Estimation of ventilation rate in uranium or thorium handling laboratories using short-lived thoron daughter activity

Natural uranium in oxide form is used as fuel in the Indian PHWR. Natural ²³⁸U fuel contains ²³²Th as an impurity to the extent of 50 - 60 ppm. This thorium impurity is converted to ²³²U in reactor during irradiation. ²³²U is converted to ²²⁴ Ra by alpha decays, ²²⁴ Ra further decays to ²²⁰Rn by alpha decays. ²²⁰Rn decays to stable ²⁰⁸Pb by emitting alpha, beta particles and gamma rays. ²²⁰Rn is inert gas but its daughter products are in particulate form. Effective half-life of Tn decay series is 10.6 hrs and four days are required to reduce the air borne activity concentration to negligible level on a filter paper sample. Uranium or thorium is handled remotely in the glove boxes with proper shielding. Glove boxes are under optimum negative pressure. Exhausts from glove boxes are connected to stack with proper filtration. Amber ares of the Lab is also supplied with conditioned air supply for human comfort and to keep the atmospheric thoron daughter concentration under control. Even after using proper engineering safety features, thoron that is in the gaseous form can came out from glove boxes due to holes on the neoprene gloves of micro or nano dimensions. Probability of thoron gas leakage is more during bagging out or bagging in operations. This gives rise to thoron daughter activity in the working atmosphere of Lab constantly and workers should be protected adequately