[en] Full text: A Design Guideline for ¹³¹I Therapy Facilities (1996) has been developed for the NSW Hospital and University Radiation Safety Officers Group (HURSOG) with recommendations on isolation rooms and delay tanks for storing waste water from the patient toilets. Shielding design for an isolation room is based on a methodology widely used in radiology and radiotherapy. The transmission factor (K) to achieve a dose limit of 20 μSv per week beyond a barrier is calculated for a specified number of ¹³¹I patients per week, average administered activity, and distance (source to recipient on further side of barrier). Default values are recommended for the workload, ¹³¹I dose rate and occupancy factors in the surrounding areas if not known. From the calculated value of K, the barrier thickness in lead or masonry can be obtained from tabulated attenuation data. Delay tanks are designed as a pair, alternately filling and holding waste water until the ¹³¹I activity discharged to the sewer complies with a limit of 20 ALI (i.e. 18 MBq) per week recommended by the National Health and Medical Research Council and acceptable to local water authorities. The Guideline has been used to design shielding for two isolation beds which were relocated to a ward with plasterboard walls. The two nearest adjacent beds were at a distance of 3.5 m. Due to space constraints, lead panels up to 25 mm thick were used for shielding rather than 30 cm thick concrete walls. A 14-week radiation survey of surrounding areas with TLD dosimeters during treatment of 33 patients per room, with an average administered activity of 5.1 GBq, showed doses at the four adjacent beds were below the design limit of 80 μSv per week for an occupancy factor of 0.25 conservatively based on length-of-stay bed data for the ward. Delay tanks installed at another hospital and sized according to the Guideline at 7 m³ per bed fill to 70-80% of capacity during a 3-month fill/3-month decay cycle