Patent Publication Number: US-2015068910-A1

Title: Apparatus and method for anodizing inner surface of tube

Description:
TECHNICAL FIELD 
     The present invention relates to an apparatus and a method for anodizing a surface that is mainly composed of zirconium such as cladding of a fuel rod and has a non-planar surface. 
     BACKGROUND ART 
     Zirconium has a melting point of 1852° C. and very strong resistance to corrosion from the outside environment due to chemical stability. As a result, zirconium or an alloy thereof and an oxide thereof have been used in equipment which should not be corroded, such as fuel rod claddings of a nuclear power plant, stainless ceramic kitchen utensils, and tools used for handling chemicals. 
     Meanwhile, a nuclear fuel is covered by an aluminum or magnesium film so that toxic substances generated in a fission process are mixed with a coolant and not exposed to the outside. In the case of a light-water reactor, low enriched uranium dioxide (UO 2 ) powder is molded and sintered as cylindrical tablets having a diameter of 2 cm and a height of 2 cm to prepare dark brown pellets, and the pellets are put in a thin metal tube, a cladding, of about 3 mm made of a zirconium alloy (zircaloy) having good corrosion resistance to a high-temperature coolant and both ends are sealed. 
     Generally, tens to hundreds of fuel rods make a fuel assembly as one bundle and are used as one unit, and hundreds of fuel assemblies are include in a nuclear pile. The cladding is prepared with a wall thickness of 1 mm or less so that heat is conducted well, and during nuclear fission, a center temperature of the fuel pellets is about 200° C., a surface temperature is about 600° C., and temperatures of the inner surface and the surface of the fuel rod are about 400° C. and 300° C., respectively. 
     As such, when the fuel rod exposed at a high temperature is broken, a nuclear fission product is exposed to the outside to cause a serious problem, and as a result, efforts for increasing stability are continuously required. 
     As one index of stability applied to an apparatus for heating an operation fluid such as the kind, a critical heat flux (CHF) value is included, and the value means a maximum heat flux at which a heater is not broken but withstands in a heating situation. On the other hand, when the heat flux value is exceeded, it means that the heater is broken, and particularly, in a facility such as a nuclear power plant, a critical accident such as the core melting may be caused, and as a result, the CHF is a very important index. 
     All machines heating the operation fluid in addition to the nuclear power plant are managed by applying a safety factor of a predetermined level or more so as to not exceed the CHF value during the operation. However, since the safety management sacrifices performance of the device, researches for maintaining the safety factor and improving the performance by increasing the CHF value itself of the heater have been conducted. In studies in the past, it is known that as the surface of the heater is more hydrophilic, the CHF value is increased, and recently, furthermore, researches of largely increasing the hydrophilic property and the CHF by using nanoparticles or a minute structure have been conducted. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     DISCLOSURE 
     Technical Problem 
     The present invention has been made in an effort to provide an anodizing apparatus and a method thereof having an advantage of forming a minute structure on a surface so that an internal surface of a tube mainly composed of zirconium has a hydrophilic property. 
     Technical Solution 
     An exemplary embodiment of the present invention provides an apparatus for anodizing a surface of a tube, DeletedTexts including: an electrolyte container storing an electrolyte solution; a first solution conduit connected with the electrolyte container to receive the electrolyte solution; a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit; a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube; a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit; and a cathode rod inserted from the second jig and extended to the first jig through the inner portion of the targeted tube, in which while the electrolyte solution passes through the inner portion of the targeted tube, a cathode is applied to the cathode rod, and an anode is applied to the targeted tube to perform an anodizing process. 
     The apparatus may further include a cleaning solution container storing a cleaning solution, in which the cleaning solution container may be connected with the electrolyte container and the first solution conduit through a three-way valve. 
     The apparatus may further include a first constant-temperature water bath receiving a coolant, in which the electrolyte container and the cleaning solution container may be installed to be immersed in the coolant of the first constant-temperature water bath. 
     A flow meter and a flow control device may be installed in the first solution conduit to control a flow of the electrolyte solution or the cleaning solution passing through the first solution conduit. 
     The apparatus may further include a second constant-temperature water bath receiving a coolant, in which the targeted tube may be installed to be immersed in the coolant of the second constant-temperature water bath. In the first jig, a solution conduit connection hole to which the first solution conduit is insert-connected may be formed at a first side portion, and a targeted tube connection hole to which one end of the targeted tube is insert-connected may be formed at a second side portion. 
     The first jig may further include a cathode rod insertion hole to which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the first jig may be formed on a concentric axis. 
     In the second jig, a solution conduit connection hole to which the second solution conduit is insert-connected may be formed at a first side portion, and a targeted tube connection hole to which the other end of the targeted tube is insert-connected may be formed at a second side portion. 
     The second jig may further include a cathode rod insertion hole into which the cathode rod is inserted, and the targeted tube connection hole and the cathode rod insertion hole of the second jig may be formed on a concentric axis. 
     The first solution conduit, the targeted tube, and the second solution conduit may be connected with each other to form a U-shaped conduit, and the targeted tube may be positioned at a downstream branch of the U-shaped conduit. 
     The electrolyte solution may be a hydrofluoric acid solution, and the targeted tube may be a zirconium tube or a zirconium-alloy tube. In addition, the cathode rod may be a stainless steel rod. 
     Another exemplary embodiment of the present invention provides a method for anodizing an internal surface of a tube using an anodizing apparatus including an electrolyte container storing an electrolyte solution, a first solution conduit connected with the electrolyte container to receive the electrolyte solution, a first jig configured to fix one end of a targeted tube to a downstream end of the first solution conduit, a second solution conduit in which the other end of the targeted tube is connected to an upstream end to discharge the electrolyte solution flowing into the targeted tube, and a second jig configured to fix the other end of the targeted tube to an upstream end of the second solution conduit, the method including: supplying the electrolyte solution to flow through the first solution conduit, the targeted tube, and the second solution conduit; inserting a cathode rod to be extended to the first jig from the second jig through the inner portion of the targeted tube; and supplying power by applying a cathode to the cathode rod and applying an anode to the targeted tube, in which, while the electrolyte solution passes through the inner portion of the targeted tube, the anodizing process is performed. 
     The method may further include supplying a cleaning solution to flow through the first solution conduit, the targeted tube, and the second solution conduit after power supply stops and the anodizing process ends. 
     Advantageous Effects 
     As described above, according to the apparatus for anodizing an internal surface of a tube with zirconium or an alloy thereof, it is possible to form a hydrophilic oxide film with a minute structure formed on the internal surface of the tube which may not be easily applied in a method such as MEMS. 
     Further, it is possible to easily prepare a cylindrical internal surface such as a fuel rod with zirconium or an alloy thereof as a hydrophilic surface through the anodizing method. As the internal surface of the fuel rod is formed as the hydrophilic surface, a critical heat flux may be increased, and as a result, it is possible to increase efficiency of a nuclear power plant. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating fuel rods configuring a fuel assembly used in a nuclear power plant. 
         FIG. 2  is a schematic diagram illustrating a fuel assembly used in a nuclear power plant. 
         FIG. 3  is a configuration diagram schematically illustrating an apparatus for anodizing an internal surface of a cylindrical tube according to an exemplary embodiment of the present invention. 
         FIG. 4A  is an exploded cross-sectional view illustrating a jig of the apparatus for anodizing the internal surface of the cylindrical tube according to an exemplary embodiment of the present invention, and  FIG. 4B  is a coupled cross-sectional view thereof. 
         FIG. 5  is a photograph illustrating a zirconium alloy (zircaloy) tube applied to the anodizing apparatus according to an exemplary embodiment of the present invention. 
         FIG. 6  is a SEM photograph of a minute structure formed on an internal surface of a zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. 
         FIG. 7  is a photograph of a spreading phenomenon of droplets of water on the internal surface of the zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. 
         FIG. 8  is a graph illustrating a critical heat flux enhancement ratio in a flow boiling situation in the case of using the zirconium alloy (zircaloy) tube prepared by the anodizing method according to another exemplary embodiment of the present invention. 
     
    
    
     MODE FOR INVENTION 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. The drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     According to previous studies (see Kandlikar, S. G., 2001, “A Theoretical Model to Predict Pool Boiling CHF Incorporating Effects of Contact Angle and Orientation,” Journal of Heat Transfer, Vol. 123, pp. 1071-1079), as a surface of a heater in a boiling apparatus is more hydrophilic, a critical heat flux (CHF) is increased. 
     When a CHF point is increased through surface modification, a heating engine may operate at a higher temperature, and as a result, energy efficiency is increased according to a principle of a Rankine cycle. Further, even though an actual operation temperature is not increased, a difference between a heat flux and the CHF during operation is increased, and as a result, stability is further increased by the difference. A cladding surface of the fuel rods exposed to a high temperature is prepared as a hydrophilic surface by applying the principle to the nuclear power plant to increase the CHF point, thereby ensuring higher stability. 
       FIG. 1  is a schematic diagram illustrating fuel rods configuring a fuel assembly used in a nuclear power plant, and  FIG. 2  is a schematic diagram illustrating a fuel assembly used in a nuclear power plant. 
     Fuel rods  10  embedding uranium dioxide (UO 2 ) pellets (a sintered body) are arranged at 16×16 and fixed by spacer grids  21  and  23  to form a nuclear fuel assembly  20 . Each fuel rod  10  has a cylindrical shape, and similarly, a plurality of pellets  12  having cylindrical shapes are accumulated therein, and in order to prevent the pellets  12  from moving in the fuel rod  10 , a compression spring  15  may be inserted into the upper portion. A cladding  17  forming an outer skin of the fuel rod  10  may be made of a zirconium alloy which is an alloy mainly composed of zirconium, and is also called zircaloy. 
     Meanwhile, in order to make the surface of a solid hydrophilic, a method of changing a chemical characteristic of the surface and a method of changing a surface shape at a microscale or a nanoscale are used, and according to the exemplary embodiment of the present invention, the method of changing a surface shape at a microscale or a nanoscale is adopted. 
     That is, in the exemplary embodiment, while oxidation on the cladding surface is electrochemically promoted by applying the anodizing method to the cladding of the fuel rod, a micro/nanoscale structure may be formed on the surface. In the anodizing method, as compared with a microelectromechanical system (MEMS) process using a photolithography method in which a cleaning facility is basically required and thus basic investment costs are largely involved and it is difficult to be applied to a large area or a curved surface, there are advantages in that the anodizing method is cheap and may be applied to the large area or the curved surface. 
     In order to apply the anodizing method to the cladding surface of the fuel rod which has a tube shape, an anodizing apparatus which is different from a plate specimen is required. That is, for the anodization, basically, an anode and a cathode which face each other and an electrolyte filling a space therebetween are required. In addition, when a voltage is applied between the anode and the cathode, the anodization proceeds, and for a continuous anodizationDeletedTextsreaction, smooth circulation of the electrolyte is required. 
       FIG. 3  is a configuration diagram schematically illustrating an apparatus for anodizing an internal surface of a cylindrical tube according to an exemplary embodiment of the present invention. An anodizing apparatus for anodizing the internal surface of the tube as illustrated in  FIG. 3  may be configured by satisfying the aforementioned condition. 
     Referring to  FIG. 3 , the anodizing apparatus according to the exemplary embodiment includes an electrolyte container  31 , and a first solution conduit  37  and a second solution conduit  39  connected thereto. The electrolyte container  31  stores an electrolyte solution, and the first solution conduit  37  receives the electrolyte solution from the electrolyte container  31 . The second solution conduit  39  is connected with the first solution conduit  37  with an anodization-targeted tube  50  therebetween, and the electrolyte solution supplied through the first solution conduit  37  flows into the targeted tube  50  to be discharged through the second solution conduit  39 . 
     The electrolyte container  31  may be installed to supply the electrolyte solution to the anodization-targeted tube  50  by using positional energy without applying separate power. 
     The first solution conduit  37 , the targeted tube  50 , and the second solution conduit  39  are connected with each other to form a U-shaped conduit, and the targeted tube  50  may be positioned at a downstream (right) branch of the U-shaped conduit. Accordingly, even if the flow of the electrolyte solution stops, the electrolyte solution may be collected in the targeted tube  50 , and the anodization may not stop but may last. 
     In order to fix one end of the targeted tube  50  to the downstream end of the first solution conduit  37 , a first jig  41  is installed, and in order to the other end of the targeted tube  50  to an upstream end of the second solution conduit  39 , a second jig  42  is installed. A structure of the jigs  41  and  42  will be described in detail with reference to  FIGS. 4A and 4B . 
     Meanwhile, a cathode rod  45  serving as the cathode in the anodizing process is installed to be inserted from the second jig  42  and extended to the first jig  41  through the internal portion of the targeted tube  50 . That is, while the electrolyte solution flows into the targeted tube  50 , the anodizing process may be performed by applying the cathode to the cathode rod  45  and applying the anode to the targeted tube  50 . A cathode terminal of a power supplier  47  that is separately provided is electrically connected with the cathode rod  45 , and an anode terminal is electrically connected with the targeted tube  50  to apply the voltage. As the cathode rod  45 , a conductive material rod having corrosion resistance to the electrolyte solution may be applied, and when the cathode rod  45  is bent in the targeted tube  50 , a short circuit may be caused, and as a result, a bent material is not appropriate. 
     The power supplier  47  may perform real-time monitoring through a computer  49 , and perform a function of inputting a pulse current or reversely applying the cathode and the anode by controlling the voltage and the current. According to a previous study (Chan Lee, Hyungmo Kim, Ho Seon Ahn, Moo Hwan Kim, and Joonwon Kim, “Micro/nanostructure Evolution of Zircaloy Surface Using Anodization Technique: Application to Nuclear Fuel Cladding Modification”, Applied Surface Science, Vol. 258, issue 22, pp. 8724-8731, 2012.09.01), in the anodization of a zirconium alloy, a surface structure varies according to a response duration, and as a result, properties of the surface including wettability are gradually changed. The power supplier  47  may monitor the current in real time, determine a reaction step based on the monitoring result, and easily selectively prepare a surface structure according to a desired purpose through a function of stopping the current supply. Further, when the power supplier  47  reversely applies the cathode and the anode, the anodizing process is changed into a plating process, and as a result, plating in the tube may be performed by using the apparatus of the exemplary embodiment. 
     The anodizing apparatus of the exemplary embodiment includes a cleaning solution container  32  storing a cleaning solution. The cleaning solution container  32  is connected with the electrolyte container  31  and the first solution conduit  37  through a three-way valve  35 . The electrolyte solution may be supplied or the cleaning solution may be supplied through the first solution conduit  37  according to an operation of the three-way valve  35 . During the anodizing process, the electrolyte solution is continuously supplied, and when the anodizing process is completed, the cleaning solution is immediately supplied. The cleaning solution may be, for example, deionized water. 
     The anodizing apparatus of the exemplary embodiment includes a first constant-temperature water bath  51  which receives a coolant, and the electrolyte container  31  and the cleaning solution container  32  may be installed to be immersed in the coolant of the constant-temperature water bath  51 . 
     Further, a flow meter and a flow control device  36  are installed in the first solution conduit  37  to control the flow of the electrolyte solution or the cleaning solution passing through the first solution conduit  37 . As a result, a reaction speed of the anodization influenced by the flow or uniformity of the formed structure may be controlled. 
     The anodizing apparatus of the exemplary embodiment includes a second constant-temperature water bath  61  receiving the coolant. The targeted tube  50  is installed to be immersed in the coolant received in the second constant-temperature water bath  61  to help a proper temperature be maintained. The coolant is circulated by a circulation device  63  connected with the second constant-temperature water bath  61  to maintain the temperature. The coolant may be, for example, water. Since the second constant-temperature water bath  61  does not have a closed structure in which the coolant completely surrounds the outside of the targeted tube  50  but has an opened structure in which an upper portion is opened, the targeted tube  50  may be easily inserted and ejected before and after an experiment. During the anodization, heat is generated due to the reaction, and because the temperature may be a factor that has a large effect on the anodization, proper temperature control is required for an optimum reaction condition. 
     In the exemplary embodiment, a temperature control device is provided in the circulation device  63  to control the temperature of the coolant supplied to the first constant-temperature water bath  51  and the second constant-temperature water bath  61 . Accordingly, the temperature control effect may be maximized by simultaneously performing indirect cooling through the heat transfer from the outer surface of the targeted tube  50  by using the second constant-temperature water bath  61  and cooling through temperature control of the electrolyte solution itself directly contacting a reaction surface by using the first constant-temperature water bath  51 . 
     The electrolyte solution discharged through the second solution conduit  39  is collected in an electrolyte receiving bath  65 , and the collected electrolyte solution is again circulated to the electrolyte container  31  to be reused. 
       FIG. 4A  is an exploded cross-sectional view illustrating a jig of the apparatus for anodizing the internal surface of the cylindrical tube according to an exemplary embodiment of the present invention, and  FIG. 4B  is a coupled cross-sectional view thereof.  FIGS. 4A and 4B  illustrate the second jig  42  illustrated in  FIG. 3 , but since the structure of the first jig  41  is similar to the structure of the second jig  42  except for only a formation direction of an opening, hereinafter, only the second jig  42  will be described. 
     Referring to  FIG. 4A , the second jig  42  includes a jig body  421  in which a solution conduit connection hole  421   a  to which the second solution conduit  39  is insert-connected is formed at a first side portion, and a targeted tube connection hole  421   b  to which the other end of the targeted tube  50  is insert-connected is formed at a second side portion. An upper cover  425  is coupled with an upper portion of the jig body  421  by a bolt  424 , and a lower cover  423  is coupled with a lower portion of the jig body  421  by a bolt  424 . A cathode rod insertion hole  421   c  into which the cathode rod  45  is inserted is formed at an upper end of the jig body  421 , and the targeted tube connection hole  421   b  and the cathode rod insertion hole  421   c  are formed on a concentric axis. At the portion where the cathode rod insertion hole  421   c  and the targeted tube connection hole  421   b  are formed, O-rings  427  and  428  are installed between the jig body  421  and the upper cover  425  and between the jig body  421  and the lower cover  423  to prevent the electrolyte solution from leaking to the outside. Threads engaging with each other are formed on an outer surface of the end of the second solution conduit  39  and an internal surface of the solution conduit connection hole  421   a  to be screw-coupled with each other. 
     Referring to  FIG. 4B , the second solution conduit  39 , the targeted tube  50 , and the cathode rod  45  are connected to the coupled second jig  42 . That is, the second solution conduit  39  is insert-connected to the solution conduit connection hole  421   a , the targeted tube  50  is insert-connected to the targeted tube connection hole  421   b , and the cathode rod  45  is inserted into the cathode rod connection hole  421   c . The inserted cathode rod  45  has a concentric axis with the targeted tube  50  and is installed to pass through the inner portion of the targeted tube  50 . 
     In the anodizing apparatus according to the exemplary embodiment, for example, the targeted tube  50  may be a zircaloy tube, and the electrolyte solution may be a hydrofluoric acid solution. The cathode rod  45  inserted into the targeted tube  50  may be a stainless steel rod, and the first solution conduit  37  and the second solution conduit  39  may be formed of perfluoroalkoxy materials. In addition, the jig body  421 , the upper cover  425 , the lower cover  423 , and the bolt  424  configuring the second jig  42  may be made of polytetrafluoroethylene (PTFE) materials, and the O-rings  427  and  428  may be Viton® O-rings. 
     &lt;Anodizing Method&gt; 
     A method for anodizing the internal surface of the tube prepared by zirconium or an alloy thereof according to the exemplary embodiment of the present invention by using the anodizing apparatus illustrated in  FIG. 3  will be described below. 
     First, the electrolyte solution stored in the electrolyte container  31  is supplied to flow through the first solution conduit  37 , the targeted tube  50 , and the second solution conduit  39 . In the exemplary embodiment, a solution of which the temperature of the electrolyte solution is in a range 0 to 15° C. may be applied, and a solution of which the concentration of the electrolyte solution is in a range of 0.01 to 1 wt % may be applied, and for example, as the electrolyte solution, a hydrofluoric acid solution of 0.5 wt % at 10° C. or less may be applied. As the targeted tube  50 , a tube of which the length is 10 cm or more may be applied, and a zirconium-alloy tube of which an outer diameter is ⅜ of an inch may be applied. In addition, a flow velocity of the electrolyte solution flowing in the targeted tube  50  is about 300 to 1000 ml/min. 
     Next, the cathode rod  45  is inserted to be extended to the first jig  41  from the second jig  42  through the inner portion of the targeted tube  50 . The cathode rod  45  may be pre-inserted before the electrolyte solution is supplied. In the exemplary embodiment, as the cathode rod  45 , a stainless steel rod having an outer diameter of 3 mm may be applied. 
     Next, power may be supplied by applying the cathode to the cathode rod  45  and applying the anode to the targeted tube  50 . In the exemplary embodiment, a voltage of 5 to 40 V may be applied between the anode and the cathode, and for example, a voltage of 15 V may be applied. The voltage may be applied for a time of 10 to 40 minutes. 
     Through such a process, while the electrolyte solution flows into the targeted tube  50 , the anodizing process is performed, and as a result, an uneven structure having a micro/nanoscale is formed on the inner surface of the targeted tube  50 . The inner surface of the targeted tube  50  has hydrophilicity due to the formed uneven structure. 
     Next, after the anodizing process ends by stopping the power supply, the cleaning solution is supplied to flow through the first solution conduit  37 , the targeted tube  50 , and the second solution conduit  39 . After the anodizing process ends (the power supply stops), a contact with the electrolyte solution (hydrofluoric acid solution) has a negative influence on the structure such that a minute structure of the surface may be rapidly broken, and therefore, the electrolyte solution needs to be rapidly cleaned. To this end, the cleaning solution may be injected immediately after the reaction ends by operating the three-way valve  35 . 
     Experimental Example 
     As described above, the anodizing method was performed by using the anodizing apparatus according to the exemplary embodiment of the present invention. As the electrolyte solution, a 0.5 wt % hydrofluoric acid solution was used, and as the cleaning solution, deionized water was used. 
       FIG. 5  is a photograph illustrating a zirconium alloy (zircaloy) tube applied to the anodizing apparatus according to an exemplary embodiment of the present invention. The zircaloy tube is made of a zirconium alloy having an outer diameter of ⅜ of an inch and a length of 53 cm, and is illustrated to be cut in a length direction so as to show the inner portion. 
     As the cathode rod, the stainless steel rod with an outer diameter of 3 mm is applied, and the voltage applied between the anode and the cathode is 15 V, while a reaction time is 25 minutes. The temperature of the constant-temperature water bath was kept at 3° C. 
       FIG. 6  is an SEM photograph of a minute structure formed on an internal surface of a zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. According to the anodizing method using the anodizing apparatus described above, it is shown that the minute structure may be actually formed on the inner surface of the large-area curved surface. 
       FIG. 7  is a photograph of a spreading phenomenon of droplets of water on the internal surface of the zirconium alloy (zircaloy) tube prepared by an anodizing method according to another exemplary embodiment of the present invention. That is, since it was observed that droplets of water placed on the surface spread within rapidly, it can be seen that the surface formed by the anodizing method according to the exemplary embodiment has a very high hydrophilic property, and accordingly, as described above, improvement of a high critical heat flux may be expected. 
     According to the exemplary embodiment, an experiment for checking a critical heat flux enhancement ratio under a flow boiling condition was performed by using the zircaloy tube with the minute structure formed at the internal surface. That is, when water flows into the tube and heat is applied from the outside to increase the temperature, the flowing water reaches the critical heat flux around a predetermined point of the tube. A thermocouple is mounted on the zircaloy tube in which the minute structure is formed on the internal surface to measure a critical heat flux (CHF) value, and the degree of enhancement of the CHF is illustrated in  FIG. 8 . 
     In  FIG. 8 , a mass flux means a flux of water flowing in the tube, and an inlet temperature means a temperature of water flowing into the tube. Referring to  FIG. 8 , about a 60% increase is shown under a condition of a mass flux of 1500 kg/m 2 s. The result verifies that the surface minute structure made under the above condition significantly increases the CHF in an actual flow boiling condition. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.