The thoria dissolver, used for separation of ²³³U from reactor-irradiated thorium metal and thorium oxide rods, is no longer operational. It was decided to carry out  assessment of the radiological status of the dissolver cell for planning of the future decommissioning/dismantling operations. The dissolver interiors are expected to be contaminated with the dissolution remains of irradiated thorium oxide rods in addition to some of the partially dissolved thoria pellets. Hence, ²²⁰Rn, a daughter product of ²²⁸Th is of major radiological concern. Airborne activity of thoron daughters ²¹²Pb (Th-B) and ²¹²Bi (Th-C) was estimated by air sampling followed by high-resolution gamma spectrometry of filter papers. By measuring the full-energy peaks counts in the energy windows of ²¹²Pb, ²¹²Bi and ²⁰⁸Tl, concentrations of thoron progeny in the sampled  air were estimated by applying the respective intrinsic peak efficiency factors and suitable correction factors for the equilibration effects of ²¹²Pb and ²¹²Bi in the filter paper during the delay between sampling and counting. Then the thoron working level (TWL) was evaluated using the International Commission on Radiological Protection (ICRP) methodology. Finally , the potential effective dose to the workers, due to inhalation of thoron and its progeny during dismantling operations  was assessed by using dose conversion factors recommended by ICRP. Analysis of filter papers showed a maximum air borne thoron progeny concentration of 30 TWLs inside the dissolver.