The final products are ISO containers which would be manufactured from radioactive carbon steel arising from the decommissioning of Spanish nuclear installations. The ISO containers would be used for transport of radioactive materials only.
ISO containers will be manufactured using 2 m wide plates. A large installation would be required in order to manufacture these plates as well as a workshop would be implemented for manufacturing the containers. The diagram of an ISO container is shown in Figure 4. An ISO container weighs around 4.5 t, approximately 2.5 t of which corresponds to plates and 2.0 t to commercial profiles. It is estimated that 10 ISO containers would be necessary per year with a plate production of 25 t per year. The minimum capability of a rolling mill installation is 200,000 t by year.
The estimated quantities of radioactive scrap arising from the decommissioning of Spanish nuclear installations from 2000 onwards is more than 20,000 t. This quantity is sufficient for supply material during 10 years to recycling plant. It is not considered that a recycling plant could be in operation in Spain before 2005. Thus for the manufacture of ISO containers only a 1.25% of the annual scrap production would be required.
Process Steps for Scenario 4
The main activities of this scenario are the following:
• Segmentation of scrap for transport.
• Transport and monitoring of radioactive material.
• Reception, sorting and storage in the melting plant.
• Preparation of scrap for melting.
• Loading of furnace and melting.
• Ingots production.
• Storage and characterisation.
• Selection of ingots for remelting.
• Reheating ingots.
• Hot rolling manufacturing.
• Finishing mill.
• Cooling bed and coiled.
• Cutter plates.
• Manufacturing of ISO containers.
• Secondary wastes transport to disposal site.
Since there is not enough scrap to justify an on site recycling installation, it has been considered that the melting and manufacturing plant will be integrated as a specific installation in an existing conventional melting/casting plant or as an independent melting, casting and manufacturing plant.
In addition to the equipment and installations of a commercial melting or casting plant, some ancillary areas or systems are also required inside the recycling plant, eg storage and handling areas for radioactive scrap, occasional cutting equipment in order to fit the size scrap to capability of furnace, radiological protection system, special ventilation and filter systems, monitoring, radiological characterisation, etc. The scrap arriving to the plant, inside transportation containers, must be temporary stored on site in order to fit the planned melting campaign. The nominal capacity of the plant could be about of 2000-3000 t per year and the furnace would be electric induction type of 4.5 t capacity.
The general acceptance criteria for metal melting in the plant are estimated to be the following (based on current literature and discussions with plant operators):
Radiation levels:
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 will also be a list of radionuclides with an upper activity limit defined for each one of them, although details of this list are not considered here.
The dose rate calculation for this reinforcing bars scenario should be developed according to the IAEA models and on the base of a maximum of 200 Bq/g in the final product. Estimated doses to workers based on the IAEA dose model are presented in Appendix 3.
Technical Details of Scenario
The size of the plates to be manufactured would be 3 mm thick, 2 metres wide and of variable lengths. The manufacture of this kind of material would be by means of a hot rolling process using an appropriate base product. The manufacturing process is similar to that used in the manufacture of bars although it must be borne in mind that the mills used in the production of flat products are fed with flat slabs rather than ingots or square slabs. The rolling cylinders are usually flat and, generally, the widths tend to oscillate between 1·5 to 2 metres, although they may even reach 4 metres. To obtain thin plates (between 1·5 and 6 mm thickness) continuous or semi-continuous mills are used.
Once the rolling temperature has been reached, the slab is taken from the reheating furnace and by means of a roller path driven by motors it is transported to a descaling mill composed of vertical edgers and a horizontal two-high mill the principal mission of which, rather than to reduce the width, is to break the layer of scale and facilitate its elimination by means of water under high pressure.
The slab, free from scale, goes along the same roller to a roughing mill where the thickness is reduced (eg from 200 mm to 25 mm) for it to move on into the finishing mill. Before the plate enters the finishing mill, the head and the tail are cut off to eliminate defective areas - cavities, segregation - and the defects in form due to the rolling - fish tail - eliminating once more the scale that has formed by means of water under pressure. The finishing mill is made up of various four-high stands in which, by means of different reductions, the final thickness is obtained. At the exit of the finishing mill, the plate is subjected to rapid cooling which can be modified by means of a series of water showers in the cooling bed. Subsequently, the band is coiled or cut for storage.
Whilst the installation would be similar to that used for rolling bars, in this case for 2 m wide plates a larger installation would be required for manufacture of these products. Basically, from the furnace to the different stands of the rolling mill the sizes, capacity and power
should be greater than those needed for rolling bars. The rolling mill may be either continuous or semi-continuous but will in both cases be more expensive than the one used in the manufacture of bars. In this case, the installation could use a semi-continuous mill. Further details of the rolling manufacturing process are given in Reference [21] and are not included here.
Due to the width of the plate to be obtained, the original ingot must be larger than in the case of bars, in order for a sufficiently large slab to be obtained for subsequent reduction. Therefore, the furnace and the rolling mill to be used will be practically identical to those used in industrial plate manufacturing plants, with the corresponding high cost and need for a great deal of available space. Consequently, this production line is not considered to be viable from a technical and economic standpoint for the quantity of production under study. Economic Assessment of Scenario 4
An economic assessment of this scenario has not been carried out because the extra capital cost required for such a rolling mill would be such that the scenario would be economically unattractive, given the low rates of utilisation of such a plant.
Scenario 4: Conclusions
This scenario has been considered to be unfeasible due to the extra capital cost incurred in the provision of a rolling mill to produce steel plates for the manufacture of ISO containers and the small utilisation of the plant for the ISO container manufacture requirements compared to its capacity. In addition the fact that an ISO container would be made of a slightly radioactive material raises problems in itself, with the administrative and radiological controls required for using this transport container. For all these reasons this scenario is not considered further.