Study of exposure to radiation in a lost wax foundry

Full text: Lost wax casting is an ancient method for making metal pieces. At the beginning of the process, the required metal piece is made in wax. After that, the wax model is covered with a silicaceous shell. When the silicaceous shell is finished, the wax is melted and replaced by liquid metal. As soon as the metal is cool the silicaceous shell is broken away. Finally a finishing process is made, and the piece is ready for utilization. The silicaceous shell is prepared with zircon sands containing varying concentrations of natural radionuclides: 238U, 232Th and 235U together with their progenies. For that reason it is very convenient to assess the dose to workers, considering the tasks they perform and the exposure time during these activities over a year. The first part of the study focuses on identifying the situations and areas where workers are exposed to radiation. The exposure pathways are: inhalation of dust, ingestion of dust, inhalation of radon, skin contamination and external irradiation. In a plant where this process is implemented the areas where workers are exposed are: the store, the shell building area, the foundry area, the shell or mold breaking area, and the waste area, including the areas close to the store. In these areas, workers are exposed while they work at their different tasks. The second part of the study deals with dose assessment. In most of the areas, external irradiation is the dominant pathway, and in order to assess doses a Monte-Carlo simulation using MCNP-4C code has been performed. The photon fluxes thus obtained are multiplied by the conversion factor of flux to kerma for air, by conversion factor to Effective Dose by kerma unit, and by the number of emitted photons per disintegration of parent nuclide. The application of this methodology to a given mill has produced the following results: Individual doses received by workers by area have a mean value of 5.9 μSv/y and a standard deviation of 7.7 μSv/y with an upper limit of 20.3 μSv/y and a lower limit of 0.2 μSv/y. The corresponding collective dose is 232.8 μSv/y and the distribution of dose by task being as follows: 1) Maintenance: carrying the zircon sands from lorry to store (0.039 μSv/y), placing the zircon sand pallet on the shelf (0.012 μSv/y), exposure while the pallet is on the shelf (2.8 μSv/y), taking the pallet off the shelf (0.022 μSv/y), taking the pallet to the mold preparation area (0.39 μSv/y). The total collective dose associated to this task is 3.263 μSv/y 2) Shell Making: shell preparation (58.2 μSv/y), shell cover (big trays) (76 μSv/y), shell cover (small trays) (9 μSv/y), exposures to molds (5.6μSv/y), loading the pieces on the wagon (2.1μSv/y), taking the wagon to the foundry area (7.4 μSv/y), taking the residues to the waste area (2.2 μSv/y), putting the pieces into the wax removing kiln (1.4 μSv/y). The total collective dose of this task is 161.91 μSv/y 3) Mold preparation (35.2 μSv/y) 4) Shell removal: waste material removal (0.4 μSv/y), and moving the cart from the foundry area (0.6 μSv/y) 5) Finishing (28.6 μSv/y) 6) Waste (0.00034 μSv) Workers in shell making are the most exposed, but the value of the collective dose is less than 1 mSv/y. Comparing with the levels of the graphical representation of the classification system, we can conclude that the processes 'do not need to consider regulation'. (author)