Recycling of Steel by Melting
Scenario 2: Carbon Steel Casting Shielded Disposal Bo
The final product is a cast iron disposal box (CIDBOX) for medium and low level waste, which would be manufactured from radioactive carbon steel arising from the decommissioning of Spanish nuclear installations. The manufactured boxes would be transported to the disposal site and used instead of concrete containers.
Quantity of Carbon Steel Required and Arisings of Suitable Material
The concrete disposal containers currently used at the El Cabril disposal facility in Spain number about 440 - 500 per year and it is envisaged that these concrete containers could be partially replaced by metal containers manufactured of recycled scrap steel. The nominal capacity of the recycling plant would be of the order of 2000-3000 t per year, giving the possibility of manufacturing in the region of 200 such boxes per year. These boxes could than be used to substitute approximately 100 concrete containers from El Cabril. The proposed cast iron disposal box (CIDBOX) has a total volume of about 5 m 3 (2.25 m x 2.25 m x 1.1 m) with 50 mm of wall thickness and allows the storage up to 9 standard 200 l drums. The net weight (including lid) is 9500 kg with an additional estimated load of 3000 kg of radioactive wastes. The CIDBOX is shown in Figure 2. The CIDBOX is such a size that two units are dimensionally equivalent to the present concrete container used for final disposal in El Cabril. So the CIDBOX could be used instead of a concrete container for the storage up to 18 standard 200 l drums.
More than 20,000 t of carbon steel scrap could be available from year 2000 from the decommissioning of nuclear installations in Spain. This quantity is sufficient to supply material for 10 years to such a recycling plant. It is not considered that a recycling plant could be in operation in Spain before the year 2005.
The manufacturing steps for the CIDBOX are the following:
• Segmentation of scrap for transport
• Transport and monitoring of radioactive material
• Reception, sorting and storage at the melting plant
• Preparation of scrap for melting
• Loading of furnace and melting
• Ingot production.
• Storage and characterisation
• Selection of ingots for melting and casting preparation
• Melting and casting iron box
• Cast iron box transportation to finishing shop and storage
• Box and secondary wastes transportation to disposal site
• Conditioning wastes in boxes and disposal
There is not enough scrap arising at any one site to justify an on site recycling installation within Spain, therefore it has been considered that the melting and manufacturing plant will be integrated as a specific installation of an existing conventional melting/casting plant or as an independent melting, casting and manufacturing plant. The nominal capacity of the plant would be 2000-3000 t per year and the furnace would be electric induction type of 4.5 t capacity.
Besides the equipment and installations of a commercial melting or casting plant, some ancillary areas or systems are also required inside the plant, eg storage and handling areas for radioactive scrap, cutting equipment for sizing scrap to the furnace size, radiological protection systems, special ventilation and filter systems, monitoring, radiological characterisation, etc. The scrap arriving at the plant, inside transportation containers, must be temporarily stored on site in order to fit the planned melting campaign.
The type of material required for CIDBOX is cast iron. The chemical content of the cast iron can be controlled in the melting process and adapted to the final product specifications. The first stage in the melting process is that of ingot production from the melted scrap. This is followed by a second melting stage where the ingots are remelted to produce the disposal box.
In this scenario the radioactive scrap must be prepared at the decommissioning site (eg first stage decontamination, segmentation, etc.) and transported in an acceptable container to the recycling plant. An average distance of 600 km has been assumed from the location where the scrap arises to the melting plant. A distance of 700 km has been assumed from the melting plant to the disposal site where the boxes will be used. The amount of secondary wastes generated during the melting and fabrication operations has been assumed to be 3.5% of the initial load of scrap.
The general acceptance criteria for metal melting in the plant are estimated to be the following (based on available literature and discussions with plant operators):
Radiation level per package < 0.1 mSv/hr contact (1 cm)
Surface contamination or activated metal < 0.1 mSv/hr average contact (unshielded) Radionuclide limits:
In general terms the average radionuclide concentration shall not exceed the 1500 Bq/g for Co-60 or gamma - beta emitters and 100 Bq/g for Alpha emitters over the package or component. There would also be a list of radionuclides with an upper activity limit defined for each one of them, however a detailed list is not given here.
Doses to workers involved in such a recycling facility have been estimated based on the IAEA dose model and are presented in Appendix 3.
Technical Details of Scenario 2
The boxes would be hollow boxes measuring 1 m x 2 m x 2 m with a wall thickness of 5 cm. Given that the boxes will be subjected to mechanical stress, atmospheric corrosion, possible vibrations, and so on, the ideal material for this application is "As-Cast" ferritic nodular cast iron. There is no need for thermal treatment given that its mechanical characteristics (resistance, toughness, coefficient of elasticity, etc) are similar to those of common steel. It has a greater resistance to corrosion than common steel and its capacity for absorbing vibrations is, likewise, greater than that of common steel.
It is possible to obtain this quality of material using a load based on common steel provided that adequate additions of graphite and silicon-iron are made so as to ensure that the final carbon equivalent of the product is equal to, or slightly greater than, 4.30%. With regard to the matrix, this may be ferritic, pearlitic or pearlitic-ferrite "as-cast", and it will have the subsequent mechanical characteristics of iron. The greater the amount of ferrite, the lower the resistance to tension and the lower the brittleness. The greater the amount of pearlite, the more the resistance to tension, but the lower the toughness and the greater the brittleness. A ferritic matrix may be achieved by the combination of two parameters: a minimum amount of pearlitizing elements (Mn, Cu, Ni, etc) in the composition or a slow cooling of the piece within the cast.
All of these metallurgical variables and parameters must be defined and established when designing the final required product with precision. At the moment, it is thought that it will be possible to obtain a ferritic nodular casting using scrap steel with adequate raw materials available on the market to be added to loads based on scrap steel if necessary.
The most suitable production technology, and the technology used in this study, is sand- casting of 2 m x 2 m x 0·05 m and 1 m x 2 m x 0·05 m pieces in Ferritic Nodular Iron. A definition of the most important technical equipment and installations required, together with an estimate for establishing a sand-casting nodular iron smelting plant is given below:
• A smelting and running out installation.
• A sand-casting installation.
Obviously, this equipment should have sufficient capacity and characteristics for manufacturing nodular iron pieces of up to 2 m x 2 m x 0·05 m. Further details of specifications for the equipment listed above are not included here, but can be obtained from [21].
Economic Evaluation of Scenario 2
In this section a comparative economic assessment between direct disposal of metallic scrap and the alternative of the recycling by means of disposal CIDBOX manufacturing is carried out.
The associated costs involved in the recycling option include :
• Cutting and packaging (in 20 ft ISO containers) of the arising scrap.
• Scrap transportation.
• Investment in a metal recycling installation.
• Operating the metal recycling installation including storage, sorting, melting, casting and finishing.
• CIDBOX and secondary wastes transportation.
• Conditioning drums in CIDBOX and disposal.
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The following cost data have been assumed for the majority of the metal melting scenarios requiring small radiological melting facilities. The data are based on an annual throughout of 2,000 t per year which is the lower range of the capacity of the furnace and operation of the melting facility for a 10 year period. The data have been obtained from detailed discussions with operators and manufacturers of such facilities:
PROJECT COST
Engineering 20 Million pts £100,000 143 k ECU
Civil Works 80 Million pts £400,000 573 k ECU
Equipment 110 Million pts £550,000 788 k ECU
Tests 10 Million pts £50,000 72 k ECU
Radiological Protection 25 Million pts £125,000 179 k ECU
Control 12 Million pts £ 60,000 86 k ECU
TOTAL 257 Million pts. £1,285,000 1842 k ECU
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OPERATION
Workers 70.8 Million pts/year £354,000 /year 507 k ECU
Technicians 24.0 Million pts/year £120,000 /year 172 k ECU
TOTAL 94.8 Million pts/year £474,000 /year 679 k ECU
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ENERGY
1 kWhr by kg of metal
OVERHEAD EXPENSES
60 Mill pts/ year or £300,000 /year or 430 k ECU / year
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DISMANTLING
190 Mill pts or £950,000 or 1362 k ECU
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OTHERS
15 Mill pts / year or £75,000 /year or 108 k ECU / year
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PROJECT
Engineering 10 Million pts £50,000 72 k ECU
Civil Works 35 Million pts £175,000 250 k ECU
Equipment 55 Million pts £275,000 395 k ECU
Tests 7 Million pts £35,000 50 k ECU
Radiological Protection 15 Million pts £75,000 108 k ECU
Control 5 Million pts £25,000 36 k ECU
TOTAL 127 Million pts £635,000 910 k ECU
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OPERATION
Workers 70.8 Million pts/year £354,000 /year 507 k ECU
Technicians 24.0 Million pts /year £120,000 /year 172 k ECU
TOTAL 94.8 Million pts /year £474,000 /year 680 k ECU
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ENERGY
1 kWhr by kg of metal
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OVERHEAD EXPENSES
60 Mill pts/ year or £300,000 /year or 430 k ECU
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DISMANTLING
80 Mill pts or £400,000 or 573 k ECU
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OTHERS
15 Mill pts /year or £75,000 /year or 107.5 k ECU
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The above costs have been used to derive the costs of disposal per tonne of material and the costs of recycling per tonne of material.
Cutting and placement of scrap in drums 215 ECU/t
Conditioning wastes 72 ECU/t
Transport 290 ECU/t
Manufacture of concrete container 3585 ECU/ box Disposal of concrete container 32250 ECU/ box
Drums 60 ECU/ drum
Container capacity 18 drums
Drum capacity 300 kg scrap
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Conditioning and transport (5.4 t) 3096 ECU
Drums (18 Drums) 1097 ECU
Disposal 36 k ECU
TOTAL 40.03 k ECU
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The cost of disposal of the scrap metal if it were not recycled has been included in the direct disposal cost in order to compare options.
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1 CIDBOX = 9.5 t of steel and 380 kg secondary wastes (2 drums)
Dismantling Activities 430 ECU/t
Melting 860 ECU/t
Casting 774 ECU/t
Transport 286 ECU/t
TOTAL 2350 ECU/t
Conditioning 1792 ECU/ box
Manufacture of CIDBOX 2332 ECU
Secondary waste disposal 3585 ECU
TOTAL 25916 ECU
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Re-evaluation of Scenario 2 with Common Cost Assumptions
The costs for each of the main stages in the scenario are summarised here using two disposal charges - one typical of a near surface disposal facility and another typical of a deep geological repository. The cost analysis below includes common assumptions relating to transport costs and scrap handling charges. The calculation of disposal costs for secondary waste and for the disposal option includes minimal volume reduction prior to disposal. Supercompaction of some wastes may be carried out but calculations have assumed a packing density of 1.2 t/m3 for secondary wastes from processing and 1.5 t/m3 for direct disposal of wastes.
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Capital cost of plant 2,752,000
Operating costs of plant (per year) 2,778,000
Decommissioning costs of plant 1,935,000 (70% of capital cost)
Costs of containers for disposal of secondary wastes (drums) 60 Costs of containers for disposal of secondary waste (concrete) 3585 Total disposal charge for secondary waste (shallow) 5,806,000 Total disposal charge for secondary waste (deep) 158,049,000 Costs for handling of wastes/scrap and transport (717 ECU/t) 14,334,000 TOTAL (shallow) 52,607,000
TOTAL (deep) 204,850,000
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Cost of buying final product (each) 3,585 Costs of containers for disposal of wastes (drums) 60 Cost of containers for disposal of wastes (concrete) 3,585 Costs for handling of wastes/scrap and transport (575 ECU/t) 11,467,000 TOTAL (shallow) 41,451,000
TOTAL (deep) 733,548,000
Cost of recycling - disposal (shallow disposal) 11,156,000 Cost of recycling - disposal (deep disposal) -528,700,000 Scenario 2 - Conclusions
The above economic assessment demonstrates that the recycling option for suitable scrap carbon steel for manufacture into steel disposal containers is economically attractive given the assumed costs for disposal and manufacture for Spain. The re-evaluation of this scenario with disposal charges appropriate to near surface and deep disposal facilities assumes a common approach for transport and scrap handling charges so that scenarios can be directly compared. This indicates that the disposal and recycling costs are very close but a possible increase in near surface disposal charges would be required to make the scenario economically attractive.