Title of Invention: REFRIGERATOR Abstract: A refrigerator includes a main body defining a storage space, a cryogenic freezing compartment having an insulation space that is independent with respect to the storage space, an evaporator disposed inside the storage space to cool the storage space, and a thermoelectric module assembly disposed at one side of the cryogenic freezing compartment so that the cryogenic freezing compartment is cooled to a temperature less than that of the storage space. The thermoelectric module assembly includes a thermoelectric module, a cold sink coming into contact with a heat absorption surface of the thermoelectric module and disposed in the cryogenic freezing compartment, and a heat sink coming into contact with a heat generation surface of the thermoelectric module. The heat sink is cooled by introducing a refrigerant supplied to the evaporator. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2017-0046453 (filed on Apr. 11, 2017), which is hereby incorporated by reference in its entirety. BACKGROUND The present disclosure relates to a refrigerator including cryogenic freezing compartment. Generally, refrigerators are household appliances that store foods at a low temperature. An inner space of such as a refrigerator may be divided into a refrigerating compartment and a freezing compartment according to temperatures for foods stored in the refrigerator. The refrigerating compartment generally maintains a temperature of about 3 degrees Celsius to about 4 degrees Celsius, and the freezing compartment generally maintains a temperature of about −20 degrees Celsius. The freezing compartment having a temperature of about −20 degrees Celsius is a space in which foods are kept in a frozen state and is often used by consumers to store the foods for a long time. However, in the existing freezing compartment, which maintains a temperature of about −20 degrees Celsius, when water within cells is frozen while freezing meat or seafood, a phenomenon in which water is exuded out of the cells may occur, and thus, the cells are destroyed. As a result, when cooking the foods after thawing, their original taste may be lost, or the texture may change. On the other hand, when meat or seafood is frozen, the temperature rapidly passes through the freezing point temperature zone in which intracellular ice is formed to minimize the cell destruction. Thus, even after thawing, meatiness and texture may be renewed or reproduced freshly to make it possible to enjoy delicious dishes. As the case stands, fancy restaurants use a cryogenic freezer that is capable of rapidly freezing meat, fish, and seafood. However, unlike restaurants that need to preserve large quantities of foods, since it is not always necessary to use the cryogenic freezer in ordinary homes, it is not easy to separately purchase the cryogenic freezer that is used in restaurants. However, as the quality of life has improved, consumers' desire to eat more delicious foods has become stronger to lead to an increase in consumers who want to use the cryogenic freezer. In order to meet the needs of such consumers, there has been developed a household refrigerator in which a cryogenic freezing compartment is installed in a portion of the freezing compartment. It is preferable that the cryogenic freezing compartment satisfies a temperature of about −50 degrees Celsius, such an extremely low temperature is a temperature that is not attained only by a refrigeration cycle using a general refrigerant. Accordingly, there has been developed a household refrigerator in which a cryogenic freezing compartment is separately provided in the freezing compartment in a manner in which cooling is performed by using a refrigeration cycle up to a temperature of −20 degrees Celsius and by using a thermoelectric module (TEM) in case of cryogenic refrigeration. However, since a temperature difference between the freezing compartment of about −20 degree Celsius and a cryogenic freezing compartment of about −50 degree Celsius is very large, it is not easy to realize a temperature of about −50 degrees Celsius by applying a structure for insulation, defrosting, cold air supply, and the like, which was applied to the design of the existing freezing compartment, to the cryogenic freezing compartment as it is. Also, when a cryogenic freezing compartment, which occupies a space of the freezing compartment itself, is provided, since reduction in volume capacity of the freezing compartment has to be minimized, it is necessary to minimize a space occupied by the structure for cooling and circulating cold air in the cryogenic freezing compartment. Particularly, when the cryogenic temperature is implemented using the TEM, heat exchange has to be smoothly performed both at a heat absorption side and a heat generation side of the TEM, cold air cooled by the heat exchange at the heat absorption side has to smoothly circulate, and heat exchange loss and flow loss should not occur while having a simple structure as much as possible. Furthermore, due to the volume occupied by the TEM and related components, which are installed to achieve the cryogenic temperature, there is a possibility that a flow rate or pressure distribution in the existing grill fan assembly structure changes, and thus, the freezing in the freezing compartment is not smoothly performed. SUMMARY Embodiments provide a refrigerator in which an independent cryogenic freezing compartment is provided in a storage space, and the inside of the cryogenic freezing compartment is in an extremely low temperature state by a thermoelectric module. Embodiments also provide a refrigerator in which a cryogenic freezing compartment having an extremely low temperature is realized so that a low-temperature refrigerant in a refrigeration cycle is cooled via a heat sink for cooling a heat generation part of a thermoelectric module. Embodiments also provide a refrigerator that is improved in cooling performance of a heat generation part of a thermoelectric module. In one embodiment, a refrigerator includes: a main body defining a storage space; a cryogenic freezing compartment having an insulation space that is independent with respect to the storage space; an evaporator disposed inside the storage space to cool the storage space; and a thermoelectric module assembly disposed at one side of the cryogenic freezing compartment so that the cryogenic freezing compartment is cooled to a temperature less than that of the storage space, wherein the thermoelectric module assembly includes: a thermoelectric module; a cold sink coming into contact with a heat absorption surface of the thermoelectric module and disposed in the cryogenic freezing compartment; and a heat sink coming into contact with a heat generation surface of the thermoelectric module, wherein the heat sink is cooled by introducing a refrigerant supplied to the evaporator. The heat sink may be provided in a refrigerant flow path connecting an expansion device and the evaporator, which constitute a refrigeration cycle, to each other. The heat sink may include: a refrigerant inflow tube connected to the heat sink to allow the refrigerant to be introduced therethrough; and a refrigerant outflow tube connected to the heat sink to allow the refrigerant to be discharged therethrough. An inflow hole and an outflow hole, into which the refrigerant inflow tube and the refrigerant outflow tube are inserted, may be defined in the heat sink, and a seating part through which the refrigerant inflow tube and the refrigerant outflow tube pass and on which a welding ring for bulging processing is seated may be disposed to be stepped on each of the inflow hole and the outflow hole. The heat sink may include: a sink body including an accommodation part providing a space through which the refrigerant flows; a plate covering an opened surface of the sink body and coming into contact with the heat generation surface; and a heat exchange fin disposed inside the accommodation part to guide a flow of the refrigerant. The sink body may include the accommodation part that passes through the sink body, and the plate may include: a front plate covering an opened front surface of the accommodation part and coming into contact with the heat generation surface; and a rear plate covering an opened rear surface of the accommodation part. The sink body may include the accommodation part that is recessed forward, and the plate may cover an opened front surface of the accommodation part. A restriction piece that is bent to the sink body may be disposed on an outer end of the plate, and a restriction groove into which the restriction piece is inserted to couple the plate to the sink body may be defined in a circumference of the sink body. The accommodation part may include a barrier that partitions the inside of the accommodation part into a first space into which the refrigerant is introduced and a second space from which the refrigerant is discharged, and the heat exchange fin may be provided in each of the first space and the second space. The accommodation part may include a fin fixing part for fixing the heat exchange fin so that the heat exchange fin is fixed to be spaced apart from an inner surface of the accommodation part. In the heat exchange fin, a plate-shaped material may be continuously bent to provide a passage for guiding a flow direction of the refrigerant. The heat exchange fin may include: a plurality of contact parts coming into contact with the plate and heat-exchanged with a surface of the plate; and a fin connection part bent from an end of each of the plurality of contact parts to connect the contact parts to each other, wherein the contact parts and the fin connection part may be continuously disposed in a width direction of the heat exchange fin. The heat exchange may include: a first passage extending in a longitudinal direction of the heat exchange fin to provide a passage through which the refrigerant flows; and a second passage of which at least a portion overlaps the first passage and which is disposed to cross the first passage and thereby to provide a passage in which the refrigerant is branched to flow, and the first passage and the second passage may be continuously disposed in the longitudinal direction of the heat exchange fin. The first passage and the second passage may be alternately provided in plurality, and the plurality of first and second passages may be alternately disposed and have lengths different from each other. The cryogenic freezing compartment may be disposed inside the storage space. A grill fan assembly disposed to a front side of the evaporator to cover the evaporator may be disposed in the storage space, and the thermoelectric module assembly may be mounted on the grill fan assembly, wherein the cold sink may be disposed in a region of the cryogenic freezing compartment, and the heat sink may be disposed in a region in which the evaporator is disposed. An insulation material accommodating the thermoelectric module may be disposed between the cold sink and the heat sink, and the insulation material may be disposed on a boundary with the grill fan assembly. The refrigerator may further include a module housing accommodating the insulation material, the thermoelectric module, and the heat sink, wherein the module housing may be fixedly mounted on the grill fan in a state of being spaced apart from a rear wall of a space in which the evaporator is accommodated. The heat sink may be disposed in a space between an inner case defining the storage space and a grill fan assembly covering the evaporator. The storage space may include a freezing compartment. The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a refrigerator with a door opened according to an embodiment. FIG. 2 is a perspective view illustrating an inner structure of an inner case of the refrigerator. FIG. 3 is an exploded front perspective view of a coupling structure of a grill fan assembly, a cryogenic freezing compartment, and a thermoelectric module assembly according to an embodiment. FIG. 4 is an exploded rear perspective view of the coupling structure of the grill fan assembly, the cryogenic freezing compartment, and the thermoelectric module assembly. FIG. 5 is a cross-sectional view taken along line A-A of FIG. 2. FIG. 6 is a schematic view illustrating a configuration of a refrigeration cycle cooling device of the refrigerator. FIG. 7 is a front perspective view of the thermoelectric module assembly. FIG. 8 is a rear perspective view of the thermoelectric module assembly. FIG. 9 is an exploded front perspective view illustrating a coupling structure of the thermoelectric module assembly. FIG. 10 is an exploded rear perspective view illustrating the coupling structure of the thermoelectric module assembly. FIG. 11 is a perspective view of a heat sink according to an embodiment. FIG. 12 is an exploded perspective view illustrating a configuration of the heat sink. FIG. 13 is a perspective view of a heat exchange fin that is a main component of the heat sink. FIG. 14 is a cross-sectional view taken along line B-B′ of FIG. 13. FIG. 15 is a cross-sectional view taken along line C-C′ of FIG. 13. FIG. 16 is a front view illustrating a coupling structure of the heat exchange fin and a sink body. FIG. 17 is a cross-sectional view taken along line D-D′ of FIG. 11. FIG. 18 is a cross-sectional view taken along line E-E′ of FIG. 11. FIG. 19 is a view illustrating a coupling structure of the sink body, a refrigerant inflow tube, and a refrigerant outflow tube. FIG. 20 is a view illustrating a flow of a refrigerant within the heat sink. FIG. 21 is a partial front view illustrating a state in which the thermoelectric module assembly is mounted on the inner case. FIG. 22 is a partial cross-sectional view illustrating a coupling structure of the thermoelectric module assembly and the inner case. FIG. 23 is a view illustrating a connection state of the thermoelectric module assembly, the evaporator, and a refrigerant tube. FIG. 24 is a schematic view illustrating a flow path between the thermoelectric module assembly and the evaporator. FIG. 25 is a view illustrating a state in which cold air is supplied while the thermoelectric module assembly operates. FIG. 26 is an exploded perspective view illustrating a structure of a heat sink according to another embodiment. FIG. 27 is an exploded perspective view illustrating a structure of a thermoelectric module assembly according to another embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS Hereinafter, preferred embodiments will be described in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present invention will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In this specification, the term “cryogenic temperature” means a temperature that is lower than about 20 degrees Celsius, which is a typical freezing storage temperature of the freezing compartment, and the temperature range is not limited numerically. Also, even in the cryogenic freezing compartment, the storage temperature may be below about 20 degrees Celsius or more. FIG. 1 is a perspective view of a refrigerator with a door opened according to an embodiment. Also, FIG. 2 is a perspective view illustrating an inner structure of an inner case of the refrigerator. As illustrated in the drawings, a refrigerator according to an embodiment includes a refrigerator main body 10 and a refrigerator door 20 disposed on a front portion of the main body 10 to open and close each spaces of the main body 10. The refrigerator according to an embodiment has a bottom freezer type structure in which a refrigerating compartment 30 is disposed at an upper side, and a freezing compartment 40 is disposed at a lower side. The refrigerating compartment and the freezing compartment include side-by-side doors 21 and 22 that rotate with respect to hinges 25 disposed on both ends to open the refrigerating compartment and the freezing compartment. However, the embodiments are not limited to the refrigerator having the bottom freezer type structure. For example, the embodiments may be applied to a refrigerator having the side by side structure in which the refrigerating compartment and the freezing compartment are respectively disposed at left and right sides and a refrigerator having a top mount type structure in which the freezing compartment is disposed above the refrigerating compartment as lone as a cryogenic freezing compartment is capable of being installed in the freezing compartment. The refrigerator main body 10 includes an outer case 11 defining an outer appearance of the refrigerator and an inner case 12 installed to be spaced a predetermined distance from the outer case 11 and defining an inner appearance of the refrigerator. An insulation material may be foamed and filled into a space between the outer case 11 and the inner case 12 to insulate the refrigerating compartment 30 and the freezing compartment 40 from an indoor space. A shelf 13 and a drawer 14 are installed in the storage space of each of the refrigerating compartment 30 and the freezing compartment 40 to store foods while improving space utilization efficiency. The shelf 13 and the drawer 14 may be installed in the storage space so as to be guided along rails 15 disposed on left and right sides. A door basket 27 is installed inside the refrigerating compartment door 21 and the freezing compartment door 22 as illustrated in the drawings to store containers such as beverage bottles. A cryogenic freezing compartment 200 according to an embodiment is provided in the freezing compartment 40. A space of the freezing compartment 40 is horizontally divided to be efficiently used. Here, the space of the freezing compartment 40 is partitioned by a partition wall 42 disposed at a center of the freezing compartment 40 and having a shape that vertically extends. Referring to FIG. 2, the partition wall 42 is installed to be fitted inward from the front portion of the main body and supported within the freezing compartment 40 through an installation guide 42-1 disposed on the bottom of the refrigerator. According to an embodiment, the cryogenic freezing compartment 200 may be disposed at a left upper portion of the freezing compartment 40 as one example. However, the position of the cryogenic freezing compartment 200, which is disposed in the freezing compartment 40, is not limited thereto. That is, the cryogenic freezing compartment 200 may be installed in the refrigerating compartment 30. However, when the cryogenic freezing compartment 200 is disposed in the freezing compartment 40, since a temperature difference between the inside and the outside (a freezing compartment atmosphere) of the cryogenic freezing compartment is more less, it is more advantageous that the cryogenic freezing compartment 200 is installed in the freezing compartment 40 in views of cold air leakage prevention. A machine room isolated from the freezing compartment is disposed in a rear lower portion of the freezing compartment 40. A compressor 71 and a condenser 73 of a refrigeration cycle cooling device 70 using a refrigerant are disposed in the machine room. A grill fan assembly 50 including a grill fan 51 defining a rear wall of the freezing compartment 40 and a shroud 56 coupled to a rear portion of the grill fan 51 to distribute cold air within a cooling chamber is installed between a space defining the freezing compartment 40 and a rear wall of the inner case 12. Also, an evaporator 77 of the refrigeration cycle cooling device 70 is installed in a predetermined space between the grill fan assembly 50 and the rear wall of the inner case 12. When the refrigerant within the evaporator 77 is evaporated, the refrigerant is heat-exchanged with air flowing through the inner space of the freezing compartment 40. The air cooled by the heat exchange is distributed into a cold air distribution space defined by the grill fan 51 and the shroud 56 to flow through the freezing compartment 40, thereby performing the cooling in the freezing compartment 40. FIG. 3 is an exploded front perspective view of a coupling structure of the grill fan assembly, the cryogenic freezing compartment, and a thermoelectric module assembly according to an embodiment. Also, FIG. 4 is an exploded rear perspective view of the coupling structure of the grill fan assembly, the cryogenic freezing compartment, and the thermoelectric module assembly. As illustrated in the drawings, according to an embodiment, the grill fan assembly 50 to which the cryogenic freezing compartment is applied includes the grill fan 51 defining the rear wall of the freezing compartment 40 and the shroud 56 for distributing the cold air, which is cooled by being heat-exchanged with the evaporator 77 on a rear surface of the grill fan 51, to supply the cold air into the freezing compartment 40. As illustrated in the drawings, cold air discharge holes 52 provided as passages through which the cold air is discharged forward are defined in the grill fan 51. In this embodiment, the cold air discharge holes 52 are defined in upper end left/right sides 521 and 522, central left/right sides 523 and 524, and lower left/right sides 526 (in FIG. 3, the cold air discharge holes 52 defined in the central left side and the lower left side are covered by the cryogenic freezing compartment). The shroud 56 is coupled to the rear portion of the grill fan 51 to define a predetermined space together with the grill fan 51. This space is a space in which the air cooled in the evaporator 77 provided in the rear surface of the grill fan assembly 50 or the shroud 56 is distributed. A cold air suction hole 58 communicating with a space defined at a rear side of the shroud 56 and a space between the grill fan 51 and the shroud 56 is defined in an approximately central upper portion of the shroud 56. Also, a fan 57 that suctions the cold air of the rear space of the shroud 56 through the cold air suction hole 58 to distribute and pressing the cold air into the space between the grill fan 51 and the shroud 56 is installed inside the cold air suction hole 58 in the space between the grill fan 51 and the shroud 56. The cold air pressed by the fan 57 flows through the space between the grill fan 51 and the shroud 56 and then adequately distributed. Then, the cold air is discharged forward through the cold air discharge holes 52 that are opened forward. A thermoelectric module accommodation part 53 in which a thermoelectric module assembly 100 for performing cryogenic cooling of the cryogenic freezing compartment 200 is installed is provided between the cold air discharge hole 522 defined in the right upper end and the cold air discharge hole 524 defined in the right central portion as the right upper portion of the grill fan 51. The thermoelectric module accommodation part 53 is disposed on a front surface of the grill fan 51, which corresponds to a position at which the cryogenic freezing compartment 200 is installed, in the freezing compartment 40. The thermoelectric module accommodation part 53 may be installed in a manner in which the thermoelectric module accommodation part 53 is integrally molded with a wall defining a rear boundary of the freezing compartment 40 that is one of the storage space in which the cooling is performed by the refrigeration cycle cooling device 70, i.e., the grill fan 51 or separately manufactured with respect to the wall and then assembled with the wall. For example, the grill fan 51 may be manufactured through injection molding. Here, the grill fan 51 may be molded together with a portion corresponding to the thermoelectric module accommodation part 53. On the other hand, even when the rear boundary of the storage space may be defined by the inner case 12, and it is difficult to mold the thermoelectric module accommodation part 53 together while the inner case 12 is molded, as illustrated in FIG. 21, the thermoelectric module accommodation part 53 may be separately manufactured and then fixed to and assembled with the wall. The thermoelectric module accommodation part 53 has an approximately rectangular parallelepiped shape (a rear side thereof is opened to the cooling chamber in which the evaporator is provided) extending to protrude forward from the front surface of the grill fan 51. When viewed from at a front side, this shape may have an approximately rectangular shape that is vertically long. When viewed from the front side, a grill part 531 through which the air cooled by the thermoelectric module assembly 100 is discharged is disposed at a central portion of the rectangular shape, and a suction part 533 that is opened forward is disposed on each of upper and lower portions of the rectangular shape. The suction part 533 may serve as a passage through which air outside the suction part 533 is suctioned into an inner space (that is a space defined at a rear side of the grill part 531 and an inner space of an outer circumferential wall of the rectangular shape defining an outer appearance of the thermoelectric module accommodation part 53) of the thermoelectric module accommodation part 53. The inner space of the thermoelectric module accommodation part 53 may communicate with a space defined at a front side rather than the thermoelectric module accommodation part 53 through the grill part 531 and the suction part 533 and be isolated from a space defined at a front side of the grill fan 51. A discharge guide 532 having a partition wall shape extending forward between the grill part 531 and the suction part 533 is provided between the grill part 531 and the suction part 533 to prevent the cold air discharged from the grill part 531 from being immediately reintroduced into the suction part 533 that is adjacent thereto. To prevent the air discharged from the grill part 531 from being immediately reintroduced into the suction part 533, the discharge guide 532 may be disposed within only a range in which the grill part 531 and the suction part 533 are adjacent to each other. However, when it is desired to further enhance an effect of the cold air discharged from the grill part 531 to flow forward, i.e., an effect of improving straightness, the discharge guide 532 may entirely surround the grill part 531 as illustrated in the drawings. Although the discharge guide 532 has a flow cross-section with a square shape as illustrated in the drawings, the discharge guide may have a flow cross-section with a circular shape like a shape of the grill part 531 or a blade of the fan disposed at the rear side of the grill part 531. The flow cross-sectional shape does not necessarily have a rectangular or circular flow cross-section, but may be modified into various shapes as long as it may improve the straightness of the cold air while preventing the cold air discharged from the grill part from being reintroduced into the suction part. Also, the formed position of the suction part 533 is not limited to the upper and lower positions of the cooling fan 190. That is, the suction part may also be disposed at right and right sides of the cooling fan 190. The installed position thereof may be provided at one or more selected positions of the upper, lower, left, and right sides of the cooling fan 190. The thermoelectric module accommodation part 53 has an opened rear side. Also, the thermoelectric module assembly 100 is inserted forward from the rear side of the grill fan 51 and is accommodated in the thermoelectric module accommodation part 53. A sensor installation part, in which a sensor for detecting a temperature and humidity of the cryogenic freezing compartment 200 is installed, continuously installed at a side of the thermoelectric module accommodation part 53. A defrost sensor is installed on the sensor installation part 54 to detect a defrosting time of a cold sink that will be described later, thereby determining whether defrosting is required. The sensor installation part 54 may be disposed at a position that may represent a state of the cryogenic freezing space when the space of the cryogenic freezing space is measured. According to an embodiment, since the suction part 533 is disposed at each of the upper and lower portions of the thermoelectric module accommodation part 53, it is advantageous for more accurate measurement that the sensor installation part 54 is installed to avoid the position. Thus, in this embodiment, the sensor installation part 54 may be installed on one side surface of the thermoelectric module accommodation part 53. Also, a through-hole is defined forward in the sensor installation part 54. Thus, an air atmosphere in the front of the sensor installation part may be transmitted to the inner space of the sensor installation part 54. The thermoelectric module assembly 100 is inserted forward from the rear side of the grill fan assembly 50 and is accommodated into and fixed to the thermoelectric module accommodation part 53. In detail, an outer circumferential surface of the cooling fan 190 having a box fan shape is disposed to face an inner circumferential surface of the thermoelectric module accommodation part 53 at the front side of the thermoelectric module accommodation part 53, and in a state in which the position is restricted, the outer circumferential surface of the cooling fan 190 is fixed to a front surface of the thermoelectric module accommodation part 53 by using a fixing unit such as a screw. Also, the thermoelectric module assembly 100 is inserted forward from the rear side of the grill fan assembly 50 so as to be disposed at the rear side of the cooling fan 190 and then coupled and fixed to the grill fan assembly 50 by using the fixing unit such as the screw. Although described below, a passage through which the refrigerant passes is provided in the heat sink 300 of the thermoelectric module assembly 100, and a refrigerant inflow tube 360 and a refrigerant outflow tube 370 through which the cold air is introduced and discharged are provided in the heat sink 300. While the refrigerator is assembled, the refrigerant inflow tube 360 and the refrigerant outflow tube 370 provided in the heat sink 300 of the thermoelectric module assembly 100 have to be welded to refrigerant tubes, through which the refrigerant flows, in the refrigeration cycle cooling device 70 of the refrigerator. Particularly, the inflow tube 360 may be connected to a rear end of the condenser, i.e., a rear side of an expansion device such as a liquid receiver and a capillary tube, and the outflow tube 370 may be connected to a front side of the evaporator. As described above, the thermoelectric module assembly 100 is fixed to be spaced a predetermined distance from the inner case 12 through a spacer 111 in the form of a module in which components (the cold sink, the thermoelectric module, the heat sink, and a module housing) illustrated in FIG. 13 are assembled. Thus, a worker may more easily perform the welding operation in the space that is secured by the spacer 111, and after the welding of the refrigerant tube is finished, the grill fan assembly 50 is installed at a rear side of the freezing compartment to fix the grill fan assembly 50 to the thermoelectric module assembly 100. The spacer 111 is fixed to the inner case 12 through a screw or is fixed to the inner case 12 in a manner in which a protrusion protruding from the inner case 12 is fitted into a hole defined in a rear portion of the spacer 111. FIG. 5 is a cross-sectional view taken along line A-A of FIG. 2. As illustrated in FIG. 5, a cryogenic case 210 has an opened front side, and an opening 211 is defined in a portion of a rear portion of the cryogenic case 210. As a result, the cryogenic case 210 has a box shape having an approximately parallelepiped shape, and a rail structure extending in a front and rear direction is provided on left and right surfaces and then fixedly mounted on the inside of the refrigerator. The cryogenic case 210 includes an outer case 213 facing the space of the freezing compartment 40 and an inside case 214 disposed inside the outer case 213 and coupled to the outer case 213 to define a predetermined space between the outer case 213 and the inside case 214. The insulation material 80 is disposed in the space between the outer case 213 and the inside case 214 to thermally insulate the inner space of the cryogenic freezing compartment and the freezing compartment 40. A foamed insulation material 81 such as polyurethane may be used as the insulation material. The foamed insulation material is configured to fix the outer case 213 to the inside case 214 in addition to the insulation function. A vacuum insulated panel 82 having better insulation efficiency may be further applied to the wall of the cryogenic case 210 that has to have a thin thickness. The opened front side of the cryogenic case 210 is opened and closed by a cryogenic compartment door 220. The cryogenic compartment door 220 has a predetermined space. Also, an insulation material is provided in the space to thermally insulate the inner space of the cryogenic freezing compartment 200 from the space of the freezing compartment 40. The cryogenic compartment door 220 may have a predetermined thickness for user's gripping feeling, and the foamed insulation material may be foamed into a hollow to securer rigidity. A cryogenic tray 526 accommodated into the inner space of the cryogenic case 210 is fixedly installed at the rear side of the cryogenic compartment door 220. The cryogenic tray 226 may be integrally behaved with the cryogenic compartment door 220. When the cryogenic compartment door 220 is withdrawn forward, the cryogenic tray 226 is slidably withdrawn forward from the cryogenic case 210. The cryogenic compartment door 220 is guided by an external rail disposed on a lower or bottom surface of the cryogenic case 210 to slidably move forward and backward. A portion of a rear wall of the cryogenic tray 226 may be opened so that the cold air that is cryogenically cooled in the thermoelectric module assembly 100 is introduced into the cryogenic tray 226 when the cold air flows forward by the cooling fan 190. Thus, when the cryogenic freezing compartment 200 is installed in the freezing compartment 40, since the opened rear surface of the cryogenic tray 226 faces the thermoelectric module accommodation part 53, the cryogenic cold air supplied to the front side by the cooling fan 190 from the thermoelectric module accommodation part 53 may be smoothly introduced into the inner space of the cryogenic tray 226. The cryogenic case 210 has a top surface that is slightly spaced apart from a bottom surface of an upper member of the inner case 12, i.e., a ceiling surface. According to an embodiment, the top surface of the cryogenic case 210 and the bottom surface of the upper member of the inner case 12 may cooperate with each other to realize a duct-like structure. Thus, the air discharged from the cold air discharge hole 522 defined in the upper end of the grill fan 51 may be guided forward along the duct-like structure to smoothly flow. Thus, even though the cryogenic case 210 is installed, the cold air may smoothly reach the door basket 27 installed in the inner upper portion of the freezing compartment door 22. To realize the above-described duct-like structure, an upper wall of the cryogenic case 210 has to have a thin thickness. That is, when the upper portion of the cryogenic case 210 has a thin thickness, the duct-like structure may be realized while securing an inner volume of the cryogenic case. In this respect, according to an embodiment, the foamed insulation material 81 may be foamed in a remaining space in state in which the vacuum insulated panel 82 is built in the upper member of the cryogenic case 210 so that the upper member of the cryogenic case 210 has the thin thickness. The foamed insulation material may be filled into the inner spaces of the outer case and the inside case 214, which are not filled by the vacuum insulated panel 82. Thus, coupling force between the outer case 213 and the inside case 214 may be improved in addition to the insulation performance. Furthermore, since the cold air discharge hole 524 that is disposed in the vicinity of the middle height of the grill fan 51 is disposed in the lower portion of the cryogenic case 210, the discharged cold air may smoothly flow forward. The thermoelectric module assembly 100 is an assembly in which the cold sink 120, the thermoelectric module 130, the insulation material 140, and the heat sink 300 are stacked and installed in the module housing 110 to form a module shape. The cold sink 120, the thermoelectric module 130, the insulation material 140, and the heat sink 300 are inserted into and fixed to an accommodation groove 113 of the module housing 110 in the state in which the cold sink 120, the thermoelectric module 130, the insulation material 140, and the heat sink 300 are closely attached and stacked by a closely attaching unit such as the screw. Also, the thermoelectric module assembly 100 may be mounted in a manner in which the module housing 110 is closely attached and fixed to a rear surface of the grill fan assembly 50. A specific structure of the thermoelectric module assembly 100 will be described below in more detail. FIG. 6 is a schematic view illustrating a configuration of the refrigeration cycle cooling device of the refrigerator. The refrigeration cycle cooling device 70 of the refrigerator according to an embodiment is a device for discharging heat inside the freezing compartment to the outside through the refrigerant passing through a thermodynamic cycle of evaporation, compression, condensation and expansion. The refrigeration cycle cooling device according to an embodiment includes an evaporator 77 in which a liquid refrigerant in a low-pressure atmosphere is evaporated by heat exchange with air in the cooling chamber (a space between the grill fan assembly and the inner housing), a compressor 71 for pressing a gas refrigerant vaporized in the evaporator and discharging a high-temperature high-pressure gas refrigerant, a condenser 73 for condensing the high-temperature high-pressure gas refrigerant discharged from the compressor 71 by heat exchange with air in the outside (machine room) of the refrigerator to discharge heat, and an expansion device 75 such as a capillary tube, which drops down a pressure of the refrigerant condensed in the condenser 73 to a low temperature atmosphere. The low-temperature low-pressure liquid refrigerant that decreases in pressure in the expansion device 75 is reintroduced into the evaporator 77. According to an embodiment, since heat of the heat sink 300 of the thermoelectric module assembly 100 has to be quickly cooled, the low-temperature low-pressure liquid refrigerant that decreases in pressure and temperature after passing through the expansion device 75 has to pass through the heat sink 300 of the thermoelectric module assembly 100 before being introduced into the evaporator 77. Thus, the refrigerant discharged via the capillary tube is introduced into the heat sink 300 through the refrigerant inflow tube 360 to cool or absorb heat generated from a heat generation surface of the thermoelectric module 130 and then is discharged from the refrigerant outflow tube 370 and reintroduced into the evaporator 77. The liquid refrigerant may quickly absorb the heat generated from the heat generation surface 130b of the thermoelectric module 130 through a thermal conductive manner using the heat sink 300 while passing through the heat sink 300. Thus, the heat of the heat sink 300 may be quickly cooled by the refrigerant circulating through the heat sink 300. In detail, the compressor 71 presses the low-temperature low-pressure gas refrigerant to discharge the high-temperature high-pressure gas refrigerant. Also, the refrigerant is condensed, i.e., liquefied while releasing the heat in the condenser 73. As described above, the compressor 71 and the condenser 73 are disposed in the machine room of the refrigerator. The high-temperature high-pressure liquid refrigerant that is liquified by passing through the condenser 73 may be introduced into the evaporator 77 in the depressurized state by passing the expansion device 75 such as the capillary tube. In the evaporator 77, the refrigerant is evaporated while absorbing heat therearound. According to the embodiment of FIG. 6, the refrigerant passing through the condenser 73 is branched into a refrigerating compartment-side evaporator 77b or a freezing compartment-side evaporator 77a. Here, the heat sink 300 of the thermoelectric module assembly 100 is disposed at the front side of the freezing compartment-side evaporator 77a and disposed at the rear side of the expansion device 75 in the refrigerant flow path. The cryogenic freezing compartment is a space in which a maximum freezing temperature of a temperature of about −50 degrees Celsius is to be maintained. Thus, when the heat generation surface 130b of the thermoelectric module 130 is maintained in a very cool state, the heat absorption surface 130a may be easily maintained in a colder state. Thus, a portion of the heat sink 300 through which the refrigerant flows may be disposed at the front side rather than the freezing compartment-side evaporator 77a in the refrigerant flow path and thus be maintained in the colder state. Particularly, since the heat sink 300 comes into direct contact with the thermoelectric module 130 to absorb heat from the thermoelectric module 130 in the conductive manner through a heat conductor such as metal, the heat generation surface 130b of the thermoelectric module 130 may be surely cooled. Also, while the cooling of the cryogenic freezing compartment 200 is performed, i.e., the refrigerant within the heat sink 300 cools the heat generation surface 130b of the thermoelectric module 130, the compressor may operate at a maximum output or an output higher than a set output to prevent the cooling efficiency of the freezing compartment from being deteriorated. When the cryogenic freezing compartment 200 is to be used at a temperature of about −20 degrees Celsius as in the normal freezing compartment without being cooled to a cryogenic temperature of about −50 degree Celsius, it is possible to be used as a general freezing compartment only by not supplying power to the thermoelectric module 130. In this case, if power is not applied to the thermoelectric module 130, the heat absorption and the heat generation do not occur in the heat sink of the thermoelectric module 130. Thus, the refrigerant passing through the heat sink 300 is introduced into the freezing compartment-side evaporator 77a in the liquid refrigerant state that is not evaporated because of not absorbing heat. That is, the cold air generated in the refrigerant cycle cooling device through the general compression manner may be supplied to the freezing compartment 40 and the refrigerating compartment 30 of the refrigerator. When the cryogenic freezing compartment operates, the refrigerant passing through the expansion device 75 may quickly absorb heat generated from the heat generation surface of the thermoelectric module 130 by passing through the heat sink 300 of the thermoelectric module assembly 100 so that the heat generated form the heat generation surface of the thermoelectric module 130 is quickly discharged and then is introduced into the evaporator 77a. Although the refrigeration cycle cooling device 70 in which the evaporators 77a and 77b are provided in plurality to individually cool the refrigerating compartment 30 and the freezing compartment 40 is described as an example in this embodiment, the embodiment may be equally applied to a refrigeration cycle cooling device in which all the refrigerating compartment 30 and the freezing compartment 40 are cooled by using one evaporator 77a. Hereinafter, a structure of the thermoelectric module assembly 100 will be described in more detail. FIG. 7 is a front perspective view of the thermoelectric module assembly. FIG. 8 is a rear perspective view of the thermoelectric module assembly. FIG. 9 is an exploded front perspective view illustrating a coupling structure of the thermoelectric module assembly. FIG. 10 is an exploded rear perspective view illustrating the coupling structure of the thermoelectric module assembly. As illustrated in the drawings, a thermoelectric module assembly 100 according to another embodiment may include a thermoelectric module 130, a cold sink 120, a heat sink 300, an insulation material 140, and a module housing 110. The thermoelectric module 130 is a device using a Peltier effect. The Peltier effect refers to a phenomenon in which, when a DC voltage is applied to both ends of two different elements, heat is absorbed into one side, and heat is generated from the other side according to a direction of current. The thermoelectric module has a structure in which an n-type semiconductor material, in which electrons are the main carriers, and a p-type semiconducting material, in which holes are carriers, are alternately connected in series. Here, an electrode portion for allowing current to flow from the p-type semiconductor material to the n-type semiconductor material is disposed on a first surface, and an electrode portion for allowing current to flow from the n-type semiconductor material to the p-type semiconductor material with reference to any one direction in which the current flows. Thus, when the current is supplied in a first direction, the first surface becomes the heat absorption surface, and the second surface becomes the heat generation surface. When the current is supplied in a second direction opposite to the first direction, the first surface becomes the heat generation surface, and the surface becomes a heat absorption surface. According to an embodiment, the thermoelectric module assembly 100 is inserted and fixed forward from the rear side of the grill fan assembly 50, and the cryogenic freezing compartment 200 is provided at the front side of the thermoelectric module assembly 100. Thus, the heat absorption occurs on a surface facing a surface defining a front side of the thermoelectric module, i.e., a surface facing the cryogenic freezing compartment 200, and the heat generation occurs on a surface defining a rear side of the thermoelectric module, i.e., a surface having a backdrop of the cryogenic freezing compartment 200 or in a direction facing the cryogenic freezing compartment 200. Also, when current is supplied in the first direction in which the heat absorption occurs on the surface facing the cryogenic freezing compartment in the thermoelectric module, and the heat generation occurs on the opposite surface, the freezing of the cryogenic freezing compartment may be enabled. In an embodiment, the thermoelectric module 130 has a flat plate shape having a front surface and a rear surface. Here, the front surface may be a heat absorption surface 130a, and the rear surface may be a heat generation surface 130b. The DC power supplied to the thermoelectric module 130 generates the Peltier effect. Thus, heat of the heat absorption surface 130a of the thermoelectric module 130 moves to the heat generation surface 130b. Thus, the front surface of the thermoelectric module 130 becomes a cold surface, and the rear surface becomes a heat generation portion. That is, it may be said that the heat within the cryogenic freezing compartment 200 is discharged to the outside of the cryogenic freezing compartment 200. The power supplied to the thermoelectric module 130 is applied to the thermoelectric module through a leading wire 132 provided in the thermoelectric module 130. The cold sink 120 may come into contact with and be stacked on the front surface of the thermoelectric module 130, i.e., the heat absorption surface 130a facing the cryogenic freezing compartment 200. The cold sink 120 may be made of a metal material or an alloy material such as aluminum having high terminal conductivity. A plurality of heat exchange fins 122, each of which has a shape extending vertically, are disposed to be horizontally spaced apart from each other on the front surface of the cold sink 120. The heat sink 300 may come into contact with and stacked on a rear surface of the thermoelectric module 130, i.e., the heat generation surface 130b facing the direction in which the cryogenic freezing compartment 200 is disposed. The heat sink 300 is configured to quickly dissipate or discharge the heat generated from the heat generation surface 130b by using the Pelitier effect. A portion corresponding to the evaporator 77 of the refrigeration cycle cooling device 70 used for the cooling of the refrigerator may be constituted by the heat sink 300. That is, when a process in which the low-temperature low-pressure liquid refrigerant passing through the
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