Patent Publication Number: US-2002004017-A1

Title: Melting crucible

Description:
[0001] The present invention relates to a melting crucible and metal alloys for use as crucibles for the melting of material for the encapsulation of waste materials particularly, though not exclusively, radioactive waste material encapsulated by a so-called vitrification process where the waste is mixed with glass forming materials which are melted.  
       [0002] The so-called vitrification process for the encapsulation of high level radioactive waste (HLW) in particular, involves the mixing of the waste material with glass forming material and the subsequent melting of the glass forming material in a crucible so as to dissolve the waste therein. The waste results mainly from the dissolution of irradiated fuel assemblies which contain, in addition to the uranium fuel and fission products per se, large quantities of iron, zirconium and chromium from the fuel cans, mainly Zircaloy (trade name) alloy and from the fuel assembly stainless steel fittings. The waste is mainly in the form of oxides formed from the calcining of the nitrates resulting from the dissolution process (the so-called “PUREX Process”, for example).  
       [0003] The temperatures at which the glass and waste are processed is in excess of 1050° C., the molten material forming an aggressive corrosive medium with respect to the crucible material. The current alloy used for the manufacture of crucibles is known as Inconel 601 (trade name) (UNS N6601) which comprises in weight %: 23 Cr; 60 Ni; 1.3 Al; &lt;0.5 Co; 0.01 Si; &lt;0.1 C; &lt;1.0 Mn; &lt;1.0 W; &lt;1.0 Cu; Balance Fe. Each crucible produces about 25 kg of the product waste per hour and has a life of about 4000 hours consisting of 500 pours on an eight hour cycle before needing to be replaced. This alloy suffers from chromium depletion in the surface region in both the alloy above the molten glass level, i.e. in air, and also in the alloy below the glass level, i.e. leaching of the chromium by the molten glass. Owing to the nature of the material being processed, the proximity of operators and direct handling of the used life-expired crucibles is precluded. Consequently, the replacement of crucibles is a difficult and time consuming process as it must be undertaken by remote handling equipment. Furthermore, the used crucible itself also constitutes active waste which further adds to the waste which must be dealt with by processing and storing. Thus, any measure which will extend the service life of the melting crucible will provide valuable economic and ecological advantages.  
       [0004] It is an object of the present invention to provide a crucible of an alloy having a longer service life for the processing of HLW waste than known crucible alloys.  
       [0005] According to a first aspect of the present invention, there is provided a melting crucible, the crucible having a composition comprising in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C; 1.8-2.4 Al; 0.1-0.2 Ti; 0.05-0.12 Y; 0.010-0.10 Zr; 0.1 max. Mn; 0.5 max. Si; 0.1 max. Cu; Balance Ni apart from incidental impurities.  
       [0006] According to a second aspect of the present invention there is provided use as a melting crucible of an alloy having a composition comprising in weight %: 24-32 Cr; 8-11 Fe; 0.15-0.25 C; 1.8-2.4 Al; 0.1-0.2 Ti; 0.05-0.12 Y; 0.01-0.10 Zr; 0.1 max. Mn; 0.5 max. Si; 0.1 max. Cu; Balance Ni apart from incidental impurities.  
       [0007] Preferably, the chromium content lies in the range 26-32 wt %.  
       [0008] The melting operation for the encapsulation of HLW is generally carried out by induction melting under an air atmosphere, the crucible being induction heated and the waste charge being heated by radiation and conduction from the crucible wall. Most alloys used for high temperature oxidation resistant applications depend upon the formation of a continuous and stable oxide film on their surface, the oxide films generally comprising chromia (Cr 2 O 3 ) and/or alumina (Al 2 O 3 ). Alumina is generally the more stable oxide at temperatures above 1050° C.  
       [0009] During the vitrification process for HLW, the glass forming materials are selected specifically for their ability to dissolve a wide range of oxides including chromia and alumina which are contained in the calcined waste prior to mixing with the glass forming materials. The alumina results mainly from aluminium added as a metallurgical addition to the so-called “Magnox” or uranium metal fuel. Alumina dissolves very easily in the glass whereas chromia dissolves more slowly. The prior art crucible material, Alloy 601, is a chromia former, which type of alloys incidentally, are not normally used for glass processing applications since the chromia turns the glass green, however, since the waste itself contains chromia this is of little importance.  
       [0010] The alloy for use in the crucible according to the present invention has compositional similarities to Inconel 601 but contains increased aluminium and further has an addition of yttrium to improve oxidation resistance due to the formation of a continuous and stable alumina film in contrast to the chromia film of Inconel 601. The alloy also contains higher chromium and carbon to provide a dispersion of chromium carbide precipitates throughout the microstructure and which produces a significant improvement in high temperature mechanical strength and creep resistance. Inconel 601 does not contain any second phases to improve high temperature mechanical properties.  
       [0011] Testing of alloys used for the crucible according to the present invention indicate that a service life in the region of 6000 hours will be achieved, i.e. an increase of 50% in the service life.  
       [0012] Thus, since the alloy for the crucible of the present invention is an alumina former, it is surprising that the life and properties are greatly improved over the prior art alloy. It has been found that, in use, above the molten glass surface level, alumina forms as expected providing oxidation resistance in the air atmosphere pertaining above the glass level, whilst below the glass surface level, chromia forms, the chromium carbide dispersion effectively providing a reservoir of chromium. Thus, surprisingly the crucible alloy according to the present invention provides both superior oxidation resistance above the glass level in air and superior corrosion resistance below the glass whilst providing improved high temperature mechanical properties in both zones which may allow a greater degree of acceptable corrosion/oxidation before the crucible needs replacing. 
     
    
    
     [0013] In order that the present invention may be more fully understood, an example will now be given with reference to the accompanying drawing which shows a schematic cross section through a crucible according to the present invention.  
     [0014] The drawing shows the crucible  10  of a waste melting plant. The main melting part  12  of the crucible is surrounded by induction heating means  14  and a lower neck portion  16  has separate induction heating means  18  therearound. Waste from a calciner (not shown) is led into the crucible indicated by arrow  20  as is glass forming frit, known as “crizzle” in the industry, indicated by arrow  22 . The mixture is melted in the main part  12  of the crucible. A plug  26  of solid vitrified material is left in the neck portion  16  from a previous melt. Once the charge  24  is fully molten and at the correct temperature, the plug  26  is melted and the molten charge  24  is run out into a stainless steel container  30  for long term storage.  
     [0015] In the drawing, the crucible  10  is made from an alloy having a composition in weight %: 25 Cr; 10 Fe; 2.1 Al; 0.2 C; 0.1-0.2 Ti; 0.05-0.12 Y; 0.01-0.1 Zr; &lt;0.5 Si; &lt;0.1 Mn; Balance Ni. Above the molten surface level  28  (and on the outside of the crucible) the oxide film (not shown) on the surface of the crucible is predominantly alumina whereas the oxide film on the crucible surface below the surface  28  is predominantly chromia. The alumina above the glass level is undepleted and the chromia below the glass level is also substantially undepleted for much longer than the Inconel 601.  
     [0016] Thus the crucible made from and using the metal alloy according to the present invention provides a large and surprising advantage over crucibles made from known alloys.