Radiological survey of a uranium pilot plant for rebuilding purpose

Changes in the process led to the necessity of rebuilding Metallic Uranium Development Pilot Plant that worked for several years with natural and depleted uranium casting salvage operations. For so, it was necessary a radiological survey of the facility. This survey followed recommendations of regional radiation safety norms and regulations (CNEN 3.01) and international recommendations of ICRP 26 and safety series 9. The purpose of this work is to present survey steps, which included workers dose rate measurements, direct and indirect surface contamination monitoring methods and air monitoring of the facility. Also it was discussed the data collected. It was noted from this radiological survey that even though health physics restriction measures, a low level and widespread contamination eventually occurred through all over the restricted area of the facility. It was convenient to divide the area in several sections for a better practical job. A general radiological survey was necessary for evaluation of essential health physics conditions before the beginning of decontamination procedures and rebuilding tasks. First thing to be done was air monitoring. It was studied the dispersion of nuclides particles considering inhalation risk. Monitoring points were chosen in agreement with worker's circulation in the area. For measurement it was used Millipore filters coupled to a vacuum pump. Nuclides concentration in the filters was determined by neutron activation technique. Equivalent dose rate was measured using a Geiger-Mueller detector considering worker as reference. Surface contamination was evaluated by direct and indirect methods. A portable Geiger-Mueller pancake and scintillation monitors were used for direct monitoring. For indirect method, dry smear test was applied by wiping 5 cm discs into a 100cm2 surface area. A scintillation counter was used for α and a Geiger-Mueller pancake counter for βcounting. Nuclides were identified by gamma-ray spectrometry in a sample collected in the section chosen for this work. It was identified 234Th and 235U radionuclides, by gamma-ray spectrometry indicating the presence of 238U and small amounts of 235U. So, contamination is basically composed by uranium compounds. Concentration in air is about a hundred times lower than more restrictive derived air concentration limit (DAC) for uranium nuclide. Dose rate measurements varies from 0,2 up to 60,0 μ Sv/h. Maximum activity per square centimeter from direct method of surface contamination monitoring, considering beta and gamma emissions is 315 Bq and alpha emission, is 24 Bq. By indirect method of surface contamination evaluation, maximum value is 16,0 Bq/cm2. Surface contamination values were not the same for direct and indirect methods, showing that most of contamination is fixed. Health physics conditions for the workers during decontamination processes must be directed considering that the contaminants are uranium compounds, although the concentration in air is low, loose contamination is present, so care should be taken to avoid intakes during the job. All procedures for local decontamination must be planned before, taking into account the variety of contaminated materials in the place and the facts already mentioned. Then, after decontamination jobs, facility will be ready for rebuilding tasks. (author)