreased by 9.23% and 9.29% in the nucleate boiling region and in a time period of 1000s.

https://ajme.aut.ac.ir/article_3432_356730cb0ae73e4190a95c392c830585.pdf

The Effects of Subcooled Temperatures on Transient Pool Boiling of Deionized Water under Atmospheric Pressure

Steady-state and transient critical heat fluxes (CHFs) were measured using a 1.0 mm diameter horizontal cylinder in a pool of highly wetting liquid, such as ethanol, due to steady and transient heat generation rate for wide range of subcoolings and pressures. Boiling CHF was assumed to happen based on a kind of hydrodynamic instability (HI) at CHF, and the model is supposed that the increase in vapor generation from the cylinder surface causes a limit of the steady-state vapor escape flow when CHF occurs. However, it has been understood that the CHF value changes systematically depending on experimental conditions according to a series of experiment. As the reason, a large part of the transition processes to film boiling at CHF highly subcooled liquid at high pressure was not explained by the mechanism of heat transfer crisis based on the HI model. Another mechanism assumed is that the transition occurs due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on the cylinder surface in liquid. This research is to find the mechanism in the transition boiling CHF, which is confirmed that it depends on the HI and HSN.

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Subcooled Pool Boiling CHF in Ethanol

/pdf/critical-heat-fluxes-caused-by-an-increasing-heat-input-and-402v8u3r86.pdf

Transient pool-boiling heat transfer

Citations

Transient flow boiling CHF under exponentially escalating heat inputs

Measurement and Prediction of the Onset of Intermittent Dryout During Blowdown Transients for Upward Annular Flow

The Effects of Subcooled Temperatures on Transient Pool Boiling of Deionized Water under Atmospheric Pressure

Subcooled Pool Boiling CHF in Ethanol

Critical Heat Fluxes Caused by An Increasing Heat Input and A Rapid Depressurization

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