One of the effective features of IVR is an external reactor vessel cooling (ERVC) strategy. CHF testing of the pool boil was performed to select the potential candidate for improved IVR-ERVC. The maximum heat flux was reduced approx. 3 times compared to that of the case without liquid metal.
Background
General literature survey
The speed of the two-phase flow would be increased and have a positive effect on the CHF improvement. Critical heat fluxes as a function of angular position on the lower head are reported and related to the observed two-phase flow regimes. Park et al.13 measured the critical heat flux (CHF) on the upper part of the lower outer wall of the reactor vessel.
Objectives and scope
POOL BOILING CHF EXPERIMENTS
- Inclination effect
- Additives effect
- Nanofluid effect
- Experimental apparatus
- Test procedure and experimental uncertainty
- Result and discussion
- Conclusion
Before flooding the coolant into the reactor cavity, this corrosion layer would have already formed on the surface of the reactor vessel. The degree of the static contact angle is affected by the deposited materials on the surface of the heater. The dispersion stability of graphene oxide nanofluid under the chemical conditions in flooding water that includes boric acid, lithium hydroxide (LiOH) and trisodium phosphate (TSP) was checked in terms of surface charge or zeta potential before the CHF experiments.
MODIFIED HYDRODYNAMIC CHF PREDICTION MODEL
- Experimental apparatus
- Test procedure and experimental uncertainty
- Result and discussion
- Conclusion
Using R-123 preserves the integrity of the heating element both before CHF and during the film boiling phase. The final surface state of the heating element in the CHF test was identical to the initial state of the heating element in the observation test. Although the main factors contributing to this deposition are not clear, roughly three factors must be involved: the properties of the dispersed particles (size, shape, zeta potential, concentration, etc.), the properties of the heating element surface (roughness) , electrical/magnetic characteristics, etc.), and the properties of the base fluid (vapor/liquid density, surface tension, etc.) The morphologies of deposit layers formed on the heating element surface from different nanoparticle materials during nucleation are shown in Fig. 3-3.
The change in wavelength corresponds to the tendency to increase CHF for all tested nanofluids. The test heating elements were designed and prepared to allow direct observation of Rayleigh-Taylor wavelengths. An increase in CHF for nanofluids was predicted as a function of reduced Rayleigh-Taylor wavelength.
One can postulate that the diameter of bubbles is half the Rayleigh-Taylor wavelength in the original hydrodynamic instability model. The Rayleigh-Taylor wavelength for the first case is half the size of the second case. In a hydrodynamic instability model, the surface area is a constant π/16 regardless of the Rayleigh-Taylor wavelength.
The change in Rayleigh-Taylor wavelength affects the diameter of the bubble, the fraction of the area of the heating element covered by the evaporating vapor, and the velocity of the vapor exiting.
FLOW BOILING CHF EXPERIMENTS
Additives effect
Nanofluid effect
Kim et al.54 studied the effect of alumina nanoparticles on CHF under flow boiling conditions at low pressure. The results of this study first reported the potential of nanofluids to improve CHF in the flow state. The wettability of the nanoparticle-coated surface was improved compared to the bare surface.
A variety of nanofluids (alumina/water, zinc oxide/water, and diamond/water) were used to conduct the CHF test in flow condition. The parametric studies on the mass flux, nanoparticle material and concentration on the boiling process were carried out to determine the most important factor for the CHF improvement. After the CHF test, a significant amount of nanoparticles was found to precipitate on the heating surface.
The conclusion of this paper is that the increase in CHF is associated with humidification. Kim et al.56 explored the possible mechanism underlying the enhancement of CHF through the application of aluminum nanofluid at low flow and low pressure. Lee et al.58 performed the CHF test on flow boiling of aluminum and silicon carbide nanofluids under low flow conditions.
The increase in CHF occurred due to the improved wettability of the liquid film on the surface of the heater due to the deposition of nanoparticles.
Experimental apparatus
Vafaei and Wen57 investigated the critical subcooled flow boiling heat flux of aluminum nanofluids in single microchannels.
Test procedure and experimental uncertainty
After the working fluid was flooded in the test facility, a degassing process was carried out to remove the non-condensable gas by heating it to 95 ℃.
Results and discussion
The mechanism of construction of the coating layer is that the nanoparticles are deposited on the dried surface, the place where the bubbles left the heating surface. The nanoparticle coating layer was formed due to the evaporation of the thin liquid microlayer on the heating surface. Surface wettability is estimated by measuring the static contact angle on the heating surface using a sessile drop test.
In general, the contact angle of the nanoparticle-coated heater decreased compared to the uncoated or bare heating surface. In this region, the CHF or liquid film dehydration (LFD) is determined by the thickness of the liquid film covering the heating surface. In this work, the wetting properties of the heating surface were analyzed by measuring the static contact angle.
The static contact angle score was increased compared to one of the base surfaces. The thickness of the coating is an important parameter when estimating or calculating the thermal activity value. The thermal conductivity of these materials is not higher compared to the heater material.
The increase in thermal activity was initially not taken into account in explaining the CHF improvement.
Conclusion
Fortunately, the heat transfer area on the surface could be easily controlled because the graphene-coated heating surface had a non-wetted property. -5 Result of the heat balance test between thermal power and electrical power. e) Surface covered with graphene oxide particles Fig. 4. 4-11 CHF mechanism in the vertical geometry (a) Normal LFD (b) Local dehydration due to hydrodynamic instability. b) Graphene oxide coated heating surface.
GALLIUM LIQUID METAL FLOODING SYSTEM
Experimental apparatus
Simulated breakdown heat was generated by cartridge heaters that were inserted into the heated object. The material of the heated object was copper which was used to produce the desired geometry. Therefore, it is possible to simulate IVR-ERVC phenomena and compare the differences between typical ERVC and liquid metal ERVC strategies more realistically.
The melting point of this liquid metal is around 30 oC while its boiling point is around 2400 oC. The thermal conductivity of gallium is about 50 times that of water at 30 oC. This liquid metal is considered as a potential candidate for the liquid metal IVR-ERVC strategy.
The input power was calculated using the current and voltage displayed on the power regulator screen. When the peak temperature exceeds the design limit of the material, the power can be automatically cut off to prevent damage to the cartridge heaters and the heated object. The built-in thermocouples were placed at an angle of 90 degrees to the stagnation point, which is the lowest point on the heated surface.
The temperature data obtained from these thermocouples are used to calculate the local heat flux and heat transfer coefficient on the surface of the heated object.
Test procedure and experimental uncertainty
Results and discussion
If the liquid metal solidifies during an injection from the storage tank of a liquid metal into the reactor cavity, the ERVC strategy itself may fail. Liquid metal plays a role as the heat transfer and storage medium in the ERVC system. The decay heat generated in the corium is transferred between the reactor vessel and the liquid metal.
The heat dissipation capacity of the system depends on the relative configuration between the liquid metal and the borated water. It ensures the safety of the reactor vessel and lowers the temperature of the liquid metal in the ERVC system. However, in the current system, large volumes of liquid metal are required to flood the reactor cavity.
In the liquid metal system, we could expect new heat transfer phenomena and prevent the occurrence of CHF. On the other hand, heat is also transferred to the liquid metal by convection. However, a high temperature gradient exists in the liquid metal because the thermal conductivity of the liquid metal is high.
Another insight is that the temperature of the outer surface of the reactor vessel is higher when the liquid metal was flooded.
Conclusion
The remaining thickness of the reactor vessel becomes thinner when the temperature of the outer surface is increased. It is expected that the reduced thickness is not a serious concern because the internal temperature reaches the melting point of the reactor vessel regardless of the liquid metal flooding. Reduction of thickness can be relatively reduced according to the temperature difference between the inner and outer surfaces of reactor vessel.
CONCLUSIONS AND RECOMMENDATIONS
CHF enhancement of nanofluids
CHF enhancement of gallium flooding system
Recommendations
Chu, T.Y.; Bentz, J.H.; Slezak, S.E.; & Pasedag, W.F.; Ex-ship boiling experiments: laboratory-scale testing and reactor of the flooded cavity nuclear containment concept Part II: Reactor-scale boiling experiments of the flooded cavity nuclear containment concept. Jeong, Y.H., Chang, S.H., & Baek, W.P.; Critical Reactor Vessel Wall Heat Flux Experiments Using 2-D Sliced Test Section. H.; An experimental study on CHF in the SA508 test heater pool boiling system under atmospheric pressure.
You, S.M.; Kim, J.; Kim, K.; Effect of nanoparticles on critical heat flow of water in pool boiling heat transfer. Kim, S.J.; Bang, I.C.; Buongiorno, J.; Hu, L.; Changing the surface wettability during pool boiling of nanofluids and its effect on critical heat flux. Buongiorno, J.; Hu, L.W.; Apostolakis, G.; Hannink, R.; Lucas, T.; Chupin, A.; A feasibility assessment of the use of nanofluids to improve vessel retention in light water reactors.
Rohsenow W.M.; Griffith P.; Correlation of Maximum Heat Flux Data for Boiling of Saturated Liquids, Cambridge, Mass.: Massachusetts Institute of Technology, Division of Industrial Cooperation, 1955. Kim, S.J.; McCrell, T.; Buongiorno, J.; Hu, L.W.; Alumina nanoparticles improve the flow boiling critical heat flux of water at low pressure. Kim, S.J.; McCrell, T.; Buongiorno, J.; Hu, L.W.; Experimental study of flow critical heat flux in alumina-water, zinc oxide-water and diamond-water nanofluids.
Park, H.M.; Jeong, Y.H.; Heo, Sun.; The effect of the geometric scale on the critical heat flux for the top of the reactor vessel bottom head.
