Radiation shielding design, verification and dose distribution calculations for industrial and insect irradiation facilities

摘要

This study focused on analysis of the new radiation shield design, verification and doseuddistribution calculations for three irradiation facilities: The analysis of the new radiationuddesign concrete shield for the Citrusdal facility, and shield verification calculationsud(radiological safety assessment) for the already constructed Stellenbosch and HEPROudirradiation facilities. Additionally, as a subordinate objective, the study reports on doseuddistributions in the irradiated insect containers of the Citrusdal irradiation facility, andudsample product boxes of the HEPRO irradiation facility.udIn 2006 a private company, Citrus Research International (CRI), began the engineeringuddesign of a new insect sterilization facility in Citrusdal in the Western Cape region ofudSouth Africa, which was then constructed from ordinary concrete density (3 ¸ 2.35udg/cm$ ) in the year 2007. This facility employs a very successful ionising radiation basedudmethod, the Sterile Insect Technique (SIT), for drastically reducing or controlling insectudpopulation, which poses a serious pest problem in the Citrusdal agricultural area. TheudSIT uses the radionuclide 60Co as the source of ionising radiation. The design-baseudactivity of the 60Co source for the Citrusdal facility was 740 TBq (20 kCi). Therefore,udthe new radiation shield design was done for this source strength. Analysis of the shielduddesign has been carried-out using the integrated point-kernel (QAD-CGGP and theudMathCAD worksheet) and confirmed with a Monte Carlo (MCNPX) radiation transportudcomputer codes. This was done by considering the concrete shielding walls, the roof asudwell as the entrance/exit labyrinth at critical points for radiological protection purposes.udHowever, only MCNPX was used to ascertain whether the labyrinth entrance/exit wasudradiologically safe because the point-kernel uses a semi-analytical, approximateudmethodology that is not able to account for radiation streaming through labyrinth.udSatisfactory agreement was found between the three computational methods.udStellenbosch irradiation facility is being operated by the Agricultural Research Counciludas insect sterilization facility at Nietvoorbij, that is situated approximately 70 km fromudCape Town. This facility uses a radiation based Sterile Insect technique; using 60Co asudthe source of ionising radiation. The facility is designed chiefly for sterilization of fruitudflies which presents a major problem in the agricultural area in South Africa. The facilityudwas designed for a 5 kCi (0.185 PBq) 60Co source, but is now operated with a 10 kCiud(0.37 PBq) 60Co source (reference source strength in the year 1999). The source activityudwas 3.32 kCi (0.12 PBq) in October 2007, i.e., shield verification calculations wasudperformed for this activity. This was carried out using QAD-CGGP and verified withudMCNPX radiation transport computer codes, and where possible, validated withudmeasurements. Dose rates were measured with a calibrated gamma-ray monitor,udGRAETZ X5C. The spatial distribution of dose rates around the shielding concrete walls,udon top of the roof and in various positions along the maze were considered. The doseudrates at the closest distance (at 10 cm) from the exterior of the concrete shielding wallsudwere slightly higher. However, good agreement was found between measured andudsimulated (calculated) values.udHEPRO is a commercial irradiation facility situated in Ferrule Avenue, Montagu Gardens, Milnerton, City of Cape Town, Western Cape, South Africa. The HEPROudfacility is mainly used for food irradiation and sterilization of medical instruments. This facility was originally designed to operate with a maximum '!Co source strength of 1udMCi (37 PBq). However, to increase the product throughput rate, the source strength hadudto be increased to a nominal 1.5 MCi (55.5 PBq) in 2005/6. Nevertheless a radiologicaludsafety assessment was performed for a source strength of 2 MCi (2 37 PBq 74 PBq)ud60Co, because any radiation safety system such as radiation shielding must have a “safetyudfactor”. Again the point-kernel method was used for radiological safety assessment andudverified with MCNPX radiation transport computer code. Observation of critical spotsudand where possible the spatial distribution of dose rates were measured using a calibratedudgamma-ray monitor, GRAETZ X5C, to provide additional confirmation of results. Theudspatial distribution of dose rates around the shielding concrete along the walls, roof,udcable penetration inside the hut on roof, inside irradiation vault, above water pool,udhotspot on wall, pneumatic pipe penetrations, sliding plug-door, and ducts of the hoistcableudsituated on the roof of the locked hut were considered. The comparison of dose rateudvalues obtained using the aforementioned methods showed a slight deviation such thatudthey were considered satisfactory.udThe use of Monte Carlo computational tools such as MCNPX in support of theudprediction/assessment of the absorbed dose distributions in the irradiated product of theudirradiation facilities can prove to be economically effective, representing savings in theudutilization of dose meters, among other benefits. The absorbed dose distributions withinudthe insect containers were calculated with the model developed in MCNPX for one hourudof exposure. The modelling was carried out for a 60Co source with a nominal activity ofud16.8 kCi, i.e., 622 TBq. The calculated values were compared to an average value fromudthe preliminary experimental (sterility test) measurements. The sterility test for theudinsects, False Codling Moths, was done during the onset of the SIT programme at Citrusdal irradiation facility. The variability of the dose distribution inside the insectudcanisters were within acceptable dose values for adequate sterilising False CodlingudMoths.udA systematic computational calculation methods was used to model/simulate the sampleudproduct undergoing irradiation in a 60Co source for HEPRO industrial irradiator facility.udThis was carried-out using the Monte Carlo code—MCNPX. The calculation was doneudfor a source-rack containing 4 25 100 60Co pins, having a total activity of about 1udMCi (37 PBq). Four boxes were stacked around about 50 cm away from the sourceudcenterline. Each square box of sample product was rotated by 90° around the horizontaludaxis (perpendicular to the source rack), i.e., three times so that each of the four side ofudthe box faced the source rack once during an irradiation session. A specific Fortran-90udcode uses an algorithm to calculate the absorbed dose distribution for equal-timeudirradiation was therefore applied. The Fortran-90 code uses the dose values calculated byudMCNPX and sums them in a suitable way in order to reproduce the mentioned rotations.udThe calculations revealed highest absorbed dose at the edges of the box, and evenudslightly higher at the four corners. The lowest absorbed dose was noticed at the centre of the sample product.