**3. Nucleate pool boiling**

Nucleate pool boiling is the first type of boiling to be used by humans and the first one to be investigated. Boiling curve for saturated water at the atmospheric pressure was obtained by Professor Shiro Nukiyama (Tohoku University, Japan) at the beginning of 1930s and for long time was called as the "Nukiyama's boiling curve" (see **Figure 1**). In the current view the boiling curve is updated with *q*max and *q*min values and melting temperatures of some common metals/alloys. Photos of nucleate pool boiling are shown in **Figures 3** and **4**.

Major nucleate-pool-boiling characteristics (see **Figure 1**) are as the following:


#### **Figure 6.**

*Wall- and bulk-fluid-temperature and pressure-loss-gradient profiles in uniformly heated vertical, bare tube at flow boiling (based on Figure 4 from Siemens: 25JahreBENSONbild\_E.doc [Accessed: February 22, 2022]).*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 7.**

*Critical-heat-flux (CHF) profiles vs. pressure in uniformly heated vertical, bare tube at flow boiling (upper solid curve) and in pool boiling (lower dashed curve).*

#### **Figure 8.**

*Flow boiling of water in vertical rectangular channel (8 12.5 730 mm; two opposite walls—st. st. and other two—transparent acrylic): Pressure 0.1 MPa; inlet velocity 0.036 m/s; subcooling temperature 90°C; heat flux 167 kW/m<sup>2</sup> ; scale—height of each photo equals to 150 mm in actual test section; width 12.5 mm; from left to right—portions of channel from lower to upper part starting from 130 mm of heated length. Vapor bubbles on photos are shown as black circles. Flow regimes from left bottom to right top (approximately): Bubbly flow; slug flow; annular flow; annular flow with entrainment of droplets; and single-phase steam flow. Liquid film on left and right st. st. walls moves up in photos 2–4.*


#### **Figure 9.**

*Bulk boiling in two-phase counter-flow thermosyphon on glass surface: Methylene chloride (R-30), atmospheric pressure, filling charge more than 100% of evaporator volume, evaporator—lower part of thermosyphon and condenser—upper part, in between—short transportation zone), and heat flux increasing from left to right.*

#### **Figure 10.**

*Nucleate boiling in two-phase counter-flow thermosyphon on st. st. surface: Methylene chloride (R-30), atmospheric pressure, filling charge more 40% of evaporator volume, and heat flux increasing from left to right up to critical heat flux (CHF).*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

**Figure 11.**

*Nucleate boiling in two-phase counter-flow thermosyphon on metal heated rod (annular-channel evaporator): Water, atmospheric pressure, filling charge 100% of evaporator volume, and heat flux increasing from left to right up to critical heat flux (CHF).*

Therefore, for all these Points /Regions we need to have the appropriate correlations (for general correlations, see, for example, Chapter 10 in [9]). In general, there are three internal boiling characteristics such as (for details, see [23–27]; and **Table 1**): (1) vapor-bubble departure diameter, *D*b; (2) frequency of vapor-bubbles departure, *f*; and (3) mean velocity of vapor-bubble growth, *u*<sup>b</sup> = *D*<sup>b</sup> *f*b. However, these internal-boiling characteristics are not easy to estimate, and their uncertainties

#### **Figure 12.**

*Nucleate boiling in two-phase counter-flow thermosyphon on metal heated rod (annular-channel evaporator): Water-ethylene-glycol mixture (water boiling temperature 100°C and ethylene-glycol—200°C), atmospheric pressure, filling charge 100% of evaporator volume, and heat flux increasing from left to right up to critical heat flux (CHF). Photo 1: no boiling—mixture not separated; photos 2–9—mixture is separated, i.e., water (liquid density—958 kg/m<sup>3</sup> ) boils in the upper part of evaporator and non-boiling ethylene-glycol as liquid (density— 993 kg/m3 ) transfers heat to boiling water with natural convection in the lower part.*

are quite high. Also, there are some theoretical approaches to boiling heat transfer, but, usually, only empirical correlations are used for various nucleate-pool-boiling characteristics/parameters, which are based on well-known and well-defined thermophysical properties. For example, the vapor-bubble departure diameter is usually replaced with [9]:

$$D\_{\mathbf{b}} \propto \sqrt{\frac{\sigma}{\mathbf{g}\left(\rho\_f - \rho\_\mathbf{g}\right)}}.\tag{1}$$

In 1952, W. Rohsenow has proposed his nucleate pool-boiling correlation, which is the most widely used correlation during the last 70+ years.

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 13.**

*Universal experimental setup for boiling experiments at various operating conditions and in different flow geometries (current view—test section (to the left) is two-phase counter-flow thermosyphon: 1—condenser; 2 cooling jacket; 3—coolant (antifreezing mixture water-ethylene glycol); 4—transportation (adiabatic) zone; 5 working fluid (WF); 6—evaporator; 7—current terminals; 8—insulation; 9—sheathed thermocouples (fluid/ vapor temperatures); 10—wall-temperature thermocouples; A—ammeter; G—electrical generator (power supply); V—voltmeter; and VP—vacuum pump.*

The main concept of this correlation is that the heat transfer from the wall directly to the liquid with an increased heat-transfer rate, due to the agitation of liquid by the departing vapor bubbles.

$$\frac{c\_{p\\_f}\ \Delta T\_b}{H\_{\rm f\underline{g}}} = \mathcal{C}\_{\rm f\\_} \left[\frac{q}{\mu\_{\rm f}\ H\_{\rm f\underline{g}}} \sqrt{\frac{\sigma}{\mathcal{g}\left(\rho\_f - \rho\_{\rm g}\right)}}\right]^m \left(\frac{c\_{p\\_f}\ \mu\_f}{k\_{\rm f}}\right)^n,\tag{2}$$

where *Csf* is constant, depending upon the nature of the heating-surface- fluid combination (see **Table 2**). However, some other well-known correlations do not include any heating-surface parameters/properties or impact of the heating-surface- fluid combination on HTC at boiling (for details, see [10]).

Detailed analysis of the data in **Table 2** has shown that information on the surfacefluid combination is too simplified and, actually, misleading. A thorough analysis of original publications in which *C*sf values were obtained is presented in the joint

#### **Figure 14.**

*Test sections for boiling experiments at various operating conditions: (a) two-phase counter-flow glass thermosyphon (bulk-boiling photos—Figure 9); (b) two-phase counter-flow thermosyphon with boiling on st. st. surface (nucleate-boiling photos—Figure 10); (c) two-phase counter-flow glass thermosyphon with multiple evaporators and horizontal condenser); and (d) two-phase counter-flow thermosyphon with boiling on st. st. internal tube (annular boiling channel) (nucleate-boiling photos—Figures 11 and 12):—fluid-expansion tank; 2—cooling jacket; 3—condenser; 4—heating jacket; 5—evaporator; and 6—current (power) terminals.*

publication by I. Pioro, W. Rohsenow, and S. Doerffer [10, 23] and by Pioro [8] together with the latest Pioro correlation on the pool-boiling heat transfer. This list of *C*sf values is the most comprehensive and detailed one so far (see Appendix A at the end of this Chapter).

The major problem with correlations, which account for a heating surface-fluid combination, is that these correlations can be used only for a particular heating surface and fluid used in experiments. Otherwise, uncertainties can be very high! On opposite, if correlations, which do not account on a particular heating surface and fluid combination, are used, it is impossible to predict uncertainties of HTCs calculated!

The most important nucleate-pool-boiling characteristic is the Critical Heat Flux (CHF), because if the CHF is reached, the boiling-surface temperature can jump to very high values (beyond 1000°C, see **Figure 1**) and, eventually, the boiling surface can be damaged or even melted. Of course, this temperature rise depends on the type of heating, i.e., for electrical and nuclear heating temperature rise can go far beyond 1000°C. However, if the boiling surface is heated with hot or high-temperature medium, the surface temperature cannot be higher than that of this medium.

The mostly used CHF correlation for pool boiling is as the following (for details, see **Figure 7**):

$$q\_{\rm cr} = \mathcal{C}\_{\rm cr} H\_{\rm fg} \left[ \sigma \mathbf{g} \rho\_{\rm g}^2 \left( \rho\_{\rm f} - \rho\_{\rm g} \right) \right]^{0.25},\tag{3}$$

where *C*cr is constant with the average value of 0.15. However, in reality, this constant can be within the range of 0.08–0.28! This correlation was obtained through a dimensional analysis by S.S. Kutateladze in Russia in 1948 and through a hydrodynamicstability analysis by N. Zuber in the United States in 1958 [9].

Therefore, in conclusion we can say that in spite of more than 100 years of active research into the pool-boiling phenomena, we have failed to develop universal HTC,

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 15.**

*a-e. Electron-microscope images (enlargement 100) of plates made of: (a) aluminum—*R*<sup>q</sup> = 4.5 μm,* R*sk = 0.47 μm; (b) copper—*R*<sup>q</sup> = 1.7 μm,* R*sk = 0.38 μm; (c) brass—*R*<sup>q</sup> = 0.7 μm,* R*sk = 1.3 μm; (d) st. st.—* R*<sup>q</sup> = 0.6 μm,* R*sk = 0.19 μm; and (e) polyethylene high density. For thermophysical properties and surfaceroughness parameters, see Tables 3 and 4, respectively, and Appendix A). Details on experimental setup with these heated plates and experimental data are presented in [8]. f. Effect of heat flux on HTC at nucleate pool boiling of R-11 on copper (*Rq *= 1.7 μm) (b) and plastic (PHD) (e) large-size plates.*

CHF, minimum heat flux, film boiling, and other correlations with a reasonable accuracy, which can be applied to various heating surfaces with different thermophysical properties, surface-roughness parameters and microgeometry, wall thickness, orientation in space, and different boiling fluids within a wide range of operating conditions!

The thermophysical properties of boiling surfaces are listed in **Table 3**.

#### **Figure 16.**

*Effect of gravity on boiling heat transfer and CHF in horizontal tube: Water,* P *= 10 MPa,* G *= 500 kg/m2 s,* D *= 24.3 mm (based on Kohler and Hein [11]) (Courtesy of NRC, USA).*


*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*


**Table 1.**

*Internal boiling characteristics of various fluid-surface combinations [23, 24].*


#### **Table 2.**

*Values of* C*sf for various surface-fluid combinations [9].*


#### **Table 3.**

*Thermophysical properties of boiling surfaces (extended plates) at 27°C (listed according to decreasing thermalconductivity values) [9].*

A laser profilometer was used to determine the surface-roughness parameters that are listed in **Table 4**. The characteristics of the laser profilometer itself were as follows:



**Table 4.**

*Average surface-roughness parameters of boiling surfaces (extended plates) (listed according to decreasing surfaceroughness (*R*q/*R*a) values) (for descriptions of all surface roughness parameters, see below or in [10]).*

#### *Explanations to* **Table 4***.*

Simple-Roughness-Amplitude Parameters.

*Mean parameters.*

*R*<sup>a</sup> average roughness: area between the roughness profile and its mean line or its integral of the absolute value of the roughness-profile height over the evaluation length. The average roughness is the most commonly used parameter in surface-finish measurements.

*R*<sup>q</sup> root-mean-square roughness (rms roughness): average roughness parameter calculated as a square root from another integral of the surface-roughness profile. Root-mean-square roughness was a commonly used parameter in the past; however, nowadays it has been replaced with *R*<sup>a</sup> in metal-machining specifications. Usually (but not necessarily), *R*<sup>q</sup> is 1.1–1.3 times larger than *R*a.

*Extreme parameters.*

*R*<sup>p</sup> peak roughness (height of the highest peak in the roughness profile over the evaluation length).

*R*<sup>v</sup> depth roughness (depth of the deepest valley in the roughness profile over the evaluation length).

*R*<sup>t</sup> total roughness (vertical distance from the deepest valley to the highest peak), *R*<sup>t</sup> = *R*<sup>p</sup> + *R*v.

*Mean-extreme parameters.*

*R*pm mean-peak roughness (average peak roughness over the sample length).

*R*<sup>z</sup> mean-total roughness (average value of the five highest peaks plus the five deepest valleys over the evaluation length).

*R*z3 mean-total roughness of third extremes parameters (average vertical distance from the third deepest valley to the third highest peak).

Mean-extreme parameters are less sensitive to single unusual features, such as artificial scratches, gouges, burrs, etc.

*Roughness-spacing parameters.*

*HPC* High-Peak Count per length (number of peaks per length that cross above a certain threshold and then go back below it).

*Mean-roughness-spacing parameters.*

*S*<sup>m</sup> mean spacing between peaks (peaks cross above a mean line and then go back below it).

*λ*<sup>a</sup> average wavelength of surface.

*λ*<sup>q</sup> rms (root-mean-square) average wavelength of surface.

*Roughness-hybrid parameters.*

*Δ*<sup>a</sup> average of absolute slope of roughness profile over the evaluation length. *L*<sup>o</sup> actual profile length (in all measurements, this was 8 mm).

Statistical parameters.

*R*sk skewness (this parameter represents the profile variation symmetry over its mean line). Surfaces with *R*sk < 0 have fairly deep valleys in a smoother plateau. Surfaces with *R*sk > 0 have fairly high spikes, which protrude above a flatter average.

## **4. Flow boiling**

Flow boiling is boiling with forced convection, which is the most used type of boiling in industry [28], especially, in thermal- and nuclear-power industry [29, 30] and air-conditioning and refrigeration industry [31, 32]. In the thermal-power industry gas-fired and coal-fired (or fossil-fuel-fired) power plants are used, many of which equipped with the subcritical-pressure Rankine steam-turbine cycle (see **Figure 17a**) [33, 34]. In nuclear-power industry of the world there are 441 nuclear-power reactors connected to electrical grids of which 60 reactors are Boiling Water Reactors (BWRs) including several Advanced BWRs (ABWRs). Moreover, all current nuclear-power reactors are connected only to Rankine steam-turbine power cycles in which boiling takes place in steam generators (in BWRs and ABWRs saturated steam is generated inside reactors (for details, see **Figures 17b**, **18**, and **19**).

More information on all current and future nuclear-power reactors and their power cycles can be found in [29].

The main advantage of using flow or forced-convection boiling is very high HTCs compared to other types of heat transfer (see **Table 5**).

Major flow-boiling characteristics (see **Figure 6**) are as the following:


In general, these flow-boiling characteristics are quite similar to those of pool boiling. It is impossible to provide correlations for all cases of pool boiling as well as of flow boiling. However, this Chapter contains a list of references and bibliography, which have quite a large number of various cases covered and correlations provided.

**Figures 20**–**23** show specifics of flow boiling in circular tubes, and the experimental setup for these experiments is shown in **Figure 24**. This study covers only two fluids: water and R-134a. To enable a comparison of CHF results between water and R-134a, the R-134a results were converted to their water-equivalent values using the following CHF fluid-to-fluid modeling relationships. It has been shown for vertical

#### **Figure 17.**

*Temperature (*T*) vs. specific entropy (*s*) simplified diagram of subcritical-pressure Rankine power cycle: (a) fossil-fuel-fired thermal power plant with superheated primary- and secondary-steam reheat and (b) advanced boiling water reactor (ABWR) with saturated primary steam and overheated secondary-steam (see Figure 19). Abbreviations: HPT—high-pressure turbine; LP—low pressure; MS—moisture separator; RH—re heater; SG steam generator; and SHS—super heated steam.*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 18.**

*Heat-transfer tubes are installed into steam generator (SG) of PWR (1110 MWel and 3060 MWth) (in total four SGs per one reactor) (courtesy of Rosatom): https://www.flickr.com/photos/rosatom/36999718643/in/album-72157675727427445/ [Accessed: December 10, 2023]; Photo by E. Lyadov, Atommash, 2016.*


*Reactor coolant is inside tubes and Rankine-cycle feedwater heated and boiling outside tubes.*

tubes (see in [36]) that if the fluid-to-fluid modeling relationships are satisfied, i.e., *L*<sup>R</sup> = *L*W, *D*<sup>R</sup> = *D*<sup>W</sup> (geometric similarity),


*xcr R* ¼ *xcr W* (thermodynamic similarity), then the dimensionless CHF expressed as *qcr G hfg* " # *R* <sup>¼</sup> *qcr G hfg* " # *W* will also be the same for both fluids. Even though the study deals

with experiments in R-134a, the water CHF look-up table is also used as a reference, as this table represents an already normalized CHF database for water.

The look-up table data were normalized to tubes with an 8 mm ID; to obtain the CHF for a different diameter, a simple correction can be applied: *CHFD CHFD*¼<sup>8</sup> *mm* <sup>¼</sup> *<sup>D</sup>* 8 � ��0*:*<sup>5</sup> , where *D* is the Inside Diameter (ID) of a circular tube in mm, *D* = 8 mm is the reference tube ID.

#### **Figure 19.**

*Simplified layout of typical boiling water reactor (BWR) NPP (courtesy of U.S. NRC): General basic features— (1) thermal neutron spectrum; (2) uranium-dioxide (UO2) fuel; (3) fuel enrichment about 3%; (4) direct cycle with steam separator (steam generator and pressurizer are eliminated), i.e., single-flow circuit (single loop); (5) reactor pressure vessel (RPV) with vertical fuel rods (elements) assembled in bundle strings cooled with upward flow of light water (water and water-steam mixture); (6) reactor coolant, moderator and power-cycle working fluid are the same fluid; (7) reactor coolant outlet parameters: Pressure about 7 MPa and saturation temperature at this pressure is about 286°C; and (8) power cycle—subcritical-pressure regenerative Rankine steam-turbine cycle with secondary-steam reheat (for details, see Figure 17b). The largest BWR has installed capacities: 1435 MWel and 4500 MWth.*


#### **Table 5.**

*Selected typical heat-transfer-coefficient (HTC) ranges of various coolants [35].*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 20.**

*CHF vs. critical quality at flow boiling in vertical bare circular tube (ID 6.92 mm, OD 7.93 mm, heated length 0.45–1.98 m, material Inconel): R-134a,* P *= 1.67 MPa, and* G *= 1000 kg/m2 s. (a) Full scale and (b) the same as in (a), but in enlarge scale. (For details, see [36]).*

The largest by scale and the most expensive experiments are performed in nuclearpower industry to determine the abovementioned flow-boiling characteristics, because any new bundle design or even updated one requires the exact knowledge of these characteristics. Samples of several bundle-string designs and fuel channel are shown in **Figures 25** and **26**, respectively. Also, in nuclear reactors usually axial and radial heat fluxes are not uniform. These specifics increase significantly the complexity of manufacturing test sections/stations (directly-heated with electrical current thin-wall tubes have to be with variable wall thicknesses) (see **Figures 27** and **28**).

**Figure 21.**

*CHF vs. critical quality at flow boiling in vertical and horizontal circular tubes (ID 6.92 mm, OD 7.93 mm, heated length 0.45–1.98 m, material Inconel): R-134a,* P *= 1.31 MPa, and* G *= 2000 kg/m2 s. (For details, see [37]).*

All experiments with bundles are performed with electrically-heated bundle strings, so-called, bundle simulators (for details, see [39, 40]). Therefore, such bundle strings are usually made of Inconel or stainless steel thin-wall tubes and can cost millions of dollars. Also, experimental setups are very sophisticated in terms of measuring devices and require quite large power supplies, e.g., for water experiments with the full-scale bundle string as shown in **Figure 26**, it can be up to 15 MWel, but if modeling fluid (usually, R-134a) is used for additional set of experiments, power requirement can be significantly lower, i.e., up to 1.8 MWel (**Figures 29** and **30**).

To be able to scale operating conditions in water into those of R-134a and vice versa to scale PDO results from R-134a into water data the following scaling laws have been used:

*For pressure:*

$$
\left(\frac{\rho\_f}{\rho\_\lg}\right)\_R = \left(\frac{\rho\_f}{\rho\_\lg}\right)\_W \tag{4}
$$

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 22.**

*CHF vs. critical quality at flow boiling in vertical and horizontal tubes (ID 6.92 mm): R-134a,* P *= 1.67 MPa and G = 500 kg/m<sup>2</sup> s.*

*For mass flux:*

$$
\left[\frac{\mathbf{G} \cdot D\_{hy}}{\mu\_{\mathbf{g}}}\right]\_{R} = \left[\frac{\mathbf{G} \cdot D\_{hy}}{\mu\_{\mathbf{g}}}\right]\_{W} \tag{5}
$$

*For PDO HTC:*

$$\left(\frac{h\_{\rm PDO} \ D\_{\rm hy}}{k\_{\rm g}}\right)\_{R} = \left(\frac{h\_{\rm PDO} \ D\_{\rm hy}}{k\_{\rm g}}\right)\_{W}; \text{where} \quad h\_{\rm PDO} = \frac{q}{T\_{w} - T\_{\rm sat}}\tag{6}$$

*x*<sup>R</sup> = *x*W, where *x* is the thermodynamic quality. *Dimensionless CHF expressed as:*

$$
\left[\frac{q\_{cr}}{G\,h\_{\text{f\text{\text\text\text\text\text\text\text\text}}}\right]\_{R} = \left[\frac{q\_{cr}}{G\,h\_{\text{f\text\text\text\text\text\text\text\text\text\text\text\textdegree}}\right]\_{W} \tag{7}
$$

It should be noted that the most important parameters for BWR/ABWR bundlestring experiments are HTC, CHF, and PDO heat transfer. Moreover, even for PWRs (the largest PWR is the EPR (Evolutionary Power Reactor) by former company Areva, currently, by EDF (France): 1670 MWel and � 4590 MWth) and PHWRs (largest

#### **Figure 23.**

*(a) CHF vs. critical quality and (b) CHF enhancement vs. critical quality—Flow boiling in vertical circular tubes (ID 6.92 mm, OD 7.93 mm, heated length 0.45–2 m, material Inconel) without flow obstructions (i.e., bare) and with various flow obstructions: R-134a,* P *= 1.67 MPa, and* G *= 3000 kg/m2 s. (For details, see [38]).*

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 24.**

*Experimental thermalhydraulics R-134a loop: 1—gear pump; 2—coriolis-type mass flow meter; 3—preheater; —dielectric fittings, 5—Current (power) terminals; 6—electrical preheater; 7—sight glass; 8—condenser; —pressurizer; 10—pressure-relief valve; 11—refrigerant filter-dryer; 12—ball valve; 13—vacuum pump; —refrigerant storage tank; 15—pressure reducer; and 16—N2 container.*

PHWR is the CANDU-9 reactor (CANada Deuterium-Uranium)) by AECL (Canada): 878 MWel and 2750 MWth), which are not cooled with boiling light or heavy water, CHF and PDO at flow boiling should still be determined.

**Figure 31** shows the surface-temperature map for Element 35 at a pressure of 0.98 MPa and mass-flow rate of 9.6 kgs <sup>1</sup> with 28% overpower<sup>1</sup> . At high overpowers, dry patches coalesced at some angular locations and the maximum axial dry patch approached the complete length of the element. A full-length axial dry patch on an element could not be measured due to the limited traveling distance of the thermocouple drive unit. Based on the variation of surface temperature with axial distance, the full-element dryout was achieved at several high-power levels.

**Figure 32** shows a new application for boiling process such as an ultimate emergency cooling of the molten nuclear-reactor core (corium) during a severe nuclear

<sup>1</sup> Overpower is defined as: channel-power / critical-power

**Figure 25.**

*Designs of fuel-bundle strings or assemblies of two pressurized water reactors (PWRs): (a) square cross section (courtesy & copyright by MHI) and (b) hexahedron cross section (courtesy of ROSATOM) (Photo by A. Antonov, 2015): https://www.flickr.com/photos/rosatom/25761756447/in/album-72157692396689951/ [Accessed: December 10, 2023].*

accident (modern feature for Generation-III<sup>+</sup> reactor designs) (for details, see [29]). This new safety feature is in response to the Chernobyl NNP severe accident (April of 1986), when a large pressure-channel reactor (RBMK-1000: 1000 MWel and 3200 MWth) was completely melted, and there was a possibility for a corium to damage a concrete foundation of the reactor).

This new application of the boiling process has started to be implemented in new reactor's designs, but this unusual type of boiling is well-known for the Mother nature *Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

#### **Figure 26.**

*3-D image of pressurized heavy-water reactor (PHWR) fuel channel with 43-element bundle (based on AECL design; prepared by Dr. W. Peiman).*

#### **Figure 27.**

*Sample of stylized axial power profile (APP) or axial heat flux profile (AHFP) used for critical heat flux (CHF) tests and pressure-tube creep profiles: 3.3% for 10–15 years of operation and 5.1% for 20–30 years of operation (based on report COG-98-311) (courtesy and copyright by COG).*

for millions of years, because it is eventually quite close to the cooling of a molten volcano lava in oceans, seas, etc.

In addition, modern Generation-III<sup>+</sup> reactors are equipped with Passive-Core-Cooling System (PCCS), which at high heat flux will operate as boiling circulation loop (for details, see [41] or [29]).

More information on boiling, its characteristics, specifics, etc. can be found in the following publications: Boiling: Research and Advances [42]; Pioro et al. [43]; Naterer

*Sample of radial power profiles (RPPs) for CANDU-reactor bundle used for critical heat flux (CHF) tests (based on report COG-98-311) (courtesy and copyright by COG).*

**Figure 29.** *Simplified layout of large (full-bundle-string) thermalhydraulics R-134a loop [39, 40].*

[44]; Pioro et al. [45–47]; Handbook of Phase Change: Boiling and Condensation [48]; Groeneveld et al. [49]; Convective Flow Boiling [50]; Collier and Thome [51]; Lahey and Moody [52]; Whalley [53]; Hanne and Grigull [54]; Davis and Anderson [55]; Thorn et al. [56]; and Bergles and Rohsenow [57].

*Advances and Challenges of Boiling Heat Transfer DOI: http://dx.doi.org/10.5772/intechopen.114095*

*General layout of horizontal test station of MR-3 R-134a thermalhydraulics loop [39, 40]: PDT—pressure differential transducer and PT—pressure transducer.*

#### **Figure 31.**

*Surface-temperature map for element 35 at 28% overpower (actual power to critical power): R-134a, 37-element bundle (for details, see [40]).*

**Figure 32.**

*Containment heat-removal system (CHRS) (courtesy and copyright by AREVA (EDF)). Two fully redundant trains with specific diversified heat sink.*
