The Leaking Cs-137 Source in the Survey Meter Calibration Room
A radiation safety officer discovers that a 37 MBq (1 mCi) Cs-137 check source used for daily constancy testing of survey meters has been dropped and its capsule is cracked. The source is in a small calibration room (4 m × 3 m × 2.5 m) with the door closed. The RSO's survey meter reads 0.8 mSv/h at 1 m from the source location, but the meter has not been calibrated in 14 months. A technician who was in the room 30 minutes earlier reports no symptoms. The room is used by three staff members. The RSO must decide whether to enter, how to characterize the hazard, and what to do with the room and the staff.
1. What is the immediate hazard assessment, and how should the uncalibrated meter reading be treated?
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A 37 MBq Cs-137 source is a small but real hazard: at 1 m the unshielded dose rate is roughly 0.3 mSv/h per GBq × 0.037 GBq ≈ 0.01 mSv/h, so a reading of 0.8 mSv/h at 1 m is far higher than expected for an intact source and suggests either the meter is wrong, the source is not what is stated, or there is contamination/other material present. The first judgement is do not trust an uncalibrated meter for a decision to enter. Treat the area as potentially contaminated and potentially higher dose than indicated. Actions: (1) evacuate and secure the room, post warning signs, (2) perform a wipe test and a proper survey with a calibrated, energy-compensated instrument, (3) use a second independent meter, (4) don PPE and dosimeter before entry, (5) notify the licensee/RSO chain and, if contamination is confirmed, the regulator per the license conditions. The cracked capsule raises a contamination concern (Cs-137 is a beta/gamma emitter; the powder can spread), so air sampling and surface wipes are needed, not just a dose-rate survey.
2. The technician who was in the room for an estimated 10 minutes asks whether she has been harmed. How do you estimate her dose and counsel her?
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Estimate the dose from the best available data, acknowledging uncertainty. If the true dose rate at her working distance was on the order of 0.1–1 mSv/h and she spent ~10 minutes (0.17 h), the external dose is roughly 0.02–0.17 mSv - a small fraction of the 20 mSv annual limit and far below any threshold for deterministic effects. If contamination was inhaled or ingested, that pathway must be assessed separately (whole-body count, urine bioassay) because Cs-137 is readily taken up. Counseling: explain that the estimated dose is very low, that no acute effects are expected, that the estimate is preliminary, and that follow-up bioassay will confirm. Document the estimate and its assumptions. The judgement is to give a quantitative, honest, uncertainty-bounded answer rather than reassurance alone, and to arrange bioassay if contamination is plausible.
3. What are the waste-management and room-clearance steps before the room can be reoccupied?
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Steps: (1) contain and package the cracked source and any contaminated items as radioactive waste per the license, segregate by activity and half-life, and label; (2) decontaminate surfaces (removable contamination below the license's derived limits); (3) perform a final survey with a calibrated instrument and wipe test; (4) document clearance against the license's release criteria; (5) if contamination exceeds limits, the room stays restricted and the regulator is notified. Cs-137 has a ~30-year half-life, so decay-in-storage is impractical; the waste goes to an authorized receiver. The RSO should also investigate why the source was dropped and why the meter was 14 months out of calibration - a corrective action that prevents recurrence is part of the waste/clearance record. The judgement is that clearance requires both dose-rate and contamination criteria, and that the root cause is part of the response.