Ir-192 Source Fails to Retract at Night
During night-shift radiography of a 508 mm pipe spool, an Ir-192 source fails to retract into its projector. The guide tube is taped along the pipe, and the crank is at the projector about 1 m from the exposed end. The radiographer surveys with a calibrated meter: 2.5 mSv/h at 1 m from the guide-tube end and 0.9 mSv/h at 3 m. The area is already barricaded at 50 m, and two radiographers are on site. A hand crank is available, but the crank handle is only 1.2 m from the guide tube. The company emergency dose constraint for a planned source recovery is 0.5 mSv per person; a 5 mSv dose requires RPO authorization. The site RPO is reachable by phone but is 40 minutes away. The radiographers must decide whether to crank the source back immediately or wait.
1. Calculate the maximum stay time at 1 m and at the 1.2 m crank position for the 0.5 mSv constraint. What does the comparison tell you about the value of a long-handled crank?
Read the worked answer
Stay time is dose divided by dose rate. At 1 m: t = 0.5 mSv ÷ 2.5 mSv/h = 0.20 h = 12 minutes. At 1.2 m, inverse-square gives 2.5 × (1/1.2)² = 1.74 mSv/h, so t = 0.5 ÷ 1.74 = 0.287 h ≈ 17 minutes. The gain from 1.0 m to 1.2 m is modest because dose rate falls only with the square of distance; a 2 m handle would reduce the rate to about 0.63 mSv/h and allow about 48 minutes. The practical conclusions are: use the longest rigid tool available, keep the body as far back as possible, and treat time as the primary control. The 3 m reading of 0.9 mSv/h does not follow inverse square exactly because of scatter and geometry; do not use calculated distances for the barricade without a survey. A single person should perform the task, with a second person monitoring dose and time.
2. Should they crank immediately or wait for the RPO? Outline the decision sequence, including dose control, and state what records are required afterward.
Read the worked answer
First secure the area and confirm that no one is inside the barricade; the source is already exposed, so waiting does not increase the dose rate because Ir-192 decay over 40 minutes is negligible. Do not approach without a plan. Call the RPO, but in parallel prepare: two calibrated survey meters, personal dosimeters with alarms, a long-handled crank or remote tool, and a pre-briefed single operator. If the source can be retracted by normal cranking from 1.2 m in a few minutes, the estimated dose is only a fraction of 0.5 mSv and the task is justified as a planned recovery. If cranking binds or the dose rate rises, stop and wait for RPO direction; forcing a kinked cable can damage the source or guide tube. Afterward, record the survey map, times, distances, individual doses, source activity and serial number, and a detailed event report; inspect the guide tube and projector before returning to radiography. Re-entry is allowed only when a survey confirms the source is shielded and the area is safe.