The Tritium Laboratory at CDTN, Brazil: construction and operation

Full text: As it is well known the tritium concentration in the atmosphere was greatly increased as a result of the nuclear weapons testing, especially in the north hemisphere reaching values in excess of 1000 TU. Modern days values declined to levels below 50 TU with the decline attributed to the elimination of atmospheric nuclear weapons testing and radioactive decay. Differently in the south hemisphere, the tritium concentration increase was significantly lower. Nowadays the concentration in the south hemisphere is around 3 to 4 times smaller. The smaller tritium concentration in the south hemisphere leads to the necessity of using adequate equipment and procedures in order to get much lower concentration limit of detection. Therefore, the CDTN/CNEN Nuclear Technology Development Center, where tritium analysis is performed since the year 1968, decided to construct a new laboratory to measure tritium at low level. A measurement chamber was constructed especially designed for the low level tritium laboratory. Thick concrete layer on the ground plus 1.2 meter thick water massive shielding all around, over and below the chamber significantly lowered natural background. Measurements show inside the chamber only 2% of the gamma radiation (E>50keV) which is observed outside. Generally elevated radiation in environment is due to higher content of uranium in the soil. Water surrounding the chamber (ca.200 tons) additionally stabilizes temperature inside. Ventilation and air condition system provides constant temperature and humidity. Small corridor at the entrance and plastic shoe cover enable significantly lower dust level inside. Counting is performed in LSC Tri-Carb model: 2770 TR/SL (Packard Instrument Company) equipped with BGO system, in operation since August 1999. The initial estimated detection limit for determination tritium content in water samples for single measurement was slightly under 0.2 TU with a measurement time of 600 min/sample and the mean background for tritium-free water of 0.84 cpm. The graphic control following the determination of the detection limit has shown a reduction of the background to values between 0.60 to 0.70 cpm. If the new background value shows consistence over a year a new detection limit will be calculated. Measurement overall error for low activity samples (composed uncertainty; 1σ level) with regard to parameters obtained for the test electrolysis was assessed to 0.3 TU. Most part of the Laboratory clients are: mineral waters and several big mineral companies, universities and research centers. Through IAEA projects the Laboratory has performed tritium analysis for several Latin American countries. The IAEA has given support to the development of the whole new laboratory project with technical orientations for the construction, commissioning and staff training within the IAEA projects, namely: - BRA/8/02: 'Assesment of Groundwater Resources in Minas Gerais' - BRA/8/036: 'Sustainable Integrated Management of the Guarani Aquifer System'. (author)