One Driver, Two Runs a Month, and a Suspicious Dosimetry Record
A small carrier's only Class 7 driver has a recorded dose of 6.4 mSv for the year to the end of July, from a chest badge. The next run is a 4-hour round trip with three packages at TI 4.0, 2.5 and 1.5, hand-loaded into a 3.5 t van with no partition. Loading takes about 45 minutes at roughly 0.5 m from the packages; unloading about 30 minutes at similar distance. The driver does two such runs a month and does nothing else radioactive. The company's internal action level is 6 mSv, and the regulator's annual limit for the driver is 20 mSv. The driver says the badge is kept in a locker in the office and sometimes left in the cab.
1. Estimate the dose for this run and decide whether the driver should be assigned to it.
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Estimate first, decide second.
Convert TI to dose rate at 1 m by dividing by 100: TI 4.0 gives 0.04 mSv/h, TI 2.5 gives 0.025, TI 1.5 gives 0.015. Total at 1 m = 0.08 mSv/h. At the stated working distance of 0.5 m the inverse-square factor is 4, giving roughly 0.32 mSv/h at the body.
- Loading, 0.75 h: about 0.24 mSv
- Unloading, 0.5 h: about 0.16 mSv
- In transit, taking 1 m as the effective distance for 3 h: about 0.24 mSv
Run total: order of 0.6 mSv, so two runs a month for the remaining five months adds roughly 6 mSv, taking the record to about 12 mSv by year end.
That is under the 20 mSv limit, so the run is not prohibited on limit grounds. But it breaches the company's own 6 mSv action level and is not ALARA with a single driver absorbing all of it. The right answer is: assign with controls, not on autopilot - a load partition or extra distance, loader rotation if any second person is available, and a written review at the action level. If no control is possible, the run is still legal but the company must document why it is acceptable.
2. The 6.4 mSv on the badge by July does not fit this workload. What are the possible explanations and what do you check?
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The number and the work are inconsistent, and the badge handling statement is the clue.
At roughly 0.6 mSv per run and two runs a month, six months of work should produce something of the order of 1 mSv, not 6.4. Discrepancies this large are usually not real dose:
- Badge stored or transported near a source - the badge left in the office or in the cab is exactly the failure mode that produces an inflated record. Ask where it lives when not worn, and whether it has ever been left in the vehicle overnight with a load.
- Badge worn by someone else, or shared, or worn over shielding on the chest while the hands take the real dose.
- Dose transferred from a previous employer, or a prior year's dose carried into this record.
- Genuine dose from work not captured in the description - a second driver, a previous contract, an unrecorded consignment.
What to check: the badge's issue and return dates and its storage location; the dosimetry service's report and whether it flags anything; the driver's dose history; the loading records for the year. Do not simply accept 6.4 mSv, and equally do not dismiss it - if it is real, the workload estimate is wrong somewhere and the annual projection changes. Both errors matter, and only an investigation separates them.
3. The driver also asks whether hand dose matters, since the badge is on the chest. How do you answer, and what does it change about the loading method?
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Hand dose matters, and it is the dose most likely to be underestimated here.
During hand loading, the hands are at perhaps 0.2–0.3 m from the package surface, not 0.5 m. Applying inverse square to the same packages puts the local dose rate an order of magnitude above the chest value, and the extremities have their own dose limits and their own dose-assessment requirements. A chest badge records none of it. That is why a clean-looking 6.4 mSv chest record is not evidence that the driver's exposure is well controlled.
What it changes:
- Assess the extremities - extremity dosimetry, or a documented dose assessment based on measured dose rates at handling distance and recorded handling times.
- Change the method, not just the badge. Use handling aids and a trolley so the body stays back; load the highest-TI package first and against the bulkhead, furthest from the driving position; keep the loading time short rather than 'taking care' slowly at close range.
- Reinforce distance as the primary control. Time and shielding are secondary here; a step back during loading is free.
The judgement point: a compliant personal dose record does not prove that the highest-exposed part of the worker is being monitored.