الفهرس 
BACKGROUND AND PROBLEM STATEMENTOnly 14 pages are availabe for public view
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Abstract
The study modelled a typical design of a medical irradiation facility and analyze its structural shielding system. MCNPX code was used to model the shielding system and analyze dose rate distribution under different dimensions and operating conditions. Dose rates were examined from three types of dimensions of the irradiation room, measuring 4m long by 4m wide by 3m high; 7m long by 7m wide by 3m high; and 10m long by 10m wide by 3m high. The operating conditions included having all the doors closed; having all the doors open; having only the shielded door closed and having only the shielded door open. A total of twenty (20) detectors were positioned at various locations for dose estimation. The shielded wall of the facility was made of ordinary light concrete of mass density 2.3 g/cm 3 . All other walls were made of burnt soil (American standard) brick of mass density 1.52 g/cm3 . The shielded door was made of 5 cm thick lead metal coated with 2.5 cm thick stainless steel. The doors to the control room and at the main entrance were made of ordinary dry pine wood of mass density 0.064 g/cm3 . All spaces in rooms contained dry atmospheric air of density 0.001205 g/cm3 . A 60Co point isotropic source with 44TBq activity and two main photon energies of 1.332 MeV and 1.173 MeV was used for the source definition (SDEF) card. Each simulation was terminated after 5,000,000 particle histories. Tally F5 was used to estimate dose rates in combination with the DE5 and DF5 cards to covert photon flux reaching a detector to dose rates using the flux-todose-rate conversion factors published in ANSI/ANS-6.1.1 1977 (also found under Appendix H of the MCNPX Manual). The estimated dose rates were multiplied by the source strength using the Fm5 card. The study used an occupational exposure design value of 2.0E-04 rem/h (2 μSv/h) and a public exposure design value of 1.0E-05 rem/h (0.1 μSv/h) to select the approximate shielding thickness that can reduce the dose rates to levels below these design values. The design values are also published by the IAEA in the International Basic Safety Standards of 2014. The results showed that increasing the irradiation room size leads to a decrease in dose rates in all the monitored locations. This decrease in dose rates was exponential and, in some locations, it was almost linear. The optimal shielding wall thickness was not the same for all locations i.e. the dose rates fell below the design value at different thickness in different locations. The study also found that operating conditions can easily alter the shielding capability of a shielding system. In all the three irradiation room sizes dose rates increased when the shielded door or all the doors were open during an irradiation procedure. It is therefore recommended that the design of irradiation facilities should take into account various operational scenarios like incidents in order to design an effective shielding system. The size of the irradiation room should also be optimized in order to provide adequate space and reduce dose rates.