CA1142353A

CA1142353A - Melting furnace for radioactive wastes

Info

Publication number: CA1142353A
Application number: CA000362774A
Authority: CA
Inventors: Toshio Adachi; Nozomu Kumata; Susumu Hiratake; Makoto Osaki
Original assignee: Daido Steel Co Ltd
Current assignee: Daido Steel Co Ltd
Priority date: 1979-11-01
Filing date: 1980-10-20
Publication date: 1983-03-08
Also published as: FR2468980A1; DE3040755A1; GB2063438B; DE3040755C2; GB2063438A; FR2468980B1

Abstract

Abstract of the Disclosure A buffer layer is interiorly disposed on the bottom of a hollow furnace body, the buffer layer having an upper surface formed with a recessed portion to melt radioactive wastes therein. The radioactive wastes are fallen into the recessed portion through a guide cylinder and heated and melted by a heating means in said portion.
The buffer layer is provided to prevent the furnace body from being damaged by a shock which occurs when the radioactive wastes are fallen into the recessed portion.

Description

3;5 3 MELrrING F[~RNACE FOR RADIOACTIVE WASTES

Background of the Invention _ __ _ 1 Field o~ the Invention ___ . ___ . This invention relates to a furnace for melt-ing radioactive wastes arising from establishments for handling radioactive materials such as atomic power plants for the purpose of volume reducing treatment thereof.

2. Description of the Prior Art The radioactive wastes emit radioactivity therefrom and thus they are dangerous, and in addition, elaborate operation is hard to be carried out. Accordingly, in prior arts, when these radioactive wastes are melted, they have been indiscriminately charged into aknown melting furnace withoùt predividing them into heavy wastes and light-weighted.wastes. I~owever, if the wastes are melted in this way, the wastes are partly caught in the furnace when they are charged into the furnace or the wastes fall down the edge within the furnace thus failing to achieve good melting. Or, if the heavy wastes are charged, an intensive shock is transmitted to the furnace body to pose a problem of danger in damaging the furnace body.
y__f the Invention _ __ . It is an object oE the present invention to provide a melting furnace which can melt radioactive wastes to volume-reduce the wastes.

It is a further object of the present inventiOn to provide a melting fu~nace in which even if a radioactive waste is dangerous and hard to be handled as described above, - when the wastes are charged into the furnace in order to melt them, such charging operation can he done in a simple procedure.
That is, in the melting furnace of the present invention, a guide cylinder is provided to introduce the radioactive wastes to a predetermined place within the furnace, that is, a place where the wastes are melted.
Accordingly, the radioactive wastes may be merely put into the guide cylinder so that the wastes reach the predetermilled place within the furnace.
It is another o~ject of the present invention to provide a melting furnace which when the wastes are melted, can efficiently transmit heat to and rapidly melt massive wastes in the form of large bulk or slender wastes such as wires in the form of a small bulk or even inorganic wastes having a high melting point such as heat insulating materials.
That is, in the melting furnace of the present invention, the wastes introduced into the furnace are immersed in a high temperature liquid melt present in the furnace.
Accordingly, the wastes irrespective of their ~ul~, large or small, all touch the high temperature liquid melt.
Further, the wastes not only touch the melt with an outer surface thereof but tne liquid melt is moved round along the recessed portions and inner port:ions o~ the wastes and thus, any partsof the wastes touch the melt. In -this way, the wastes touch the high tempera-ture melt over the wide contact area. For these reasons, heat is always transmitted effic:iently to the wastes irrespective of their shape and melting is rapidly carried out.
It is yet ano-ther object of the present invention to provide a melting furnace in which this furnace is the melting ~urnace for radioactive wastes and is designed so that said wastes are charged into the furnace through a guide cylinder, whereby irregular heavy and light-weight wastes such as wire materials, columnar material and die steel materials are indiscrimi-nately charged into the guide cylinder and even if extremely heavy wastes are possibly fallen heavily in-to ~S the furnace through the guide cylinder at times it is possible to prevent a part of the furnace body rom being damaged as a result of a great shock directly received because of said falling, thus extending the service life of the Eurnace ~ody.
It is still another object of the present invention to prevent the need of operation for replacing refrac-tory and heat insulating materials which have been consumed within a short period of time~ and to prevent generation of the secondary waste.s (the residue from the above-mentioned refractory and heat insulating materials) resulting from such operation fo~ replacement.
Thus and in accordance with the broad concept of the invention, there is provided and claimed herein a melting furnace for radioactive wastes comprising:
a hollow furnace body; a buffer layer interiorly dispo-sed on the bottom of the furnace body; a recessed portion, in order to store a melt of the radioactive wastes, formed in the upper surface of the buffer layer;
a guide cylinder arranged in a state which in the furnace body, extends through a portion positioned .
; .

upwardly of the recessed portion, the guide cylinder having an ope~ing at the upper end thereof positioned externally of the furnace body and an opening at the lowe,r end thereof opposed to the recessed portion, the radioactive wastes thrown into the opening at -the upper end being guided toward the recessed portion; and a heating means mounted on the furnace body to heat the melt of the radioactive wastes stored in the recessed portion.
Other objects and advantages of the invention will become apparent during the following description of a preferred embodiment havin~ reerence to the accompanying drawings.
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- 3a -., Brief Description of the Drawings FIGS. 1 and 2 are respectively perspective views of a volume reducing system;
FIG. 3 is a longitudinal sectional view of a melting furnace and a pulverizing device;
FIG. 4 is a sectional view taken along line IV-IV
of FIG. 3;
FIG. 5 is a longitudinal sectional view of a solidifying devicei and FIG. 6 is a longitudinal sectional view showing another embodiment of the solidifying device.
Description of the Preferred Embodiments FIGS. 1 and 2 show the entire volume reducing system for wastes such as radioactive materials. In FIGS, 1 and 2, the wastes such as various filter, pipes, steels and other radioactive materials, which are packed into a container 1 (for example, a drum can of 200 Q), are carried into a processing room by means of a transport means 3 such as a roller conveyor through an inlet 2 of the processing room. A cover la of the thus carried container 1 is removed by means of a cover-removing device 4. This cover-removing device 4 comprises an expansible arm 5 and an attracting device 6 mounted on the end of the arm 5, the c~ver la being attracted and removed by the attracting device 6.
The container with the cover la removed is then fed to a laterally turning device i by means of the transport means 3. The laterally turning device 7 comprises a turn table 8, two support~posts 9, 9 and a container holder 10 .

s~
journalled on bo-th support posts 9, 9. The container holder 10 may be tilted in a direction of allow lla with the container held thereon. Then, wastes 13 within the container are discharged onto a hed 14. After the wastes 13 have been discharged, the holder 10 is returned in a direction of arrow llb. Next, the turn table 8 is rotated through 90 in a direction of arrow 12a. After rotated, the emptied container is a transferred on a transport means 15 such as a roller conveyor, and is carried out of an outlet 16.
L,arge wastesl~a out of those discharged onto the bed 14 are carried through a power manipulator 17 to a cut-ting device 18 where the wastes are cut into small wastes.
This miniaturization means that the wastes are formed into sizes enough to be charged into a processing container described later. The power manipulator 17 is movable in a horizontal plane as constructed in the ~ollowing. That is, two rails 19 (one of which is not shown) disposed in parallel have a movable frame 20 mounted movably in a direction of arrow 21, and the manipulator 17 is provided movably in a direction of arrow 22 with respect to the movable frame 20.
The manipulator 17 comprises a body 2~, a vertically moving rod 24 designed movably up and down with respect to the body 23, a plurality of links 25 connected to the lower end of the vertically movin~ rod 24, and a grip member 26 mounted on the end of the link 25. Thus, the grip member 26 can grip the waste 13a which can be carried to a suitable place oE a three dimensional space. The cutting device 18 ~23~;3 comprises a fixing device 27 and a plasma cutter 28. The plasma cutter ~8 is designed so tha~ it may be m~ved towards the arrow. The large waste 13a carried by the manipulator 17 is secured by means of the fixing device 27 and cut by the plasma cutter 28 into small sizes. The thus miniaturized wastes are returned onto the bed 14 by the manipulator 17.
It is noted that closed wastes, which are possibly exploded when heated, are bored by -the aforesaid plasma cutter 28.
In this cutting device 18, a suitable cutting mechanism can be used in place of the plasma cutter 28.
The wastes miniaturized or bored as described above and originally small wastes are charged into a processing container 32 through a charging chute 31 disposed on the end of the bed 14 by operation of the manipulator 17.
The processing container 32 is in the form of a cylinder as shown. The container 32 has its dimension, for example, outer diameter 390 mm, inner diameter 381, outer height 51 mm, inner height 487 mm, and capacity 50 Q. The container 32 is formed of materiàls such as metal plates such as iron or metal plates bored with numerous holes or netting.
The container 32 has its size determined in accordance with the succeeding melting furnace. The container 32 is to be pre-transported by the transport means 33 such ~s a roller conve~or to a position below the chute 31.
The container 32 with the wastes packed therein . is further carried by the transport means 33 and transferred onto a roller portion 33' mounted on a truck 35. The truck .

~Z3~3 35 may be reciprocated along rails 36 and 36 to feed the container 32 into an introducing device 38. The introducing device 38 comprises a cylindrical body 39 and a door 40 free to open and close disposed at an inlet thereof. When the door 40 is opened, the truck 35 moves forward towards the inlet of the body 39 to transfer the container 32 onto transfer means 41 such as a roller conveyor disposed within the body 39. After transfer, the truck 35 is with-dra~n and the door is closed. The container 32 is carried by the transfer means 41 and held by means of a holdinq frame ~2. A vertically moving device 43 is located above the holding frame 42. The vertically moving device 43 has a vertically moving rod 44 at the lower end of which is mounted a gripping device 45. When the container 32 is gripped by the gripping device 45, the holding frame 92 is withdrawn and a slide door 46 is opened. Then, the verti-cally moving rod 44 is moved down to charge the container 32 wi'th the wastes packed therein into a melting furnace 50.
In the melting furnace 50, the thus charged container 32 is melted by means of a heating torch 52 disposed within the furnacè body 51. A melt 53 obtained therefrom is taken out from a take-out hole 54 in the furnace body 51 through a take-out device 55 and fed into a pulverizing device 57. The aforesaid melt 53 is thrown into water ~ithin the body 58 and formed into particles 60.
- The particles 60 stay in the bottom of the body 58. When a bottom cover 59 is opened, the particles stayed in the 2~3 bottom of the body enter a bucket 61 together with water.
Since the bucket 61 has its bottom 62 in the form of a netting (a porous plate can be used), only the water is discharged.
The bucket 61 is fed to a next station by means of a truck 64 which is moved along rails 63. In the midst during movement of the bucket, the particles within the bucket 61 are dried by means of a dryer 65.
When a charging device 66 at the lower end of the buc~et is opened, the dried particles are ~acked into a storing container 67, which is then carried out by means of transport means 6~ such as aroller conveyor for storage.
For the storing container 67, a drum can, for exarrlple, of 200 Q may be used.
Next, the above-mentioned melting furnace 50 and the pulverizing device will be.further described in detail with reference to FIGS. 3 and 4 in addition to the preceding drawings. The furnace body 51 comprises a recessed water-cooled hearth 71 and an upper ~rame 72 covered thereon.
The water-cooled hearth 71 has its internal surfac~ coated with heat insulating materials 73 and 74. The heat insulat-ing material 73 used inclildes graphitic oxide (which is composed of 10 to 30 % of graphite and the remainder compris.ing a].umina or magnesia) also called the carbonaceous brick. This is used to enable energization between the hearth 71 and the melt 53 because a plasma torch is used as the heating torch 52, which will be described in detail :
.

~Z3~
later. Known refractorles may be used for the heat insulating material 7~ The heat insulating materials 73, 74 has a buffer layer 75 disposed internally thereof.
This buffer layer 75 is provided to prevent the heat ¦1 S insulating materials 73, 74 from being dama~ed as a result of direct touch of the melt 53 with the ~eat insulating materials 73, 74. The buffer layer 75 comprises a ballast layer 75a and a solid layer 75b positioned thereon. The ballast layer 75a is designed so that a number of massive buffer materials are disposed at random in a state where cleararlces are formed therebetween and they may be moved one another. For the buffer material, scrap-icon of the size about pebbles and cobble stones may be used, for example. The solid layer 75~ has its upper surface in the form of a recess which forms a reservoir 76 of the melt 53.
It will be noted that the solid layer 75b results from a mixture of the melted buffer material and the melt 53, said mixture being solidified.
A guide cylinder 80 is suspended from the middle portion of a top plate 72a of the upper frame 72. This guide cylinder 80 is of the water-cooled construction. The upper end of the guide cylinder 80 is in communication with the .
interior of the body 39 in the introducing device 38 through a cornmunicating pipe 81. Around the communicating pipe 81 in the top plate 72a of the upper frame 72, there are dis-posed vertically moving devices 82 at positions where the whole circumference is divided into three or four sections.

3~j3 Each of these vertically moving devices 82 has a vertically moving rod 83. The heating torch 52 is mounted on the lower end of the vertically moving rod 83. In the illustrated embodiment, an annular plasma torch is used for the heating torch 52. This heating -torch 52 is annually formed coaxial with the center axis of the guide cylinder 80 and has an annular arc discharge opening 84. The arc discharge opening 84 is directed in a direction in which the plasma arc at a temperature above 10000K is discharged toward the neigh-bourhood of the lower end of a ~ortion exposed from the meltand toward the upper surface of the melt 53 within the container 32. The vertically moving rod 83 is internally provided with a passage for supply of electrie power and gas to the torch 52, said passage having one end connected to the torch 52 while the other end being eonnected to a DC
power source 85 and an operating gas souree 86. A eoil 87 positioned in the outer periphery of the upper frame 72 is provided to apply the magnetie field to the annular are discharge opening 84 to rotate the arc along the diseharge opening, the magnetie field being adapted to discharge the arc from the entire zone of the arc discharge opening 84 in the toreh 82. It is noted that the aforesaid heating torch 52 may be replaced by a plurality of normal torches (for exarnpel, toreh 100 deseribed later) widely used or a gas burner or an oil burner. In this ease, the aforesaid coil 87 cannot be required. The upper frame 72 is partly formed with a gas outlet 88. ~nnecessary gases in the ~Z~3~3 furnace body 51 are delivered to a duct 89 through the outlet 88 and then discharged into atmosphere through a heat exchanger 90, a filter 91 for removing dusts con-taminated by the radioactive materials,blower 92 and stack 93.
In a part of the furnace body 51 positioned sideward the reservoir 7~ of the melt, the take-out hole 5~ is formed extending through the water-cooled hearth 71 and heat insulating material 73. In the water-cooled hearth 71, a forced cooling pipe 95 is provided, which is illustrated as forced cooling means, of which inner side forms a take-out hole. The forced cooling pipe 95 is of the dual cylinder, between which cooling water flows. Gases such as air or nitrogen may be used in place of water.
A take-out devicè 55 is connected to the forced cooling pipe 95. This take-out device 55 comprises a body 96 formed of metal (generally, iron) and a heat insulating material 97 lined on the internal surface thereof. The heat insulating material 97 is similar to the above-described heat insulating material 73. The take-out device 55 has at its lower part an outflow opening 98 through which the melt from the take-out hole 5g is fed into the device in next stage. The route from a port in communication with the ta~e-out hole 54 to the outflow opening 98 forms a down-flow passage 99 (surface of the heat insulating material 97) of the melt 53. The take-out device 55 further comprises first and second plasma torches 100, 101, ~ 11 --.
.
' respectively mounted on the body 96. The first plasma torch 100 is directed so as to irradiate the plasma arc into the take-out hole 54. The second plasma torch 101 is directed so as to i~radiate the plasma arc toward the outflow opening 98 and the do~m-flow passage 99. These plasma torches 100, 101 are also connected to a DC power sources 102, 103 and an operating gas source 86 similarly to the aforementioned torch 52. The take-out device 55 further has a gas outlet 104. Unnecessary gases are delivered to the duct 89 through 'the outlet 104. The body 58 of the pulverizing device 57 is connected to the body 96 of the take-out device 55 through a connecting member 107, the body 96 having an inlet 108 communicated with the outflow opening 98. In the pulverizing device 57, the inlet 10~ has a water pipe 109.
The water pipe 109 is partly formed with a flow-down opening 110, and water from the flow-down opening 110 flows down along the inclined surface 111 for pulverization and stays in the lower part of the body 58. The thus stayed water 'flows out of a water outlet 112 and is cooled by the heat exchanger 113, dusts ~such as particles or grains mixed into water) are removed by means of a filter 114, and water is then fed by a pump 115 to t'ne water pipe 109 for reuse.
The abode-described structure operate~ as follows:
The container with the wastes packed therein is charged into the guide cylinder 80 in the melting furnace 50 from ..... .the aforementioned in~roducing device 38. The containers may be suitably charged into the guide cylinder in a state ~2~:~5~

where a plurality of containers are stacked or one by one.
The thus charged container 32 together with the wastes is immersed in the melt 53 of which lower part is already melted. Thus, heat of the arc released from the torch 52 is not only transmit-ted directly to the containers 32 but transmitted to these containers 32 or wastes 13 even through the melt 53, ~hereby the containers and wastes may be heated in an extremely smooth manner. Because of this, the containers 32 and the wastes 13 may be melted rapidly into melt. Since the plasma arc emitted from the torch 52 is maintained at a temperature above lOOOOOK, electric power to the torch 52 may be regulated so that the melt 53 may assume a suitable high temperature. In this way, metal as well as wastes composed of inorganic material having a high melting point may be formed into a melt.
In charging these containers 32, even if they are violently lowered, the provision of the buffer layer 75 prevents the heat insulating materials 73, 7~ and water-cooled hearth 71 from being damaged. That is, even if the container 32 is heavily fallen down to produce a shock which is locally applied to a part of the solid layer 75b, the shock is dispersed into a wide region in the lower surface of the solid layer 75b and the shock transmitted to parts of the ballast layer 75a becomes -weak. Furthermore, the weak shock applied to the ballast layer 75a i5 absorbed by the ballast layer 75d itself and thus the shock transmitted to the heat insulating materials 73, 74 is very weak.

~1~23~

~ t the time of meltinq as described above, heating by means of the torch 52 and cooling by means of the water-cooled hearth 71 are simultaneously carried out. However~
since the heat insulating materlals 73, 74 are provided between.the buffer layer 75 and the water-cooled hearth 71, these heat insulating materials serve as walls for control-ling ~ransfer of heat to prevent heat applied from the torch 52 from being excessively taken away by the water-cooled hearth 71 or conversely to prevent the buffer layer 75 from being excessively melted by the heat from the torch 52. Conversely, the provision of the buffer layer 75 can prevent the heat insulating material from being overheated or from coming into direct contact with the melt, thus preventing consumption of the heat insulating material.
As for example, temperatures in normal state in the case of the foregoing are l,500C for the melt 53 in the vicinity of the center of the reservoir 76, 1300 - 1400C for the solid layer 75b, 700 - 1200C for the ballast layer 75a (the temperature in a portion c].oser to the solid layer 75b is high whereas the temperature in a portion closer to the heat insulating materials 73, 74 is low), and 610C
for the heat insulating materials 73, 74.
The melt 53 formed by melting the con~ainers 32 and the wastes 13 is taken out of the take-out hole 54 and drop out of the outflow opening 98 via the down-flow passage 99 of the take-out device 55. The dropping dew falls down on the pulverizing inclined surface 111 through .

.

~23~3 the inlet 108 of the pulveri~ing device 57, and the melt is then rapidly cooled by water into particles 60, which stay in the bottom of the body 58.
Next, where the melt 53 is stopped to be taken out, coolant such as cooling water is supplied to the forced cooling pipe 95. Then the melt present inside, that is, present in the take-out hole 54 is solidified and the solidified melt is formed into a plug which stops outflow of the melt 53.
Further, where the melt 53 is again taken out, the plasma arc is irradiated toward the aforesaid soli-dified material plugged in the take-out hole 54 from the plasma torch 100. Then the solidified material is melted and the melt 53 again flows out of the reservoir 76 through the take-out hole 54. In this case, where the melt previously moved out of the take-out hole 54 becomes solidified when it flows down through the down-flow passage 99 or the melt at the time of previous taking-out becomes solidified at the down-flow passage 99 so that the solidified material possibly impedes the flow-down of the melt which comes later or the solidified melt blocks the outlet 98, the plasma arc is irradiated toward the down-flow passage 99 or the outlet 98 from the plasma torch 101 disposed upwardly of the outlet 9~ to melt even those solidified and adhered thereto, thus effecting the smooth - flow-down of the aforementioned melt. For the above-described pulverizing device, there can be used/ for examp~e, a device ~4~
of the construction in which a melt drops on the rotary disk to scatter it in the form of a melted drop, and other devices.
Next, FIG. 5 shows a different embodiment of a device for Fosteriorly treating the melt 53 formed by melting the wastes 13 within the melting furnace 50.
This post-treating device is the solidifying device in which the melt 53 is solidified by a water-cooled mold 121, and the device is used in place of the above-described pulverizing device 57. The body 122 of the solidifying device is connected below the connecting member 107. A
vertically moving bed 123 on which the water-cooled mold 121 is placed is moved up and down by means of a lift not shown, and in its up position, the bed blocks a lcwer opening of the body 122 and positions the water-cooled mold 121 at a predetermined position to receive the melt 53.
The body 122 is provided with a gas outlet 124 through which exhaust gas is delivered toward the duct 89. Cooling water within a water tank 127 is circulated by means of pump 128 in the water-cooled mold 121 through cooling water pipes 125 and 126.
The melt within the melting furnace 50 is fed into the water-cooled mold 121 through the take out device 55 and solidified therein. In this case, if those not melted are present within the water-cooled mnld 121 or include an inflation of solidiEied material resulting from occluded gas at the solidification, the plasma arc 35~3 from the plasma torch 101 is irradiated toward the unmelted material within the water-cooled mold 1 1 through the outflow opening 98 to melt it.
FIG. 6 shows another solidifying device. In this device, in place of the aforementioned water-cooled mold there is provided a graphite crucible 132 interiorly of a storing container 131. The body 133 and vertically moving bed 134 are constructed equall~ to those of the aforementioned solidifying device. The body 133 has a plurality of cooling pipes 135 disposed therein. These cooling pipes 135 blow cooling gases, which are fed therein, against the container 131 from a number of nozzles to cool and protect the container 131. ~he gases ~at a high temperature) within the body 133 are moved out of the ~as outlet 136, after which the gases are cooled by means of a heat exchanger 1`37, and dusts are removed by means of a filter 138, the gases being compressed by means of a compressor 139 and fe;d into the cooling pipe 135. Surplus gases are moved out of the compressor 139, and thereafter the dusts are again removed by another filter 14~ and released into the atmosphere through a stack 141.
With the arrangement as described above, the melt fed into the crucible 132 is solidified therein.
When the crucible 132 is filled with the melt, the crucible 132 together with the container is carried out from the solidifying device. After carried out, a clearance 142 between the container 131 and the crucible 132 is filled ~'23~3 with concrete, and the solidified material within the crucible 132 together with the crucible 132 is sealed into the concrete and a cover is placed thereon.
It should be noted when the rnelt is put into the crucible 132 that the plasma arc from the plasma torch 101 is pre-irradiated ayainst the crucible 132 to preheat the crucible 132 so that all the melt introduced into the crucible 132 m~y be solidified in the form of a lump.
It should be also noted when concrete need not be sealed, the clearance 142 can be prefilled with heat insulating material such as magnesia particles.
As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A melting furnace for radioactive wastes comprising:
a hollow furnace body;
a buffer layer interiorly disposed on the bottom of said furnace body;
a recessed portion, in order to store a melt of the radioactive wastes, formed in the upper surface of said buffer layer;
a guide cylinder arranged in a state which in said furnace body, extends through a portion positioned upwardly of said recessed portion, said guide cylinder having an opening at the upper end thereof positioned externally of said furnace body and an opening at the lower end thereof opposed to said recessed portion, the radioactive wastes thrown into said opening at the upper end being guided toward said recessed portion; and a heating means mounted on said furnace body to heat the melt of the radioactive wastes stored in said recessed portion.

2. The melting furnace for radioactive wastes according to Claim 1, wherein said buffer layer comprises a ballast layer in which a number of massive buffer materials are disposed in a state where clearances are formed there-between and they may be moved one another, and a solid layer superposed on said ballast layer so as to completely cover the upper surface of the ballast layer, said recessed portion being formed in the upper surface of said solid layer.

3. The melting furnace for radioactive wastes according to Claim 2, wherein said furnace body is formed at its lower portion from a recessed water cooled hearth, said water cooled hearth being internally applied with a lining of heat insulating material, said buffer layer being positioned internally of said heat insulating material.

4. The melting furnace for radioactive wastes according to Claim 3, wherein said heating means comprises a plasma torch arranged around the opening at the lower end of said guide cylinder and adapted to release a plasma arc toward the melt of the radioactive waste stored in said recessed portion.

5. The melting furnace for radioactive wastes according to Claim 3, wherein a portion positioned sidewise of said recessed portion in said water cooled hearth and heat insulating material is partly formed with a take-out hole adapted to discharge the melt stored in the recessed portion outside, said take-out hole being provided with a controllable cooling means for cooling and solifify-ing the melt discharged through the take-out hole and a controllable heating means for melting a solidified material solidified within the take-out hole.

6. The melting furnace for radioactive wastes according to claim 4, wherein a portion positioned sidewise of said recessed portion in said water cooled hearth and heat insulating material is partly formed with a take-out hole adapted to discharge the melt stored in the recessed portion outside, said take-out hole being provided with a controllable cooling means for cooling and solidifying the melt discharged through the take-out hole and a controllable heating means for melting a solidified material solidified within the take-out hole.

7. The melting furnace for radioactive wastes according to Claims 5 or 6, wherein a down-flow passage is provided in the vicinity of said take-out hole in the outer surface of said water cooled hearth to permit the melt discharged out of the take-out hole to flow downwardly, an outlet is associated with the lower end of said down-flow passage to downwardly discharge the melt having passed down through the down-flow passage, and a pulverizing device is disposed at the lower position of said outlet to pulverize the melt having passed down.

8. The melting furnace for radioactive wastes according to Claims 5 or 6, wherein a down-flow passage is provided in the vicinity of said take-out hole in the outer surface of said water cooled hearth to permit the melt discharged out of the take-out hole to flow downwardly, an outlet is associated with the lower end of said down-flow passage to downwardly discharge the melt having passed down through the down-flow passage, and a solidifying device is disposed at the lower position of said outlet to form the melt having passed down into an ingot.

9. A melting furnace for radioactive wastes comprising:

a hollow furnace body interiorly provided with a vacant space for melting radioactive wastes;
a heating means disposed on said furnace body to heat and melt the radioactive wastes in said vacant space;
a take-out hole bored in a part of a portion positioned sidewise of said vacant space in the side wall of said furnace body in order to discharge a melt of the radioactive wastes melted in said vacant space;
a controllable forced cooling means mounted on said take-out hole to cool and solidify the melt positioned within said take-out hole; and a plasma torch disposed at a position opposed to said take-out hole externally of said furnace body in order to melt a solidified material solidified within said take-out hole so as to irradiate a plasma arc toward said solidified material.

10. The melting furnace for radioactive wastes according to Claim 9, wherein said forced cooling means comprises a forced cooling pipe of a dual-pipe construction which comprises an inner pipe encircling said take-out hole and an outer pipe encircling the outside of said inner pipe, a vacant space being formed between said outer pipe and inner pipe so that a coolant may be passed therethrough.

11. The melting furnace for radioactive wastes according to Claim 10,wherein said heating means comprises a plasma torch.

12. The melting furnace for radioactive wastes according to Claims 9, 10 or 11, wherein a down-flow passage is provided in the vicinity of said take-out hole in the outer surface of said furnace body to permit the melt discharged out of the take-out hole to flow downwardly, an outlet is associated with the lower end of said down-flow passage to downwardly discharge the melt having passed down through the down-flow passage, a plasma torch is disposed at the upper position of said outlet to irradiate a plasma arc toward the outlet in order to melt a solidified material solidified and adhered to said outlet, and a pulverizing device is disposed at the lower position of said outlet to pulverize the melt having been passed down.

13. The melting furnace for radioactive wastes according to Claims 9, 10 or 11, wherein a down-flow passage is provided in the vicinity of said take-out hole in the outer surface of said furnace body to permit the melt discharged out of the take-out hole to flow downwardly, an outlet is associated with the lower end of said down-flow passage to downwardly discharge the melt having passed down through the down flow passage, a plasma torch is disposed at the upper position of said outlet to irradiate a plasma arc toward the outlet and toward downwardly of the outlet through said outlet in order to melt a solidified material solidified and adhered to said outlet, and a solidifying device is disposed at the lower position of said outlet to form the melt having passed down into an ingot.